ISSN 1526-5757
55. Two
patterns of monomineral replacement in granites
Rong Jiashu
(Beijing Research Institute of Uranium Geology)
Abstract: Metasomatic
phenomena of single minerals in granites can be basically classified into two
major patterns: 1) hetero-orientation replacement (nibble replacement) and 2) co-orientation
replacement. The major formation mechanisms of mineral replacement are the dissolution-precipitation (for hetero-orientation [nibble] replacement pattern) and the cation exchange (for sheet silicates of co-orientation replacement pattern). The simultaneous presence of both the replaced mineral on one
side of a grain boundary and the same or similar mineral
as the replacive mineral on the other side is a major necessary condition of formation mechanism for hetero-orientation (nibble) replacement pattern.
Clear rim and intergranular albite are formed by replacement of
K-feldspar by albite under the effect of pure Na-bearing hydrothermal gas or fluid. If Ca
accompanies the Na-bearing gas or fluid, excess
SiO2 from the replaced K-feldspar remains and takes the shape of
vermicular quartz in replacive sodic plagioclase.
Perthitic albite lamellae look like they have formed
either by metasomatic processes or by exsolution from the primary K-feldspar
crystal, but they are probably formed mainly by simultaneous crystallization.
The key evidence for replacement is the presence of
relics from the replaced mineral in the newly formed replacive mineral.
However, the replacement phenomenon should be distinguished from simultaneous
crystallization and normal inclusions in igneous rocks.
Generally, a phenocryst can simply be distinguished
from a porphyroblast. When several metasomatic processes are superimposed, the
steps in the replacement process can possibly be determined and understood in
the light of the replacement rule.
Keywords: metasomatism
(replacement), co-orientation, hetero-orientation, dissolution-precipitation,
cation exchange, clear rim, intergranular albite, myrmekite, perthitic albite,
porphyroblast, phenocryst, simultaneous crystallization.
Figures: For those
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63.
Introduction
After formation of granite, gases and solutions
(either relevant or irrelevant to magmatic processes) can penetrate into
solidified granitic rocks, resulting in instability or change of an individual
primary mineral followed immediately by deposition or crystallization of a more
stable new mineral, thereby partly changing the primary mineral into a new
substance with different local structure.
Lindgren (1925)
stated: “Replacement in solid rocks
consists in solution of the host mineral, followed immediately by deposition of
an equal volume of the guest mineral……The volume of the replacing mineral
equals the volume of the mineral replaced.
Deposition follows so closely upon solution that at no time can any open
space be discerned under the microscope.”
Mineral replacement,
such as cation exchange, deuteric alteration as well as pseudomorphism,
chemical weathering, leaching, diagenesis and metamorphism are all linked by
common features in which one mineral is replaced by a more stable one (Putnis,
2002).
Such replacements imply that metasomatism occurs in an
open system that is different from classical metamorphism in which in situ
mineralogical change occurs in a rock (generally accompanied by loss of water)
without appreciable change in the chemistry of the rock.
It is necessary to
have gas-liquid fluid from outside to participate in replacement. Many
petrographers emphasized that the original rock should have been
cataclastically deformed before the gas-liquid fluid can penetrate and
circulate in the rock and cause the metasomatism. Other petrographers observe that replacement
really occurs in undeformed rock. The
original rock texture has been basically preserved even so far. They hold that
under a certain temperature and pressure the gas-liquid fluid surely could have
moved into and out of such a solid rock.
The period needed for the gas-liquid fluid to reach a certain distance
in the rock must be long because the rate of flow is slow, but in terms of
geologic time, the duration is short.
The metasomatic phenomena surely could have occurred in essentially
undeformed rocks for most of the examples illustrated in this article.
Moreover,
under new physical-chemical circumstances, the compositional transformation of
an old mineral (via cation exchange) locally or wholly into a new one without
dissolution-reprecipitation by gas-fluid is also a reaction product of
metasomatism.
Generally speaking,
the metasomatic process is characterized by the following features:
1) A rock remains in solid state as the whole
metasomatic process proceeds.
2) Transformation
(reformation) or dissolution of a previous mineral occurs almost simultaneously
with formation of regenerated minerals without any evidence of open space.
3) The volume of the
replacive guest mineral is equal to the volume of host mineral that is
replaced.
If one mineral is partially dissolved by one solution
and if after an interval during which open spaces exist and new minerals are
deposited in these spaces by solutions of a different composition, this process
should not be treated as replacement but as a type of filling.
Metasomatism is conceptionally different and
distinguished from isomorphism that occurs during magmatic crystallization.
Metasomatic phenomena are similar to cotectic ones when two minerals
simultaneously crystallize, so they may often be confused. Besides, metasomatism is easily
indistinguishable from unmixing (exsolution) in a solid solution.
The replacing
phenomena discussed in this paper apply to single individual mineral
replacements in granites that are observed during microscopic thin section
studies. The replacement of a primary
mineral by an aggregate of new minerals is more complicated and not discussed
here.
Two major patterns of
single mineral replacement in granites
According to the author’s observation (based on
similarities or differences of crystallographic lattice and orientation of
replacive and replaced minerals), metasomatic phenomena that occur in single
minerals in granites can be basically classified into two major patterns: 1) hetero-orientation replacement (nibble
replacement) and 2) co-orientation replacement.
Hetero-orientation replacement (nibble replacement)
occurs in undeformed intrusive rocks soon after rock-formation, as well as in
broken or gneissic rocks.
Nibble replacement occurs at the boundary between two
mineral grains. Taking the crystallographic lattice orientation of the same or
similar mineral upon which the replacive mineral contacts, the replacive
mineral gradually grows into the adjacent mineral capable of being replaced, as
if nibbly eating it. The unreplaced part of the replaced mineral survives in
the replacive mineral without changing its crystallographic orientation.
The crystallographic lattice orientations of replacive
and replaced minerals must be different or discordant. That is, the nibble
replacement occurs at the boundary between two minerals with different
orientation. No nibble replacement is
possible along the boundary between two minerals (plagioclase and K-feldspar,
for example) where they have the same and parallel crystallographic
orientation.
Only where no identical or similar mineral is present
to allow growth, an impurity in the rock could serve as a crystal nucleus.
Nibble replacement phenomena that are commonly
observed in granites are albitization, K-feldspathization, muscovitization,
quartzification, as well as berylitization, calcitization and pyritization.
1)
Nibble
replacement albitization
Nibble replacement albitization in granites,
especially rich in alkali, quite often occurs at the grain boundary between
plagioclase and K-feldspar as well as two K-feldspars.
a)
Nibble replacement albitization at contact of plagioclase with
K-feldspar
Taking the crystallographic lattice of the plagioclase
as crystal nucleus, the replacive albite grows nibbly towards the adjacent
K-feldspar.
A K-feldspar megacryst may always contain inclusions
of plagioclase. Albite rim (named ‘clear rim’ by Phemister,
1929) often surrounds the plagioclase inclusion. The width of albite rim is
commonly <
“Clear rim” occurs only at contacts of plagioclase
with differently oriented K-feldspar, but is absent where plagioclase contacts
co-oriented K-feldspar (Pl
Small relicts are found in the clear rim in some
places where the rim is thicker and the perthitic albite lamellae are more
developed (Fig. 1E, 1F
and Fig.
2).
b) Nibble
replacement albitization between two K-feldspar grains
The albite grains often found at a grain boundary
between two K-feldspars (K1 and
Both clear albite rim and intergranular albite are
situated at contacts with K-feldspars.
Their thicknesses in a given rock are similar, although intergranular
albite has generally less continuity than that of a clear albite rim.
There are several explanations for the origin of clear
albite rims and grain boundary albite grains:
(1) Unmixing
(exsolution) of adjacent K-feldspar (Phemister, 1926;
Tuttle, 1952; Ramberg, 1962; Phillips, 1964; Hall,
1966; Carstens, 1967; Haapala,
1997);
(2) Late stage
crystallization of magma (Rogers, 1961; Peng, 1970;
Hibbard, 1995);
(3) The
sericite in the rim of sericitized plagioclase is dissolved away by sodic
hydrothermal solution (Cheng, 1942, 1962);
(4) Plagioclase
is replaced by K-feldspar (Deer, 1935; Schermerhorn,
1956).
Both clear albite rim and intergranular albite are
situated at contacts with K-feldspars.
Their thicknesses in a given rock are similar, although intergranular
albite has generally less continuity than that of a clear albite rim.
The author considers that the following relationships
are favorable for determining the genesis of the feldspars.
(1) Tiny
relicts of perthitic albite of outside K-feldspar can be found in some places
in a thin clear albite rim or intergranular albite (Fig. 1E, 1F, 2,
3A,
3B, 5). The relicts strictly maintain the
primary orientation, although they are finer and clearer. Similar but coarser relicts of perthitic
albite have also been found associated with clear albite where Na-metasomatism
has modified K-feldspar in the
(2) A clear albite rim appears at a contact of
plagioclase with K-feldspar no matter whether the plagioclase is sericitized or
unsericitized (Fig. 1D, 1E).
(3) Very fine
vermicular quartz can be found in intergranular or clear rim albite with An
>2 (Fig. 2),
but is absent in a pure albite rim (An≈0).
(4) A clear
rim is blocked where contacts occur with quartz or another plagioclase
(Fig. 1A, 1C).
Although the continuity of intergranular albite is
less than that of the clear rim, the phenomena are identical, indicating that
they are of the same origin (nibble replacement).
It is difficult to determine the relation between
perthitic albite and a clear rim, as well as intergranular albite, because
⑴ the chance contacts between them are rare due to their limited
development; ⑵ perthitic albite in the form of a wedge or comb penetrates
the clear rim (Fig.3C) or intergranular albite, and in some
places even as a veinlet that penetrates the
intergranular albite (Fig. 4) as if the perthitic albite replaces the
intergranular albite (also see Figs. 428,
Although it is difficult for albite to replace an
entire plagioclase crystal, the replacive albite may still replace tiny perthitic
albite. Moreover, the presence of tiny relicts of perthitic albite in the
replacive albite is easily missed if the quartz plate is not inserted.
2)
Nibble
replacement K-feldspathization
The objects of nibble replacement K-feldspathization
are mainly plagioclase and K-feldspar.
a) New
K-feldspar nibbly replaces old plagioclase
Plagioclase crystals are often enclosed by K-feldspar.
