39. Scientific errors that can result when myrmekite
and geologic evidence
Lorence G. Collins
April 18, 2001
The present studies of granitoid rocks from the
1980s into the year 2001 are
dominantly directed toward chemical analyses and experimental and theoretical
petrology rather than the determination of textures and petrography as seen in
thin sections. Values for chemical studies of the granitoids are that they (a)
support classifications into source types (S-I-A-M), (b) demonstrate liquid line
of descent toward the temperature minimum of the Ab-Or-Qtz system, (c) enable
the plotting of Harker diagrams of major oxides and spider diagrams of major and
trace elements to show differentiation trends, (d) serve to discriminate among
possible tectonic settings, and (e) provide data to enable the determination of
ages by using various isotopes (Winter, 2001). For most granitoids this method
of investigation is without reproach. However, when only chemical
studies are used to study a granitoid, many changes in the solidified magmatic
rocks may be hidden or unsuspected. Therefore, scientific errors in
interpretation can result.
Recent work suggests that where the granitoids
contain rim and/or wartlike
myrmekite, improper assumptions can be made concerning the complete history of
these rocks (Collins, Internet website articles, 1997-2001). Following
solidification, deformation and modification by fluids containing K and Si can
cause differentiation progressively toward the temperature minimum of the
Ab-Or-Qtz system. This occurs because recrystallized minerals in granitic rocks
formed by replacement processes below melting temperatures are stable there just
as those formed from a melt. Moreover, the elemental characteristics of their
primary magmatic sources and much of their physical features (e.g., enclaves,
dikes, zoning, hypidiomorphic textures) are inherited by the metasomatized rocks
so that internally and outwardly they retain much of their magmatic ancestry.
The chemical analyses can look the same (or nearly so) in both magmatic
or metasomatically altered granitoids, not only because of inheritance, but also
because the changes in chemical compositions during metasomatism parallel what
is observed in a liquid line of descent. It is only by analyzing textures, as
seen in thin sections, that extensive changes in chemistry and mineralogy can be
detected that are unrelated to magmatic processes. Understanding the
significance of myrmekite is important because its presence in a thin
section is an indication that additional study is needed in order to fully
comprehend what the chemical data mean (Collins, 1988a). The historical
background for the shift to modern styles of granitoid investigation and its
consequences are presented below as well as problems caused by sampling bias.
Because of these problems, guidelines for sampling granitoids are suggested.
An examination of articles published since
Michel-Lévy (1874) first
discovered myrmekite shows trends which illustrate progressive changes in
attitudes towards this unusual plagioclase-vermicular quartz intergrowth (see
references listed by decades). Becke (1908) was the first to propose that
myrmekite was formed by Na- and Ca- metasomatism of K-feldspar. Then, Schwantke
(1910) countered with the idea that myrmekite resulted from exsolution of Na,
Ca, Al, and Si from high temperature K-feldspar. Because of the conflict between
these two models, myrmekite became a subject of great research interest, and
investigators presented many arguments as to which one was correct. In the
1940s, 1950s, and later years, new kinds of myrmekite were described, having
different kinds of textural patterns and mineral relationships. Because some
textural patterns did not seem to be best explained by either of the above two
models for myrmekite origin, other models for myrmekite formation were
formulated. All of these early models were discussed and evaluated by Philips
(1974), but none had sufficient support to say that the origin of myrmekite had
In the 1940s through the 1960s, authors of
articles on granitic rocks and
gneisses commonly reported myrmekite in their petrographic descriptions, listing
it as an accessory or alteration mineral along with chlorite, sericite, epidote,
calcite, clay, iron oxides, apatite, allanite, titanite, and zircon. In this
same period, many authors also included discussions, indicating their favored
model for the origin of myrmekite. In the 1940s and 1950s, however, a great
debate raged about the origin of all granite bodies, as to whether
they were entirely formed (a) by subsolidus replacement processes of sedimentary
rocks ("granitization") or (b) by magmatic processes. See papers by Read,
Buddington, Grout, Goodspeed, and Bowen presented at a meeting of the Geological
Society of America held in Ottawa, Canada, December 30, 1947 (Gilluly, 1948).
The debate was subsequently won by the experimentalists, and their conclusion
that all granites of large size are magmatic in origin modified
opinions about myrmekite. If granite bodies were entirely magmatic in origin,
then myrmekite could only be a deuteric alteration mineral that was produced
during the final stages of crystallization of a magma. Moreover, because in the
1950s and 1960s, no definitive criteria had been developed to decide whether (a)
Ca- and Na-replacement of K-feldspar or (b) exsolution was the correct origin of
myrmekite, and because the presence of myrmekite did not seem to indicate any
important relationship affecting the petrology of the granite in a major way,
many authors of articles about granite in subsequent years no longer even
mentioned myrmekite nor promoted their preference for its origin. Myrmekite’s
tiny modal volume (generally less than 0.2 volume percent), its uncertain
origin, and its relegation to a mineral formed by alteration during the last
stages of crystallization of a magma seemed to suggest that myrmekite could be
ignored without any serious consequences.