When nibble K-feldspathization occurs, the grain
boundaries of these plagioclases are complicated and circuitous. Some remnants
of plagioclase were situated at borders with K-feldspar (Fig. 6).
This phenomenon is always treated as an important
evidence of intense K-feldspathization and, therefore, the megacryst of K-feldspar
has a porphyroblast origin. Surely, the plagioclase is really replaced by
K-feldspar, but the replaced part is mainly focused at the small limited area
around the relicts of plagioclase nearby. It is hard to judge whether the whole
uniform K-feldspar away from the complicated boundaries has a metasomatic
origin.
Fig.
6 shows
that the newly formed (metasomatic) K-feldspar penetrating into the relict
areas is comparatively pure and containing less perthitic albite than that in
the outside surrounding K-feldspar. The
author considers that the latter is the preexisting primary K-feldspar and the
former has a metasomatic origin. If the
primary K-feldspar (K
Note that the orientation of the replaced plagioclase
inclusions is obviously different from that of the surrounding K-feldspar (Pl2,
Pl3, Pl
b)
New K-feldspar nibbly replaces old K-feldspar
The appearance of
K-feldspar grains in the rock is quite unique at first glance under
cross-polarized light. The K-feldspathization that occurs at a contact border
between two K-feldspar grains is characterized by the nibble replacement of
primary K-feldspar by newly formed K-feldspar. The latter usually contains tiny
perthitic plagioclase vermicules with the same orientation as the host
replacive K-feldspar, radially pointing toward the border (Fig. 9)
instead of the normal perthitic albite stringers enclosed in primary
K-feldspar. In some places, the relicts
of primary K-feldspar (Fig. 9) or its perthitic albite lamella (Figs. 10,
11) may be enclosed in the newly
formed replacive K-feldspar, which can be seen when a quartz plate is inserted.
Fig.
10 shows that the metasomatic K-feldspars
(K1,
Cathodoluminescent
image (Fig. 10C) shows that the back ground of all
K-feldspar luminesces
dead-wood tan, that the replacive K-feldspar in many places has bright sky-blue
luminescence, and that the primary K-feldspar luminesces
pinkish purple. But part of the bright sky-blue color also extends to the
primary K-feldspar.
Both the pinkish
purple (for primary K-feldspar) and bright sky-blue (for replacive K-feldspar)
disappear at any intensely argirillized part with dark color of K-feldspar.
The orientation
of the metasomatic K-feldspar (Kx') is different from that of the replaced
K-feldspar, but coincides with the adjacent K-feldspar (Kx) on which it
epitaxially grows. Because the size of the replacive K-feldspar may be as much
as
An unusual
perthitic K-feldspar, containing vermicular plagioclase lamellae in monzonitic
rocks from the Maronia pluton in northern
Note that
vermicular albite occurs in newly formed K-feldspar replacing the primary
K-feldspar, while no vermicular albite occurs in newly formed K-feldspar where
it replaces the primary plagioclase.
3)
Muscovitization
Many petrographers consider that muscovite is
metasomatic in origin. However, some
muscovite could have a primary origin (under conditions of >1500 Pa and
a) Muscovite replacing K-feldspar
Nibbly replacive muscovite occurs always in branched
form towards and into K-feldspar (Fig. 15).
b) Swapped rims of muscovite between two biotite
grains
At the grain boundary between two leucocratic biotite
grains (protolithionite) in leucogranite, two rows of small grains of muscovite
can be seen to nibbly replace the two adjacent biotite grains in some places (Figs.
16, 17). The two rows of muscovite have the
same crystallographic orientation as the opposite biotite respectively
(according to the consistency of cleavage and the difference in pleochroism and
interference color). This phenomenon, however, could easily be absent or hardly
exist due to the limited contact of two protolithionites and the tiny growth of
replacive muscovite.
4) Quartzification
K-feldspar can
partly be replaced by quartz in leucocratic granites rich in silica and alkali
and poor in calcium, magnesium and ferrous iron. The quartz outside the
idiomorphic or hypidiomorphic K-feldspar can be inferred as primary in
origin. The quartz (Q
After intense quartzification (and muscovitization),
K-feldspar can completely be replaced resulting in residual skeletal perthitic
albite and more idiomorphic albite (Fig.
19).
Quartz newly-formed may nibbly replace calcite, after
the calcite has replaced primary quartz in sodic metasomatite from Jiling
granite,
5) Berylitization
In some places small miarolitic cavities appear in
apical or periphery facies of leucogranite rich in silica and alkali which is
formed at late stage of magmatic evolution. In the center of such miarolitic
cavities anhedral beryl (Ber) is surrounded by idiomorphic K-feldspar and
albite. The anhedral beryl should be primary in origin. The beryl (Ber’)
distributed irregularly in K-feldspar with the same crystallographic
orientation as that of the anhedral beryl outside the K-feldspar has a nibble
replacement origin (Figs. 21, 22).
Albite and biotite are comparatively stable during berylitization.
The above-mentioned nibble metasomatic growth of a
rock-forming mineral occurs in granitic rocks where a lot of identical or
similar minerals would naturally serve as a crystallographic nucleus for nibble
replacement. In those places where no
identical or similar mineral occurs, an impurity in a rock could serve as a
crystal nucleus for growth; for instance, in sericitization (even
muscovitization), calcitization (carbonitization) and pyritization.
6) Sericitization
and muscovitization of plagioclase
Numerous randomly distributed schistose sericite, even
muscovite (Ms’) without any relation and contact with muscovite and biotite
outside the plagioclase may be formed inside plagioclase after alteration (Fig. 23).
7) Calcitization (calcite replaces quartz and
K-feldspar)
Calcitization is one of the processes of alkali
metasomatism in granites. Primary quartz is replaced by calcite (branch-like) (Fig. 24)
and then becomes relicts in calcite (Fig.
25), even disappears. If
quartz disappears, calcite can further replace K-feldspar resulting in isolated
relicts of perthitic albite lamellae in calcite (Fig. 26).
At that time, the crystal grains of chlorite (changed from biotite) and albite
(transformed from plagioclase) are essentially retained.
8) Pyritization
Pyrite crystals are usually absent in fresh
(unaltered) granite. Perhaps a very small amount of pyrite may exist as an
accessory mineral. However, a lot of idiomorphic cubic crystals of pyrite
(d≈
Co-orientation
replacement means that the replacive mineral has the same or nearly the same
crystallographic lattice as the replaced mineral; i.e., the replacement process
proceeds along the lattice orientation of a replaced mineral, resulting in
partial or even complete transformation of the original mineral into a new
one. Included in the co-orientation
replacement pattern are deanorthitization of plagioclase (transformed in situ
to albite), co- orientation albitization of K-feldspar, as well as
muscovitization and chloritization of biotite.
1)
Co-orientation albitization (deanorthitization) of plagioclase
Deanorthitization
of plagioclase is often seen in altered granite. Because of alteration
(sericitization for oligoclase and less calcic andesine and saussuritization
for more calcic andesine and labradorite), the plagioclase is changed in situ
into albite while preserving the original twinning. During deanorthitization, it seems as if the
Na ion enters as the Ca ion is subtracted from the crystallographic
lattice. In addition to Na, this pattern
of replacement needs SiO2 to be introduced. The surplus Ca and Al from the replacement
process as well as other impurities can move out, forming zoisite,
clinozoisite, epidote, sericite and even calcite in altered plagioclase. The
core with higher An values of normally zoned plagioclase is generally changed
to obvious albite while the rim composition remains unchanged
(Fig. 28).
Tiny grains of topaz and fluorite contained in altered plagioclase
(albite) of granite rich in F are also reported (Haapala,
1997). Thus, the whole plagioclase turns into or approaches stable albite. The
distribution of sericitization in plagioclase is not homogeneous. The
sericitized part of plagioclase is albitized, while the unsericitized
plagioclase may retain its original composition (Fig. 29).
2) Co-orientation albitization of K-feldspar
(K-feldspar is transformed into albite)
With intensive action of Na-bearing gas-liquid the
sodium ion may seemingly penetrate into the crystal lattice of K-feldspar and
substitute Na for K, resulting in the transformation from K-feldspar to albite.
The K-feldspar with grid twinning under crossed polars was changed to albite
with chessboard twinning. The compositional plane of chessboard twinning is
characterized by zigzag irregularity which is clearly distinguishable from that
of common albite twinning.
The intersect line of the three planes (Np1
Np2, Nm1 Nm2, Ng1 Ng2)
of corresponding axes of the optic indicatrix of two individual chessboard
albite twins is the twin axis. Universal stage study shows the twin axis is
b-axis. Therefore, the chessboard albite twin can be called “b-axis twin.”
Fig.
30A, 30B show
that chess-board albite can replace K-feldspar in a patch or block pattern.
Note that the boundary between the unchanged K-feldspar and the chessboard
albite is quite sharp without a blurred transition under the optical
microscope, at least. Moreover, the shapes and distribution of the chessboard
albite that the boundary between the unchanged K-feldspar and the chessboard
albite is quite sharp without a blurred transition under the optical
microscope, at least. Moreover, the shapes and distribution of the chessboard
areas are different from the perthitic albite, although they have the same
crystallographic orientation (Fig. 30C).
Some co-orientation replacement albite is very similar to the primary perthitic
albite (Fig. 31),
except in its distribution.
3) Biotite is
transformed to muscovite
Both muscovite and biotite have a common basal plane
(001) and similar crystallographic lattices.
Hot gas-liquid fluid containing Si and Al can penetrate along cleavage
into biotite, displacing Fe, Mg, and Ti, resulting in transformation from
biotite to muscovite (Fig. 32).
4) Biotite is transformed to chlorite
The crystallographic lattice of chlorite is different
from that in biotite, but still in many respects resembles the lattice in micas
(Deer, 1962). Both contain basal cleavage (001). After alteration, biotite is prone to be
replaced by chlorite in pseudomorphic form (Fig. 33).
Formation
mechanism for mineral replacement
Replacement
mechanism
The major mechanisms of mineral replacement are the
dissolution-precipitation (or –reprecipitation, in fact,
dissolution-crystallization) and the cation exchange.
Cation exchange mechanism means that the old mineral
is partly or even wholly transformed to newly replacive mineral by the cation
exchange without partial or whole dissolution of the old mineral.
Dissolution-precipitation mechanism implies that the
new mineral precipitates (crystallizes) in the space where the old mineral is
locally dissolved. Obviously, the mechanism aims at the hetero-orientation
replacement (nibble replacement) pattern.
The author considers that the two different mechanisms
of replacement should occur separately rather than simultaneously; i.e., no
cation exchange occurs when dissolution-precipitation happens, and no
dissolution-precipitation occurs when cation exchange proceeds.
For the dissolution-precipitation mechanism some
researchers suggest that as a replacive (guest) mineral grows, it must exert an
excess pressure (Ostapenko, 1976), an induced stress
(Carmichael, 1986), a force of
crystallization (Maliva and Siever,
1988), or a growth-driven stress (Merino et al., 1993) on its neighbor (host)
mineral and promote its dissolution.
Merino et al. (1993) stated that in a rigid rock, the
growth-driven stress between guest and host minerals has two immediate effects:
1) increase the solubility and rate of pressure solution of the host mineral
and 2) decrease the rate of growth of the guest itself (due to decrease of the
supersaturation because of increase in the solubility of the guest mineral) so
that the value of the two volumetric growth-rates of guest B and dissolution of
host A become equal.
The authors seemed to stress that appropriate
dissolution of replaced mineral is promoted by a force driven by growth of the
replacive mineral. Merino et al. even emphasized (1998): “any mineral may in
principle replace any neighbor, provided there is sufficient affinity for its
growth, regardless of whether the two have an element in common, of whether the
two are isostructural or not, and of which mineral has the greater formula
volume.”
According to
the author’s observation, during a certain mineral replacement in a
dissolution-precipitation mechanism, some minerals are easily replaced, while
others are not (Tab. 1). It is by no means certain that any mineral may be
replaced by a particular replacive mineral. Therefore, whether a mineral may be
replaced or not depends not only on the growth-driven stress of replacive
mineral but also on the appropriate dissolubility of the replaced mineral under
a new kind of gas-liquid composition and P-T condition.
Table 1. Replacive minerals versus minerals easily
and hardly replaced in nibble replacement
|
Replacive minerals |
Minerals easily replaced |
Minerals hardly
replaced |
|
albite, quartz ,
beryl |
K-feldspar, fine perthitic albite, calcite |
entire plagioclase, quartz, biotite |
|
K-feldspar |
K-feldspar
, plagioclase |
quartz, biotite,
hornblende |
|
muscovite |
K-feldspar, biotite |
quartz |
|
calcite |
quartz , K-feldspar |
entire plagioclase,
biotite |
Also, there are great discrepancies in different
patterns of replacement. For example, albite can easily replace an entire
plagioclase in a co-orientation replacement pattern, but can only partially
replace tiny fine perthitic albite in a hetero-orientation replacement pattern
(nibble replacement pattern).
Necessary conditions for replacement mechanism
The necessary conditions for both the cation exchange
and precipitation mechanisms are the following:
1)
Participation of the replacive material, such as K
for K-feldspathization, Na for albitization, Si for quartzification, Al for
muscovitization, and so on. These materials occur as ions in solution in a
gas-liquid state, probably mainly in gas.
Introduction of gaseous-liquid fluid from outside into
the rock needs to pass through microfractures, grain boundaries and cleavage
planes of minerals.
There would be no microfractures in rock unless stress
occurred.
2) Presence of an opening for
gas-liquid to enter and migrate
A grain boundary is always present in a rock. Microscopically, subtle openings are bound to
be present either between two different minerals or between two similar (in
component and crystallography) minerals with different orientations. Fig. 34 displays
a compact boundary (a parallel arrangement of crystallographic lattices)
without a micro crevice between A and C (contact boundary between two
co-oriented minerals or twin plane) because they have the same crystallographic
orientation. There must be an opening along the boundary* between B and C (of
the same, similar or different minerals) because they have different
orientations.
It is hard to imagine that such a narrow width of a
grain boundary (only at nanometer scale) can still be circulated by a hot
gas-liquid fluid. So many researchers are deeply convinced that the rock must
have been broken before a gas-liquid can enter and circulate causing a
replacement.
Microscopic observation indicates that the real
replacement phenomena have surely occurred in a rock without obvious traces of
deformation. So the author believes that under the increasing pressure of
gas-liquid and surrounding pressure the hot gas-liquid from outside could have
entered and circulated along the grain boundary, although very narrow.
*Width of grain boundaries in granitic mylonites has been studied by
various researchers using transmission electron microscope (TEM) and
high-resolution electron microscope (HREM). The width ranges from <100nm (Behrmann, 1985), 3~5nm (Farver
and Yund, 1995) to 0.5nm (Hiraga
et al., 1999).
3) Presence
of mineral prone to be replaced on one side of the opening
With the three above mentioned conditions, the mineral
replacement by cation exchange mechanism can proceed; for example,
muscovitization and/or chloritization of biotite.
The crystallographic structure of muscovite is close
to that of biotite. When the cations (Fe2+, Mg2+) are
exchanged by cation Al3+, the biotite is transformed to muscovite.
Although chlorite has a different crystallographic
structure from that in biotite they are similar. According to a scanning
electron microscope study, chloritization of biotite occurs mainly through the
exchange of two potassium interlayer sheets and two tetrahedral sheets ((Si,Al)O4) in biotite by the brucite-like
interlayer (Mg3(OH)6). The crystallographic lattice of
biotite has not been destroyed (Veblen and Ferry, 1983; Toshihiro Kogure and Banfield, 2000).
The replacive guest mineral formed by cation exchange
mechanism naturally maintains the orientation of the host mineral.
However, only
with the above mentioned three conditions the hetero-oriented (nibble)
replacement by dissolution-precipitation mechanism still would not occur.
With different orientation from that of the replaced
mineral, the replacive mineral should have its own crystallographic orientation.
Without nucleation substrate, the replacive mineral would hardly grow.
Therefore for the dissolution-precipitation mechanism
the following fourth condition is indispensable.
4) Presence
of the crystal nucleus on the other side on which the replacive mineral can
epitaxially grow
The common crystal nuclei are the same or similar kind
of replacive minerals.
When they are absent in the rock, an impurity (lattice
defect, interstitial foreign element, etc.) may act as a nucleation substrate
if the metasomatic process should have occurred.
Compared with the cation exchange the
dissolution-precipitation is the most common and extensive replacement
mechanism, which fits the replacement of different and the same or similar kind
of minerals, especially the hetero-orientation (nibble) mineral replacement.
The co-orientation replacement should also probably occur by the
dissolution-precipitation if it does not happen on account of cation exchange.
Description
of nibble replacement of albitization
According to the rule of dissolution-precipitation
(nibble replacement), albitization can be explained as follows:
When Na-rich gas-liquid enters into the boundary
between K-feldspar and differently oriented plagioclase, the replacive albite
should grow on the plagioclase, unilaterally nibbly replacing the K-feldspar
and forming a so-called “clear rim” of albite, because plagioclase is more
stable than K-feldspar in that case.
As Na-rich gas-liquid penetrates the boundary between
two K-feldspars K1 and
The surface of the grain boundary into which a
gas-liquid infiltrates can be named as the metasomatic active front. Because
the crystallography of the newly-formed replacive albite coincides with the
crystallography of the K-feldspar on which the albite grows, the original crack
disappears, and nibble replacement can no longer take place there. Because the crystallographic orientations of
the replaced K-feldspar and the nibbling albite are invariably discordant,
there is always an opening along the grain boundary between them. Therefore,
the metasomatic active front would continuously advance towards the replaced
K-feldspar during the process. The neighboring metasomatic active fronts can be
merged into one row and extend farther.
Thus, the so-called “swapped rims (or rows)” of albite are formed (Fig. 35).
The fact that a clear albite rim may be present along
the periphery of some small plagioclase inclusions in a K-feldspar megacryst
shows inevitably that Na-bearing gas or liquid fluid can surely penetrate
K-feldspar along its cleavage. However, the lack of replacive albite along the
cleavage is probably due to the orderly arrangement of the crystal lattice of
the K-feldspar and because much more energy is needed to dissolve the
K-feldspar along both sides of the cleavage.
No nibble replacement growth of albite can occur at
boundaries between K-feldspar and quartz, two plagioclases, plagioclase and
quartz, biotite and quartz, etc., because replacive albite can neither grow on
the lattice of quartz or biotite although K-feldspar can be replaced; nor on
K-feldspar because quartz cannot be replaced.
In most places the substrates for nibble replacement
are identical with or similar to minerals which are widely distributed in
granitic rocks. Thus, there are sufficient minerals that may act as substrates
for nibble replacement. It is unnecessary and unlikely that a replacive mineral
will select an impurity as a substrate rather than the abundant preexisting
identical or similar minerals. If there are no identical or similar minerals in
rocks, or the identical or similar minerals are not present at an adjacent
side, an impurity, such as a crystal-lattice defect, would act as a substrate;
for example, the sericitization or muscovitization in plagioclase.
Whether the co-orientation replacement albitization of
plagioclase and/or K-feldspar* is by cation exchange or
dissolution-precipitation mechanism has not been determined.
______________________________________________________
*Whether there is
co-orientation K-feldspathization of plagioclase (or albite) should be proved
later on.
Presence
of micropores is probably one of the key factors to allow alteration of
plagioclase.
In the last 30 years more detailed studies of
feldspars (mainly plagioclase) in common igneous rocks have been made by using
the electron microscope, scanning electron microscopy and high-resolution
transmission electron microscope. These
studies have been made either on fractured surfaces (Dengler,
1976; Meideno Que and
Alistair R. Allen, 1996) or on polished and ion-milled surfaces (Montgomery and
Brace, 1975) and have shown the presence, in addition to micro cracks, of
numerous now-empty micropores (within altered plagioclase) (Fig. 36). These micropores range from several tens to
< 1 μm (Worden et al., 1990). Small crystals
of sericite have grown in the micropores, and the plagioclase surrounding the
pores is transformed to albite. The clear part of the plagioclase (mainly in
the rim, but also in the inner part
Fig. 28),
being free of either micropores or sericite crystals, retains its original
composition. Perhaps, the presence of micropores is one of the key factors to
allow the alteration of plagioclase.