On that basis, interest in myrmekite waned after
the 1960s, and the arguments
for magmatism as the primary origin of all granite were seemingly
so powerful that support for large-scale "granitization" also disappeared in
most educational institutions and geological surveys and in undergraduate
geology textbooks. Anyone supporting "granitization" was automatically regarded
as being "behind the times." Thereafter, experimental work on granitic melts and
chemical studies became the dominant accepted ways to investigate granites.
Petrography and textural analysis of the rocks, as obtained through studies of
microscopic thin sections, were relegated to the "trash heap," and editors of
journals were not interested in publishing such information. In place of thin
section studies, chemical analyses of major oxides and trace elements, and
theoretical and experimental petrology based on thermodynamics became the
dominant emphasis. Cathodoluminescent techniques, fluorescent x-ray, and
electron-microprobe methods of analysis soon replaced wet-chemical methods to
determine chemical compositions, and the addition of isotopic age-dating
methods, using various isotopes, became the accepted ways to study granitic
rocks (Winter, 2001).
On the basis of these modern methods of studying
granitic rocks, published
articles in the decades since the 1970s contain little mention of petrography.
Once it had been decided that all granites were magmatic in origin, the study of
petrography was no longer needed, and any hypothesis suggesting that not all
granites were totally magmatic in origin was not even considered. With this mind
set, many articles, describing the chemistry or isotopic compositions of
granitic rocks, mention only the main component minerals. No detailed
descriptions of textures occur, and the barest information about accessories or
alteration minerals is given, if at all. Even in other articles not emphasizing
chemistry, progressively through the decades since the 1960s, myrmekite commonly
is omitted from the petrography sections, even when it is relatively abundant.
This is in spite of the fact that other alteration minerals are reported. When
other alteration minerals are mentioned but not myrmekite, the lack of
noting myrmekite is an example of scientific negligence. All mineral
data, including myrmekite, should be reported when describing rocks even if the
authors do not regard myrmekite as being important.
In the 1960s and 1970s, research on myrmekite was
in limbo. Because it was
felt by editors of journals that myrmekite had been thoroughly studied, articles
on this topic were generally rejected. Even abstracts submitted for presentation
at professional conventions were rejected because the reviewers felt that
attendees would not be interested in them. However, many geologists were still
not satisfied with the then-available models for the origin of myrmekite, and,
surprisingly, a few articles proposing new models continued to appear in the
literature in the 1980s and 1990s as new kinds of myrmekitic textures were found
in different rock types. See the reference list by decades.
Beginning in 1972, my own work supported the
general feeling that all
large masses of granitic rocks have been emplaced by magmatic processes and that
no large granite mass has formed by subsolidus "granitization" of sedimentary
rocks. However, in the late 1970s, 1980s, and 1990s evidence became
available to me that some large plutonic igneous masses, after
emplacement and solidification, have been modified by K- and
Si-metasomatism to change them into rocks of a more-granitic composition, having
increasing percentages of K-feldspar and quartz (Collins, 1988a; Hunt et al.,
1992; Collins website articles at http://www.csun.edu/~vcgeo005). In
few places, even adjacent metasedimentary wall rocks had undergone similar and
simultaneous K- and Si-metasomatism as the magmatically emplaced pluton was
metasomatized. This metasomatism required deformation of solid
rocks below melting temperatures in order to create tiny fractures in
which fluids could enter and interact with the primary minerals in the rocks.
These fluids utilized local or outside sources of K and Si and removed some Ca,
Al, Fe, and Mg among other elements from the rocks. No migration of these
elements was by solid-state diffusion through undeformed rocks across vast
distances (meters and kilometers) as was assumed to occur by some geologists
during the old style "granitization" process. The only amount of
solid-state diffusion was on a scale much less than a millimeter and only
through half the diameter between closely-spaced, microscopic fractures within a
mineral grain. Significantly, the tiny fractures created by deformation
generally have gone unnoticed by petrologists because such brittle breakage of
plagioclase crystals cannot be seen in thin section under plane or
cross-polarized light. Only strong deformation that bends albite twin
lamellae or cataclasis that rotates broken fragments is observed under
cross-polarized light. Seeing these tiny fractures requires cathodoluminescence
imaging, which is a technique that petrologists have not commonly applied to the
study of granitic textures (Hopson and Ramseyer, 1990a; Collins, 1997b).