There are also numerous micropores in alkali feldspar,
basically along the boundary of microperthitic lamellae, and much less or no
micropores in the K-feldspar phase (Fig. 37). Perhaps, this may be one of the reasons why
K-feldspar is nonsericitized. Only when the rock is deformed (micro fractured)
and subjected to intense action of sodium-bearing fluid may the K-feldspar be
thoroughly transformed to co-oriented albite.
There are two major hypotheses for the origin of
micropores in feldspar:
a) primary origin (Roedder and Coombs, 1967; Lofgren, 1974; Montgomery and
Brace, 1975, and b) secondary origin (Smith and Brown, 1968; Parsons, 1978). Que (1996) suggests that most micropores occur during
crystallization of the plagioclase, but may subsequently have been enlarged by
later fluid action. The researchers supporting the primary origin of micropores
suggest that the sericitization and albitization of plagioclase is formed soon
after the melt crystallized and earlier than the clear rim, intergranular
albite and myrmekite, while the researchers preferring the secondary origin of
micropores suggest that the sericitization and albitization of plagioclase must
occur later. The writer supports the latter opinion. However, although the
sericitization and albitization of plagioclase are distributed widely, such
alterations do not necessarily occur everywhere as do the clear rim and
intergranular albite. This observation logically indicates that the partial
sericitization and albitization of plagioclase is formed later than the
pervasive clear rim and intergranular albite. Therefore, the author deduces
that the sodic gaseous-liquid at first penetrates along grain boundary and
cleavage, resulting in the nibble replacement of clear rim and /or
intergranular albite, and then passes through the microfractures and micropores
into the inner part of the plagioclase, giving rise to sericitization and
albitization of plagioclase.
The chessboard albitization of K-feldspar, not easily
seen even in alkali metasomatites (in
Dissolution-precipitation and
cation exchange are the two different replacement mechanisms. Generally
speaking, a sharp boundary is produced by the dissolution-replacement
mechanism, while a blurred boundary is probably generated by the cation
exchange replacement mechanism.
The hetero-orientation replacement pattern occurs conforming to
the dissolution-precipitation mechanism. As for the co-orientation replacement
pattern, some sheet silicate minerals utilize the cation exchange mechanism,
but framework silicates may probably be modified by the
dissolution-precipitation mechanism. The boundary between sericitized and
unsericitized plagioclase in the same plagioclase is not very clear under the
microscope, but, in fact, has a sharp demarcation (no more than tens of
nanometers between them) observed with microprobe, SEM and TEM (Engvik, et al., 2008). The interface between the K-feldspar
and the co-orientation chessboard albite in the same K-feldspar is also
suddenly changed. Therefore, the co-orientation albitization of plagioclase
and/or chessboard albitization of K-feldspathization are probably caused by
dissolution-precipitation rather than by cation exchange.
Entering the rock, the hot
gaseous-liquid fluid at first circulates along the grain boundary (and
cleavage) causing hetero-orientation (nibble) replacement. Later the gas-liquid
may enter the inner part of some mineral, giving rise to co-orientation
replacement. So, the two replacement mechanisms, as well as the two replacement
patterns happen sequentially. Possibly, the dissolution-precipitation mechanism
(hetero-orientation pattern) occurs first, and the cation exchange mechanism
and co-orientation pattern replacement happens later.
Discussion of origin
of mineral textures in granite
Origin of cleavelandite albite
A Li-F
granite in which Li, Be, Nb, and Ta mineralizations occur is the latest
intrusion of a multicyclic batholith.
The typical L-F granite is characterized by having a small rock body and
vertical zonation. From the bottom up the composition of plagioclase is
gradually changed from sodic plagioclase to albite, and the abundance of the
plagioclase increases. The small platy
albite is enriched in the epical and top facies as the volumes of K-feldspar
and quartz decrease. Also, upwards the
mica compositions changes from lithium-bearing biotite, protolithionite or two
micas, to muscovite or lepidolite. The
small platy albite (cleavelandite) crystals are distributed at random (Fig. 38)
and in some places have interzonal arrangement (Fig. 39).
Masgutov (1960) considered this kind of albite to be random
albitization (or chaotically replacement), one of the three types of
albitization he summarized (the other two are perthitization of K-feldspar and
deanorthitization or saussuritization of plagioclase).
Beus (1962), however, suggested that the interzonal
small platy albite laminae and the enclosing megacrysts of K-feldspar and
quartz all were metasomatic in origin.
Many researchers consider that Li-F granite rich in
cleavelandite (named apogranite) is formed by post-magmatic hydrothermal
alteration from normal granite (Beus, 1962), resulting in the separation of
rare metals from mica and feldspars and the formation of ore deposits (Masgutov, 1960; Beus, 1962; Aubert, 1964; Burnol, 1974; Hu Shouxi, 1975; Wang Defu, 1975;
Hong Wenxing, 1975; Stemprok, 1979; Imeopkaria, 1980; Yuan Zhongxin
et.al., 1987; Wang Zhonggang et. al., 1989; Xia Hongyuan, 1991).
Other researchers, however, insist on the magmatic origin of the Li-F
granites as well as the primary genesis of rare metal mineralization (Wang Liankui et.al., 1970; Kovalenko,
1971; Liu Yimao, 1975; Eadington,
1978; Rong Jiashu, 1982; Du Shaohua, 1984; Raimbault, 1984; Zhang Jintong,
1985; Cuney, 1985; Xia Weihua,
1989; Zhu Jinchu, 1992; Taylor, 1992).
According to the rule of nibble replacement, if the
nibble replacement pattern is accepted as a believable and ubiquitous
metasomatic rule, the origin of the small platy albite lamellae can be judged
as primary rather than metasomatic. The nibbly replacive albite should occur
regularly at the grain boundaries of plagioclase with K-feldspar or between two
K-feldspars rather than randomly or zonally in quartz or K-feldspar.
The discovery of ongonite, the study of fusion
inclusions and an experimental petrological study have also shown that Li-F
granite can directly be formed from a granitic melt rich in Li-F at the top of
an intrusion during decreasing temperature.
Origin
of myrmekite
Myrmekite, an intergrowth of plagioclase and quartz
vermicules, often occurs in felsic-to-intermediate, calc-alkali plutonic and
gneissic rocks. Myrmekite is found along the grain boundary between plagioclase
and K-feldspar (Fig. 40).
No myrmekite appears at the contact of plagioclase with quartz
(Fig. 41).
The quartz in myrmekite is vermicular (tapered,
curved, and/or sinuous) and rod like with round to oval sections and elongated
toward the myrmekite border. The volume percent of quartz vermicules is
directly proportional to the An content of myrmekitic plagioclase
(Fig. 42).
1) Hypotheses for explaining the origin of
myrmekite
There are at
least five major hypotheses for the origin of myrmekite.
a) Replacement of preexisting plagioclase by
K-feldspar (Drescher-Kaden, 1948)
Drescher-Kaden considered that myrmekite is formed
from replacement of plagioclase by a fluid bearing K and Si, and that some
vermicular quartz was once contained in K-feldspar. Myrmekite is obviously older than K-feldspar.
If replacement proceeds thoroughly, the plagioclase can be replaced by
K-feldspar. It is just this theory
granitizers hold that the K-feldspar is produced by K-feldspathization of
plagioclase. The theory is also supported later by Collins (2002b).
b) Replacement of preexisting K-feldspar by sodic
plagioclase (Becke, 1908)
Because the SiO2 content needed for the
formation of the anorthite end member is less than that contained in K-feldspar
and albite, the surplus SiO2 (after the replacement of K-feldspar by
sodic plagioclase) then was precipitated as vermicular quartz. For each molecule of anorthite that is
introduced, four molecules of quartz are produced.
2KAlSi3O8 + 2Na+1
→
2NaAlSi3O8 + 2K+1
2KAlSi3O8 +
Ca+2 → CaAl2Si2O8 + 4SiO2
+ 2K+1
c) Exsolution or unmixing of K-feldspar (Schwantke,
1909)
Both (NaAlSi3O8) and the
so-called Schwantke molecule [CaAl2Si6O16 or Ca(AlSi3O8)2] are
supposedly contained in a former high-temperature K-feldspar (Schwantke, 1909;
Spencer, 1945). With decrease of temperature the exsolution of NaAlSi3O8
and CaAl2Si6O16 occurs, resulting in the
formation of myrmekite. Each Schwantke molecule recrystallized to form CaAl2Si2O8
and 4SiO2 are released to form vermicular quartz in the myrmekite.
The inner diffusion drives the exsolved material onto the plagioclase nearby.
d) Recrystallization of plagioclase (Collins, L., 1988)
Plagioclase is
altered by hydrothermal solutions after cataclastic deformation and prior to
introduction of K-bearing fluids. In local places in the altered plagioclase
lattice, the loss of Ca and Al and the retention of Na are accompanied by an
increase in excess silica, resulting in formation of quartz vermicules in
myrmekite when these places recrystallized. The An
content of plagioclase of myrmekite is about half the An content of the primary
plagioclase (in some kinds of granitic rocks that are modified by
K-metasomatism but not all).
e) Complex
hypothesis (Ashworth, 1973; Phillips, 1974, 1980)
Myrmekite can be formed by both exsolution and
replacement. The exsolution origin is suitable for an undeformed high level
intrusion, forming rim myrmekite, grain boundary myrmekite and enclosed
myrmekite. The replacement origin is appropriate for deformed metamorphic
rocks, forming wartlike myrmekite (especially where the K-feldspar is quite
small compared with adjacent large myrmekite).
2)
Discussion of the origin of myrmekite
Myrmekite is
characterized by the stable correlation of volume percent of myrmekitic quartz
with An content of myrmekitic plagioclase (Fig. 42).
The correlation is obviously not accidental.
Hypotheses b), c), d) and e) are consistent with this correlation.
Drescher-Kaden (1948) concluded that
some vermicular quartz in K-feldspar may be relicts of replaced myrmekite,
occurring as "ghost myrmekite" (Fig.
43). This relationship might serve as tenable evidence indicating
that K-feldspar replaces the plagioclase of former myrmekite, and, therefore,
the whole K-feldspar must be metasomatic in origin. This phenomenon, however,
can be explained by the so-called superimposed reversed K-metasomatism,
which takes place after the formation of myrmekite ceased. The newly formed
K-feldspar (being of nibble replacement pattern, too) grows on the preexisting
K-feldspar behind, taking its crystallographic orientation and nibbling at the
myrmekite in front. In this way, the myrmekite is locally destroyed by the
second reversed K-feldspathization. The quartz vermicules of myrmekite remain
as relicts because they are only partly or even hardly ever replaced by
K-feldspathization.
Many small-sized myrmekite grains are
often developed around the border of a K-feldspar megacryst in granite-gneiss,
but no obvious plagioclase occurs against which the myrmekite abuts. An
outstanding example of a myrmekite corona occurs as a rim on a residual rounded
K-feldspar porphyroclast (Fig. 44) in ultramylonite, Cima di Vila,
The hypothesis d) (recrystallization of altered
plagioclase) does not seem to apply to granitic rocks containing myrmekite in
(1) The shape of myrmekite does not look
like primary plagioclase.
(2) The granitic rocks containing myrmekite
are not necessarily affected by deformation, alteration, and recrystallization.
(3) Myrmekite is not formed during
saussuritization and sericitization.
(4) Myrmekite may occur regularly in swapped
grains at the contacts between two K-feldspar crystals.
(5) An number of
myrmekitic plagioclase is not necessarily equal to the half An-number of
primary plagioclase, but is generally lower than that of primary plagioclase.
Exsolution is an attractive hypothesis because no
penetration of fluid material is needed from outside. That is consistent with
the idea of those researchers who do not believe (hence do not consider) that
solid granitic rocks might have been circulated by Ca- and Na-bearing fluid
from outside.
Moreover, the presence of the Schwantke molecule has
not yet been proved. In addition, in
some places the large size of myrmekite coexisting with a small K-feldspar
crystal makes it impossible for the myrmekite to have formed by exsolution from
the adjacent smaller volume of K-feldspar.
The origin of perthite is also commonly explained by
exsolution. The crystallographic
orientation of both guest perthitic albite and host K-feldspar coincides
consistently with each other, while the orientation of myrmekitic plagioclase
is regularly different from that of the host K-feldspar, but identical with
that of the plagioclase against which the myrmekite abuts. Moreover, no
myrmekite occurs along the borders of perthitic albite, as well as on the
border between K-feldspar and plagioclase with the same orientation.
Swapped myrmekite (Figs. 45, 46),
consisting of double rows, although rare, still occurs on the boundary between
two K-feldspar crystals with different orientations. The fact that each row of
myrmekite has the same orientation as that of the opposite K-feldspar strongly
indicates that myrmekite is formed by nibble replacement; i.e., K-feldspar is
replaced by Na-bearing gas-liquid, involving calcium.
The more calcium that is involved in the
sodic metasomatic process, the more volumetric percent of quartz would be
included in myrmekite and the greater the An number of
myrmekitic plagioclase.
The plagioclase against which a myrmekite
grain abuts may not be necessarily cut by the thin section, if the width or
size of that myrmekite is large enough (Fig. 47).
Serial thin sections through the rock, however, would show this plagioclase in
the third dimension.
Compared with replacive albite, the myrmekite
is more capable of replacing the perthitic albite. Therefore, relicts of
perthitic albite are rarely seen in myrmekite.
Origin of perthite
K-feldspars in granites or pluton rocks often contain perthitic lamellae
of albite.
1) Content and
shape of perthitic albite
The content of albite lamellae in perthite varies with the composition of
the rock type. Generally, the perthitic albite lamellae are fewer and scattered
in plutonic rocks rich in Ca, Fe, Mg and poor in Si and alkali, while more and
abundant in rocks rich in Si and alkali and poor in Ca, Fe, and Mg. The volumetric percent of perthitic lamellae
ranges from trace amounts to 30-50 % and can even surpass the content of K-feldspar (Fig.
48).
Alling (1938) depicted various types of perthite as
stringlets, strings, rods, beads, films, bands, patch, plume, flame, vein,
braid, etc.
2) Distribution
of perthitic albite
The distribution and orientation of perthitic albite lamellae in
K-feldspar are more or less homogeneous. The elongate plane of perthitic albite
lamella is basically parallel to the b-axis and close to (100), near 64° to
(001) or along the Murchison plane (-601), (-701), or (-1-502), near 65~73° to
(001), which is clearly seen in the (010) section. The (100) and the Murchison
plane are close to the projective line of Nm of perthitic albite in the (010)
section. In addition, the perthite
lamellae are also oriented parallel to (110), (1-10) (forming a braided texture) and rarely
parallel to (010).
3) Hypotheses for origin of perthitic albite
There are several hypotheses
for the origin of perthitic albite:
a) Exsolution from solid solution of K-Na
feldspar
b) Replacement
c) Simultaneous crystallization.
Some authors consider that
there might be perthite of various origins.
a) Exsolution from solid solution of
K-Na feldspar
The exsolution hypothesis is
the most popular so far, because it has theoretical and experimental support.
Moreover, the distribution of perthitic albite in K-feldspar can be consistent
within the same facies. The perthitic albite in a phenocryst of K-feldspar is
more abundant than in smaller K-feldspar crystals in the ground mass. It is
unnecessary to consider the effect of a gas-liquid from outside to explain how
the perthitic albite may form inside the K-feldspar.
b) Replacement
A replacement hypothesis is
also supported by many researchers, particularly when the perthitic albite
lamellae are vein-like or branch-like.
Such lamellae look like they penetrate
the twin composition plane (010) (Fig. 49), or have
irregular distribution, especially when the thin section intersects the
perthitic lamella at a small angle (Fig. 50). The fact that polysynthetic twinning of
perthitic albite coincides with the grid twinning of K-feldspar looks
morphologically as if the perthitic albite is formed by replacement in the same
way that the co-orientation replacement of albitization can surely occur.
Nevertheless, the shapes and
distribution of perthitic albite in K-feldspar in some granitic rocks are
similar, identical or consistent, which is quite different from that produced
by co-orientation albitization of K-feldspar. The albite formed by
co-orientation albitization appears abruptly and abnormally, i.e., locally in
many places in blocky or stacked arrangements or thoroughly transformed to
chess-board albite that is absent in normal perthitic K-feldspar.
In some places vein or
string perthitic albite penetrates the K-feldspar and even transects the
In (010) section of the
Manebach twin (Fig. 51), the intersection
angle of the perthitic albite lamellae in both twin sides can be as much as
60~70° (or 110~120°) and their optical orientations are obviously
different. On that basis, naturally the
impression that vein-type perthitic lamellae are cutting through the twin
composition plane (001) would not appear.
As mentioned above, when a
clear rim or grain boundary albite is in contact with perthitic albite, some
tiny relicts of perthitic albite can be enclosed in the clear rim or grain
boundary albite, indicating that the relicts are formed by replacement and are
earlier than the clear rim or grain-boundary albite.
The metasomatic model for
the origin of perthitic lamellae is doubtful because the perthitic albite
lamellae generally do not contain vermicular quartz although their An values may be as much as 5~14, obviously higher than that
of the nibbly replacive albite (An3-5) which contains fine
vermicular quartz. Only if the vermicular or water-drop-like quartz is
contained in perthitic albite lamellae (such an example may be rare but exist
somewhere), the perthitic albite might be formed by replacement processes.
c)
Simultaneous crystallization
The simultaneous
crystallization hypothesis suggests that perthitic albite lamellae are
synchronously formed with host K-feldspar. Plagioclase-sanidine intergrowths
that resemble perthite have been grown directly from ternary feldspar melts by
Lofgren (1977). The intergrowths are of two general types: regularly-spaced,
parallel lamellae up to 10 μm wide that resemble
microperthite and irregular or patchy intergrowths that resemble patch
perthite. The plagioclase lamellae in the intergrowths are An25-15Or5-20
and mainly elongated along (100).
4)
Directional inclusions in feldspar
Below are some examples
showing that the simultaneous crystallization hypothesis is possible.
a)
Pencil-like albite crystals contained in K-feldspar
Several fine pencil-like
albite crystals are enclosed in a K-feldspar phenocryst. The albite inclusions
are generally scattered randomly (Fig. 52), but some (more
than 12 grains) as well as small drop-like inclusions (vertical pencil-like
albite) are distributed with the same orientation and
nearly parallel to the orientation of perthitic albite lamellae
(Murchison plane).
b) Needle-shaped inclusions of apatite
enclosed in albite are arranged along the Murchison plane
Needle-shaped inclusions of
apatite in vein albite were reported by Smith and Stenstrom
(1965). The needles tended to align in the Murchison direction.
Note that many parallel
acicular crystals of apatite are also found in primary albite in the Shanbei leucogranite, Taishan county,
5) Possible
origin of perthitic albite
The orientations of
pencil-like albite in K-feldspar as well as oriented needle-like apatite in
plagioclase are comparable to oriented perthitic lamellae in K-feldspar. The
pencil-like albite and oriented needle-like apatite, however, are obviously
explained by neither exsolution nor metasomatism. The similarities of these
orientations imply that the pencil-like albite and acicular apatite were
normally enclosed in feldspar under the control of the strongest crystallizing
force of the feldspar in a stable environment.
Therefore, the author
deduces that the perthitic lamellae distributed semi-regularly along the
Murchison plane might be formed in the following way:
A few albite crystals could
have grown epitaxially and dispersively on a crystallizing K-feldspar. During
further crystallization it was generally easier for albite to grow on its own
mineral than on K-feldspar, and K-feldspar, similarly, grew easier on its own
mineral than on albite. Because the direction of the strongest crystallization
force of feldspar is approximately identical to that of the arrangement of the
needle-like apatite, perthitic lamellae developing roughly along (-1-502) might
thus be formed.
Therefore, the perthitic
lamellae distributed semi-regularly along the Murchison plane result probably
from simultaneous crystallization, while extremely thin, straight and strictly
parallel lamellae of perthitic albite perhaps are caused by exsolution
(Fig.
54).
One of the key points
regarding the genesis of granitic rocks is the origin of feldspars, in
particular, K-feldspar. The controversy over granite actually is a controversy
regarding the origin and formation of K-feldspar.
Correctly distinguishing the
difference between a porphyroblast and a phenocryst is one of the key points
with regard to the genesis of granitic rocks, whether they have a metamorphic
or magmatic origin.
Autocathasis
of porphyroblast
Autocathasis described by
Harker (1950) in his classical book “Metamorphism” as follows: “Growing
crystals endeavour to clear themselves by expelling
foreign inclusions of any kind, but their power to do so depends upon their
inherent force of crystallization…..a growing crystal has been able to brush
aside foreign material, resulting in the grains of inclusions along certain
directions in which the force was least effective”.
These points of view are
doubtful. The unreplaced relicts should be preserved in situ without changing their own primary crystallographic
orientations, because the metasomatism process is carried out while a rock is
in the solid state. However, the unreplaced residue might be deviated and/or
slightly rotated, because plastic deformation could take place during
metasomatism. Nevertheless, it is still impossible to push the unreplaced
relicts aside and to arrange them zonally or
in oscillatory patterns.
A serial drawing (Hippertt,
1987), showing the metasomatic replacement of plagioclase by microcline and the
development of microcline porphyroblasts in augen gneiss, Niteroi, Brazil, was
cited in “Rock-forming minerals” (Deer, Howie,
Zussman, 2002 edition, page 585, Fig. 417). In the
drawing (see Fig. 55)
(a) the original metamorphic rock is composed mainly of plagioclase crystals,
(b) during the beginning of microclinization the small plagioclase crystals
recrystallized around microcline, (c) some adjacent crystals of microcline have
merged into one porphyroblast enclosing some plagioclase inclusions arranged
with long dimensions roughly parallel to lattice planes of the microcline, and
(d) the porphyroblast is transformed into an
idiomorphic shape and able to replace thoroughly the enclosed plagioclase
inclusions.
The major problem with this
model is how several small microcline crystalloblasts
with different orientations can merge to form a large one with a uniform
orientation.
Megacryst of K-feldspar may occur in melt
Crystallization of granitic
melts occurs only when they are undercooled below the liquidus temperature. The various values of undercooling degree ΔT affect the
different nucleation density and growth rates of major rock-forming minerals
and, therefore, the rock texture.
At ΔT=100-120ºC,
nucleation densities of plagioclase and K-feldspar are nearly the same (103~104),
while the growth rate (1x10-6(cm/sec)) of K-feldspar may be over 100
times more than that of plagioclase, resulting in the formation of a K-feldspar
megacryst, either euhedral or anhedral, with much smaller inclusions of plagioclase,
zonally arranged in some megacrysts (Swanson, 1977).
Many points of evidence,
especially igneous microstructures and structures resulting from solid-state
deformation, indicate that K-feldspar megacrysts in deformed granites are
residual phenocrysts, not porphyroblasts (
Some residues of a replaced
mineral might be preserved without changing its original orientation in a
porphyroblast even if the porphyroblast has a very strong force of
crystallization (such as an idiomorphic porphyroblast of pyrite in cataclastic
granite). The author holds that during porphyroblastization the residues can
remain only in situ, and it is
impossible to push them aside and to make them be rearranged as Augustithis
said.
Major
distinction between phenocryst and porphyroblast
According to the T-P
conditions for mineral crystallization in magma and to the rule of nibble
replacement, the author considers that the major distinctions (only by
microscopic observation) between a phenocryst and a porphyroblast may be
summarized as follows:
Those megacrysts, which
enclose a residual texture of the primary rock, such as schistosity and
gneissosity, and keep the orientation of this texture unchanged or hold a
recognizable change of its orientation while growing simultaneously to enclose
the texture of the schistosity or gneissosity should be regarded as a porphyroblast (Fig.
56a).
Those megacrysts, which
include zonally arranged minerals, should be considered as phenocrysts
(Fig.
56b).
Those megacrysts, which are
obviously zonal in structure due to difference in composition or optical
properties, are probably magmatic in origin.
Either phenocryst or
porphyroblast is possible if each encloses other small mineral inclusions
randomly.
Inside a megacryst of
K-feldspar, the phenomenon of nibble replacement of enclosed plagioclase
crystals can certainly be observed. Many petrographers insist on treating this
phenomenon as an important criterion to determine that the whole K-feldspar has
a porphyroblastic origin. However, according to the rule of nibble replacement,
an original K-feldspar should be selected as substrate on which the nibbly
metasomatic K-feldspar can grow towards plagioclase with different orientation.
Moreover, the perthitic albite develops poorly in many places in the newly
formed K-feldspar (commonly <0.n mm wide) where isolated relicts of
plagioclase are distributed. Fig.
12 shows that the nibbly metasomatic
K-feldspar (K’) and the primary K-feldspar (K) have the same crystallographic
orientation without obvious boundary between them. The sole distinction might
be the different content of perthitic lamellae, indicating that the major part
of K-feldspar is present prior to the K-feldspathization. A false impression,
however, that the whole K-feldspar is of metasomatic origin could easily be
made, especially when primary K-feldspar has few perthitic albite lamellae (Figs.
7, 8).
Whether
the most part of predominant pure K-feldspar has thoroughly replaced the
inclusions of plagioclase as well as biotite and even quartz?
Quartz and biotite can
hardly be replaced during K-feldspathization according to extensive observation
under the microscope. Where the replacement of plagioclase by K-feldspar is
observed, the residual amount of plagioclase in K-feldspar is comparable with
the amount within the thin section. There is no gradual or gradational
transition from plagioclase being slightly replaced to that being thoroughly
replaced in thin section; i.e., either plagioclase is slightly or locally
replaced by K-feldspar with small relics of plagioclase in it, or K-feldspar is
pure and clear, without any tiny inclusions of plagioclase. Therefore, the
author holds that the pure and clear part of K-feldspar without any relics of
plagioclase represents the primary K-feldspar rather than the metasomatic
product of complete replacement of plagioclase.
The texture of a certain
rock becomes complicated due to the formation of newly formed minerals after
several metasomatic processes.
The successive sequence of
mineral replacement can be deduced according to the rule of monomineral
replacement and the contact relationship between the replacive minerals.
Nibbly metasomatic albite should occur earlier than
the co- orientation albitization of K-feldspar
Figure
57 shows that swapped rows of nibbly metasomatically-growing
albite occur at the contact between two chess-board albites (transformed from
two primary K-feldspars). The swapped
albite would not have been produced if the two K-feldspar crystals had been
transformed into chessboard albite. On that basis the nibbly metasomatic albite
must have been produced earlier than the co-orientation albitization of
K-feldspar. This relationship is in
accordance with geological observation because swapped albite is found
everywhere in the whole body and must have been produced during a postmagmatic
process soon after one episode of intrusion.
In comparison the transformation of K-feldspar into chessboard albite
occurs only in local regions after all stages of the granitic magmatic
intrusion have ceased.
Triple
nibble replacement (albitization, K-feldspathization and slight realbitization
proceed sequentially)
A complicated texture (Fig. 58) can be seen at
the contact between two K-feldspars in the earlier granitic intrusion
(Huangshatang medium-fine grained porphyritic two-mica granite, formed during
the third period of the Indosinian cycle, in the Zhuguangshan batholith,
Another example exists where
several small albite crystals appear at the contact of two K-feldspars
(Fig. 59).
These albite crystals are divided into two groups. Each group is separately
located in one individual K-feldspar and each has similar or nearly the same
crystallographic orientation as the perthitic albite of the opposite
K-feldspar. Two major replacement
processes occur here. First, nibble metasomatic albitization occurs, resulting
in the formation of swapped rows of Ab
Nibble
swapped albite occurs first, followed by quartzification
Both albitization and
quartzification have nibbly replaced K-feldspars (Fig. 60). On the basis of the contact characteristics
of the swapped albite rows with the newly formed quartz, the nibbly metasomatic
albite should be formed in the condition that the original grain boundary between
the two primary K-feldspar crystals K1 and K2 (i.e., the
grain boundary between Ab
Swapped albite rows are formed first,
followed by berylitization
Small pegmatite miarolitic
cavities are distributed in the apical facies of a leucogranite intrusion (Shanbei granite), Taishan county,
Chloritization and muscovitization of biotite
A complicated phenomenon
appears at the contact of the primary euhedral biotite (transformed into
chlorite) with muscovite (Fig.
62). The muscovite is divided into two parts (Ms and Ms’).
The clear Ms contacted by quartz in many places is primary, while the Ms’
penetrating irregularly into biotite-chlorite is metasomatic. As the biotite is
replaced by Ms' and Chl (individually or separately),
there is no direct relationship between them. It is difficult to judge which
occurs first, chloritization or muscovitization. But, as a general rule, the
author postulates that the nibble muscovitization should have occurred prior to
the co-orientation chloritization.
The replacement texture
sometimes is easily confused with other textures.
The presence of small
isolated grains of mineral B with identical orientation in mineral A is one of
the major characteristics of replacement. However, it does not necessarily mean
that B grains are relics of replacement and A is the replacive mineral. The phenomenon might be caused by the
following conditions.
Intergrowth
and cotectic crystallization of feldspar with quartz
During the late stage of
melt crystallization at the cotectic point, two minerals (K-feldspar and
quartz) crystallized nearly simultaneously. Quartz may be more idiomorphic than
K-feldspar, showing that the end of quartz crystallization is earlier than K-feldspar (Fig.
63A).
The grain boundary is zigzag
or tortuous, resulting in a false impression* that quartz is replaced by
K-feldspar (as quartz is penetrated by K-feldspar) or vise versa, if the thin
section is cut along the grain boundary.
_______________________________________________________
*The inclusion relationship
among minerals observed in individual thin sections may produce a false
impression because of the limitation of a single thin section. Therefore,
attention must be paid to the vast majority of the phenomena in many thin
sections rather than focus on the isolated cases observed by chance.
Micrographic or
micropegmatic texture due to the specific cuneiform shapes of quartz grains is
hardly mistaken for replacement intergrowth, whereas the granophyric texture
(i.e., several small rounded, spherical or irregular micrograins of quartz with
the same orientation as that of the outside quartz grain which are enclosed in
feldspar, mainly K-feldspar) is more easily confused with replacement (Fig. 63B, 63C)
Overgrowth or epitaxial growth of
potash-sodium feldspar
The overgrowth or epitaxial
growth of potash and sodium feldspars (Fig. 63D) formed
in melt is easily confused with metasomatic texture because they all have the
same orientation. The morphologic and distribution patterns of epitaxial growth
of K-feldspar with plagioclase are more or less stable or similar in a certain
rock facies and even in a whole rock body, whereas the co-orientation
albitization of K-feldspar takes place abruptly and irregularly, and the whole
K-feldspar is rapidly transformed into thoroughly co-orientation metasomatic
chessboard albite.
Coarse myrmekite
Coarse myrmekite means that the vermicular
quartz grains contained in myrmekite are coarse. Several vermicular quartz grains with the
same crystallographic orientation are seemingly replaced by plagioclase. In
fact, the vermicular quartz grains are formed from excess silica after nibble
replacement of K-feldspar by relatively calcic plagioclase. Coarse myrmekite can also be formed by
Ca-metasomatism (nibble replacement) of sodic plagioclase (Dymek
and Schiffries, 1987) or by K-metasomatism of
relatively calcic plagioclase (Collins, 1988) where local residual altered
plagioclase lattices contain much Ca that combines with much residual Al to
leave excess Si in coarse quartz vermicules. In such places the coarseness of
the quartz vermicules is proportional to the An
content of the primary plagioclase. The
higher the An content of the primary plagioclase, the
coarser are the quartz vermicules.
Perthite and antiperthite
The perthitic, vein-like or
small blocky guest crystals contained irregularly in a host crystal are easily
mistaken as being formed by replacement because both guest and host minerals
have the same orientation.
The distribution of
irregular patches of K-feldspar in co-orientation plagioclase is treated as
evidence of replacement of plagioclase by K-feldspar (Collins,
Unmixing (exsolution) of solid solution
A mineral with solid solution,
which crystallized at high temperature, would exsolve at lower temperature,
resulting in the formation of regular intergrowth of two phases (host and guest
minerals) (orthoclase or microcline and albite), i.e., perthite. The texture of regular intergrowth thus
formed is called exsolution texture, and the corresponding mineral paragenesis
phenomenon, exsolution paragenesis.
The phenomenon seems to be the co-orientation
replacement product of either the replacement relict (guest mineral) replaced
by the host or the host mineral replaced by the vein like guest mineral because
they have the same crystallographic orientation. The
exsolved guest mineral, however, usually occurs in form of dense, flat, uniform
and homogeneous thin films, which differs from either the irregular or the vein
like perthitic albite.
The above mentioned phenomena can be easily
confused with replacement texture.
Therefore, the mineral intergrowth should be
observed and analyzed comprehensively in order to exclude a non replacement
texture from a replacement texture.
The author considers that all intergrowths of
feldspar (K-feldspar or plagioclase) with several quartz crystals enclosed in
feldspar are probably primarily formed by
simultaneous crystallization rather than by replacement of quartz by feldspar,
except myrmekite. All intergrowths of several feldspar fragments enclosed in
quartz are possibly caused by replacement of feldspar (perthitic) by quartz.
Besides, monzonitic texture (several euhedral or
subhedral plagioclase crystals are enclosed in allomorphic K-feldspar) usually
observed in monzonite, quartz monzonite, monzogranite and granodiorite should
be distinguished from the phenomena of plagioclase crystals replaced along
cracks by K-feldspar, as noticed by Collins (1998b).
Conclusion
After formation of granite, the gases and
solutions either relevant or irrelevant to magmatic process can penetrate into
solidified granitic rocks, resulting in partial dissolution of unstable primary
minerals and local crystallization of more stable secondary minerals; i.e.,
during mineral replacements that change the primary minerals and local
structure. Metasomatic phenomena, especially feldspathization, are observed in
granitic gneisses and granites in many places, even in solidified and
undeformed granites.
According to the author’s
observation (based on similarities or differences of crystallographic lattices
and orientations of replacive and replaced minerals), metasomatism can be
classified into two patterns: 1)
hetero-orientation replacement (nibble replacement) and 2) co-orientation
replacement. The metasomatic growth by nibble
replacement would occur at a grain boundary where there is a mineral prone to
be dissolved (replaced) on one side while the same or similar mineral can act
as a nucleation site of a replacive mineral on the other side. An impurity may serve as a nucleation site
for metasomatism if the same or similar mineral that could act as a nucleation
site is absent.
On the basis of the rule of
nibble replacement, the fine-grained platy crystals of albite (cleavelandite)
in Li-F granite are primary rather than being formed by “chaotic” replacement
processes.
Clear rim albite and grain boundary albite are
formed by replacement of K-feldspar where hydrothermal gas or fluid is
introduced with pure Na. If Ca
accompanies Na in the gas or fluid, excess SiO2 from replaced
K-feldspar would remain and take the shape of vermicular quartz in replacive
sodic plagioclase.
Perthitic albite looks like
plagioclase that has formed either by metasomatic processes or by exsolution
from a solid solution; however, such perthitic albite in rocks studied by the
author in
The key evidence of replacement
is the presence of relics of the replaced mineral in the newly formed replacive
mineral. However, such relics should be distinguished from crystals formed by
simultaneous crystallization and from normal inclusions in igneous rocks.
Generally, a phenocryst can
simply be distinguished from a porphyroblast. When several metasomatic
processes are superimposed, the steps preceding each replacement can possibly
be analyzed and understood in the light of the replacement rule.
Acknowledgements
The writing of this paper
was the result of encouragement and financial support by the leaders and former
colleagues in The Geological Center and Remote Sensing Center of Beijing
Research Institute of Uranium Geology.
My special thanks go to a host of friends, in particular, Dr. Shen Qihan, Academician of
Chinese Academy of Sciences, Researcher Du Letian,
whose expressed interest in offering financial support encouraged me to
initiate the writing of this paper. I
would like to extend my gratitude to Huang Shijie,
Zhou Weixun, Guo Yueheng,
Wu Jiashan, and He Jianguo. I am indebted to Researcher Li Yuexiang and Engineer Wang Fenggang
for their technical assistance in computer applications.
I extend my heartfelt thanks to Dr.
References
Aubert, G. and Burnol,
L., (1964), Observations sur les mineralisations en beryllium du massif
granitique d’Echassiers decouverte de herderite, Academie des Science (Paris) Comptes Rendus, v. 258, pp. 273-276.
Ashworth, J. R., (1972), Myrmekite of exsolution and
replacement origins. Geological
Magazine, v. 109, pp. 45-62.
Augustithis, S.
S., (1973), Atlas of the textural patterns of granites, gneisses and
associated rock types. Elsevier,
Banes, J. O. and Amourc, M.,
(1984), Biotite chloritization by interlayer brucitization as seen by HRTEM, American Mineralogist, v. 69, pp.
869-871.
Becke, F., (1908), Über
Myrmekit. Tschermaks Mineralogie und
Petrographie Mitteilung, v. 27, pp. 377-390.
Beus, A. A., Severov, E.
A., Sitnin, A. A., and Subbotin,
K. D., (1962), Albitized and greisenized granites (apogranites).
Nauka,
Bøggild,
O. B., (1924), On the laboradorization of the
feldspars. K. Dansk vidensk.selsk.Mat.-fys.Meddel.,
v. 6, no. 3, pp. 1-79.
Burnol, L., (1974), Geochimie
du beryllium et types de concentration dans les leucogranites du massif
central francais, Bureau de Recherches
Geologiques et Minieres Memoires, v. 85, pp. 137-168.
Carstens, H., (1967), Exsolution in ternary
feldspars; (II) Intergranular precipitation in alkali feldspar containing
calcium in solid solution. Beiträge zur Mineralogie und Petrologie, v. 14,
pp. 316-320.
Cesare, B., Marchesi, C., and Connolly,
J. A. D, (2002), Growth of
myrmekite coronas by contact metamorphism of granitic mylonites in the aureole
of Cima di Vila, Eastern Alps,
Cheng, Y. Q., (1942), A Hornblendic Complex, Including Appinitic Types
in the Migmatite Area of North Sutherland, Scotland, Proceedings of Geologist Association, v. 53, (2), pp.67-85.
Collins, L. G., (1988), Hydrothermal Differentiation and Myrmekite - A
Clue to Many Geologic Puzzles:
Collins, L. G., (1997), Sphene, Myrmekite, and Titanium Immobility;
Implications For Large-Scale K- and Na-metasomatism And The Origin of
Magnetite Concentrations: Myrmekite, ISSN 1526-5757, electronic Internet
publication, no.18, http://www.csun.edu/~vcgeo005/sphene.htm.
Collins, L. G., (
Collins, L. G., (1998b), Primary microcline and myrmekite formed during
progressive metamorphism and K-metasomatism of the Popple
Hill gneiss, Grenville Lowlands, northwest
Collins, L. G., (
Collins, L. G. and Collins, B. J. (
Collins, L. G. and Collins, B. J, (2002b),
K-metasomatism and the origin of Ba- and inclusion-zoned orthoclase megacrysts
in the Papoose Flat pluton, Inyo Mountains, California, USA, ISSN 1526-5757,
electronic Internet publication, no. 44, http://www.csun.edu/~vcgeo005/Papoose.htm.
Collins, L. G., (2003), Transition from magmatic to K-metasomatic
processes in granodiorites and Pyramid Peak granite, Fallen Leaf Lake
15-Minute Quadrangle, California, ISSN 1526-5757, electronic Internet
publication, no. 48, http://www.csun.edu/~vcgeo005/fallen.htm.
Deer, W. A., Howie,
R. A., Wise, W. S., and Zussman, J., (1963),
Rock-Forming Minerals. Vol. 4, Framework Silicates:
Deer, W. A., Howie, R. A., Wise, W. S. and Zussman,
J., (2002), Rock-Forming Minerals. Volume 4B. Second Edition. Framework Silicates:
Dengler, L., (1976), Microcracks in
crystalline rocks. In Wenk, H.R.
(Ed.) Electron Microscopy in Mineralogy. Springer, pp. 550-6.
Drescher-Kaden,
F. K., (1948), Die Feldspat-Quartz-Reactionsgefüge der
Granite und Gneise, und ihre genetische Bedeutung.Springer, Heidelberg, 259 pp.
Du Shaohua, and Huang Yunhui, (1984), On study of
Xianghualingite, Science
Sinica, part B (11): pp. 1039-1047. (in
Chinese)
Dymek, R. F., and Schiffries,
C. M., (1987), Calcic myrmekite: possible evidence
for the involvement of water during the evolution of andesine anorthosite
from
Eadington, P. T., and Nashar,
B., (1978), Evidence for the magmatic origin of quartz-topaz rocks from the
Engvik, A.
K., Putnis A., Fitz Gerald, J. D., and Austrheim, H., (2008), Albitization
of granitic rocks: The mechanism of replacement of oligoclase by albite. The Canadian Mineralogist, v. 46, no.
6, pp.1401-1415.
Farver, J.
R. and Yund, R. A., (1995), Grain boundary diffusion of oxygen, potassium
and calcium in natural and hot-pressed feldspar aggregates. Contributions to Mineralogy and Petrology,
v. 12, pp. 1645-1659.
Haapala,
Hall, A., (1966), A
petrogenetic study the Rosses granite complex,
Donegal. Journal of Petrology, v. 7, pp. 202-220.
Harker,
A., (1950), Metamorphism. Methuen.
Heinemann,
S., Wirth, R., and Dresen, G., (2003), TEM study of a special grain boundary
in a synthetic K-feldspar bicrystal: Manebach Twin, Physics and Chemistry of Minerals, v. 30, pp. 125–130.
Hellmann, R. et
al., (2004), Chemical alteration of feldspar: a
comparative study using SIMS and HRTEM/EFTEM, Proceedings of the Eleventh
International Symposium on Water-Rock Interaction, …RB Wanty, RR Seal - - AA Balkema.
Hibbard, M. J.,
(1995), Petrography to petrogenesis. Prentice Hall,
Hiraga, T., Nagase, T., and Akizuki, M., (1999), The structure of grain boundaries in granite-origin
ultramylonite studied by high-resolution electron microscopy. Physics and Chemistry of Minerals, v.
26, pp. 617-623.
Hong Wenxing (1975),
Discussion of characteristics and metallogenesis of several TaNb REE-bearing
granites in
Hu Shouxi,
(1975), Igneous rock of sodic and potassic series
and alkali metasomatism to metallogenesis for rare elements, Colloquia of
conference for rare elements in
Imeopkaria, E. G., (1980), Ore-bearing potential of granitic
rocks from the Jos-Bukuru complex, Northern
Nigeria, Chemical Geology, v. 28,
pp. 67-70.
Kovalenko, V.
Kovalenko, B. E., Kuzmin, M. E. , and Antapin,
B. S., (1971), Topaz-bearing quartzic keratophyre
(ongonite) of subvolcanic vein magmatic rocks. Report of
Lee, M. R., Waldron, K. A., and
Parsons,
Li Fuchun,
Zhu Jinchu, Rao Bing,
Jin Zhangdong and Zhang Linsong,
(2004), Origin of Li-F rich granites: Evidence from high P-T experiments, Science in China Ser. D Earth Sciences,
v. 47, no.7, pp. 639-650.
Lindgren, W., (1925), On metasomatism. Bulletin of the American Geological
Society, v. 36, pp. 247-262.
Liu Yimao, Li Huamei, Ling Deson, et al., (1975), Space
distribution characteristics of endogenetic rare element deposits in
Lofgren, G. E. and Gooley,
R. G., (1977), Simultaneous crystallization of feldspar intergrowths from
the melt, American Mineralogist, v. .62, pp. 217-228.
Maliva, R.G., and Siever,
R. (1988), Diagenetic replacement controlled by force of crystallization, Geology, v. 16, pp. 688-691.
Masgutov,
R.V., (1960), Yisvestia Akademi Nayk. Geol. Ser. Tom 3.
Merino,
E., Nahon, D., and Wang, Y., (1993). Kinetics and mass transfer of
pseudomorphic replacement: Application to replacement of parent minerals and
kaolinite by Al, Fe, and Mn oxides during weathering, American Journal of Science, v. 293, pp. 135–155.
Merino, E., and Dewers, T., (1998), Implications of replacement for
reaction-transport modeling. Journal of Hydrology, v. 209, pp. 137-146.
Nauman, K. F., (1873), Lehrbuch der
Mineralogie.
Olives,
J., Amouric, M., De Fouquet, C., and Baronnet, A., (1983), Interlayering
and interlayer slip in biotite as seen by HRTEM. American Mineralogist, v. 68, pp. 754-758.
Ostapenk, G. T., (1976), Excess pressure on the solid
phases generated by hydration (according to experimental data on hydration
of periclase). Geochemistry
International, v. 13 (3), pp. 120-138.
Peng, C. C., (1970), Intergranular albite in the
granite and syenites of
Phemister, J., (1926), the geology of Strth
Okykell and lower Loch Shin (Explanation of Sheet
102):
Phillips, E. R., (1964), Myrmekite and albite
in some granites of New England Batholith, New South Wales: Journal of the Geological Society of
Australia, v. 11, p. 49-60.
Phillips, E. R., (1980), On
polygenetic myrmekite: Geological
Magazine, v. 117, no. 1, p. 29-36.
Putnis, A., (2002), Mineral replacement
reactions: from macroscopic observations to microscopic mechanisms. Mineralogy Magazine, v. 66, pp. 689-708.
Putnis A., Hinrichs
R., Putnis C.V., Golla-Schindler U., Collins L.
G., (2007), Hematite in porous red-clouded feldspars: Evidence of
large-scale crustal fluid–rock interaction, Lithos, v. 95, p. 10–18.
Que Meideno, and Allen Alistarir, R., (1996), Sericitization of plagioclase in
the Rosses granite complex,
Ramberg, H., (1962), Intergranular precipitation of albite
formed by unmixing of alkali feldspar.
Neues Jahrbuch für Mineralogie
Abhandlung, v. 98, pp. l4-34.
Raimbault,
L., (1984), Geologie, petrographie et
geochimie des granites et mineralization associees de la region de Meymac
(Haut Correze, France), Ph.D. Thesis,
Paris, Ecole de Mines, 482 pp.
Rogers, J. J. W., (1961), Origin of albite in granitic rocks. Amererican Journal of Science, v. 259, pp.186-193.
Rong Jiashu, (1982), Microscopic observation on the
metasomatic phenomena of rock-forming minerals in granites, Petrological Research, Vol.1, pp.
96-109. (in Chinese)
Rong Jiashu, (2002 ), Myrmekite formed
by Na- and Ca-metasomatism of K-feldspar, ISSN 1526-5757, electronic
Internet publication, no. 45, http://www.csun.edu/~vcgeo005/Rong.htm.
Rong Jiashu, (2003), Nibble metasomatic K-feldspathization,
SSN 1526-5757,
electronic Internet publication, no. 46, http://www.csun.edu/%7Evcgeo005/2Rong.htm.
Schermerhorn, L. J. G.., (1956), The
granites of Trancoso (
Smith, J. V., and Stenstrom, R. C., (1965), Electron-excited luminescence
as a petrologic tool. Journal
of Geology, v. 73, pp. 627-635.
Smith, J. V., (1974), Feldspar
Minerals. Volume 2. Chemical and
textural properties. Springer-Verlag Berlin Heidelberg New York, 690 pp.
Schwantke
A, (1909), Die Beimischung von Ca im Kalifeldspat und die Myrmekibildung. Contributions
to Mineralogy and Geology, pp.
311-316.
Spilde, M. N., et al., (1993), Alteration of plagioclase and pyroxene
phenocrysts in a fissure fumarole, Valley of Ten Thousand Smokes, Alaska, American Mineralogist, v. 78, pp. 1066-1081,
Sprunt, E. S. and Brace, W. F. (1974) Direct
observation of microcavities in crystalline rocks. International Journal of Rock Mechanics and Mining Sciences, v.
11, pp. 139-50.
Stempro, M., (1979), Mineralized
granites and their origin, Episodes of
Geology News Letter, v. 3, pp. 20-24.
Swanson, S. E., (1977), Relation of
nucleation and crystal growth rate to the development of granites textures. American Mineralogist, v. 62, pp.
966-978.
Taylor, R. P., (1992), Petrological and geochemical
characteristics of the peasant ridge zinnwaldite-topaz granite, southern
Tschermark, G., (1864),
Chemisch-mineralogische Studien. I. Die Feldspathgruppe. SAWW, v. 50, pp. 566-613.
Tuttle, O. F., (1952), Origin of the
contrasting mineralogy of extrusive and plutonic salic rocks. Journal of Geology, v. 60, pp.
107-124.
Veblen,
D. R., and Ferry, J. M., (1983), A TEM study of the biotite-chlorite reaction and comparison with petrological observations. American Mineralogist vol.
68, no. 11-12, p. 1160-1168.
Wang Defu, (1975), On classification of rare metal mineralization granites
and genesis of mineralization in
Wang Liankui, et al.,
(1970), Summary report of
Wang Zhonggang, Yu Xueyuan, Zhao Zhenhua, et al.,
(1989), Geochemistry of rare metals, Science Press, pp. 225-245. (in Chinese)
Worden, R. H.,
Xia Hongyuan, Liang Shuyi, (1991), Genetic series of W-Sn-rare metal-bearing
granites in
Xia Weihua, Zhang Jintong, Feng Zhiwen et al., (1989), Geology of rare metal ore
deposits in granites in Nanling. Publishing House of Chinese Geological University, 14-115.
(in Chinese)
Yuan Zhongxin, Bai Ge, Yang Yueqing, (1987), Discussion of metallogenesis of rare
metal-bearing granites, Geology of ore deposits, (1), p. 88-94. (in Chinese)
Zhang Jintong, Xia Weihua,(1985), Preliminary research on geology and
metallogenic factors of WSnNbTa in Songshugang
deposit, Collected work of geology and ore deposits in Nanling,
Vol. 1, Geological Publishing House, 145-148. (in
Chinese)
Zhu Jinchu, Liu Weixin, Zhou Fengyin, (1992),
Ongonite in Xianghualing, annual report of state
key laboratory of ore deposits of Nanjing university, Publishing House of
Nanjing University, 12~19. (in Chinese)