Consequently, these clues to the K-metasomatism have been missed. At any rate,
the main means of elemental migration must be in fluids moving through fractures
and by ionic diffusion through these fluids rather than by solid-state
diffusion. The residual effects of ion exchanges along the fractures during
these movements are readily seen in cathodoluminescent images.
Subsequently, the element identification along such fractures can be verified
with scanning- and electron-microprobe analyses (Collins, 1997b).
Resistance to this new K- and Si-metasomatic
model was enormous and
increasingly so after the 1970s to the year 2001 because the general feeling
among most granite petrologists is that "granitization" had been
disproved. K- and Si-metasomatism must have sounded like "granitization"
all over again, even though it was entirely different. Assumptions were made by
most petrologists that when a magmatically-emplaced granitic body became
crystallized, it was static, and, henceforth, it did not change (except by
weathering), even for billions of years. Therefore, any hypothesis suggesting
large-scale changes by metasomatic fluids was wrong. While agreeing with the
conclusion that all granitoids are emplaced and crystallized from magmas, I
suggested that the thin section evidence, field studies, and chemical analyses
(including electron-microprobe analyses) showed that large-scale K- and
Si-metasomatism is valid in some deformed plutonic masses, and myrmekite is the
clue to this metasomatism (Collins, 1988a). Nevertheless, the increasing
resistance to a model that sounded like "granitization" has continued in the
1970s, 1980s, and 1990s, regardless of the evidence that was provided.
Objection to large-scale K- and Si-metasomatism is
surprising for the
(1) Granite petrologists have already accepted
during fenitization (Winter, 2001), and the chemistry of Na is not much
different from that of K so that both should behave in the same way. Examples of
large-scale Na-metasomatism in granitic rocks are demonstrated in northern New
York (Collins, 1997r).
(2) Large-scale K-replacement of plagioclase
crystals (from the exterior
inward) is known to occur throughout some granitic plutons during late-stages of
the crystallization of the magma (Collins, 1997s). Therefore, lowering the
temperature just a few degrees below melting conditions should not change the
capability of K to replace plagioclase. However, the style of replacement
changes to that of replacing the plagioclase crystals from the interior outward
rather than from the exterior inward. The difference in style occurs because in
magma, not totally solidified, the ability of fluids to flow between
early-formed crystals creates space for the expanded, lower-density lattice of
K-feldspar to grow as it replaces the denser lattice of plagioclase. In
contrast, in completely crystallized granitoids, no expansion between sealed and
interlocking solid crystals can occur. First fracturing must occur, and then the
extra needed space for the K-feldspar lattice to form must be produced by
removal of elements from the interiors of the deformed plagioclase crystals. As
replacement occurs, it is not mass-for-mass, as in balanced chemical equations,
but volume-for-volume (Collins, 1988a, 1997a, 1997b).
(3) Experimental work of Orville (1962, 1963)
has already shown that
K-metasomatism can be demonstrated and that it occurs at rates that permit
the conversion of plutonic rocks containing plagioclase as the only feldspar
into granite containing K-feldspar during the geologic time frames that are
available (Collins, 1999b).
And finally, (4) economic geologists have observed
broad extent of
metasomatism by large quantities of introduced fluids to form many different
kinds of metallic ore deposits. Having seen the evidence for such great volumes
of "metal" replacements in deformed but solid rocks, our colleagues find it
surprising that granite petrologists do not accept large-scale
K-metasomatism (Tim H. Bell, email communication, 1997).
The problem for accepting large-scale K- and
Si-metasomatism to form
some granitoids is partly because granite petrologists have not seen the
evidence in thin section via cathodoluminescent imaging and because they have
been looking in the wrong places, as is suggested in the next section.
There is an anecdote about a person in the middle
of the night coming upon an
old man on his knees under a street lamp looking for his glasses. After learning
of the man’s loss, this person decides to help in the hunt. After several
minutes of looking, however, and finding nothing, this person asks: "Where did
you lose your glasses?" The old man points to the dark shadows and says: "Over
there." "Then, why are you looking here?" To which the man replied: "Because the
light is here."
This anecdote illustrates an analogous problem in
Myrmekite-bearing granitic rocks generally form resistant outcrops because of
the abundance of quartz and feldspars. The outcrops of granite ("where the light
is") stand above the valleys ("the shadows where the origin of myrmekite can be
found"). In this analogy the investigators of myrmekite have looked only in the
granite where the final product has been produced and not where the progressive
stages that led to the formation of the myrmekite have taken place. One must
look beyond the granite in the transition rocks where biotite is commonly
abundant to find the answer to the origin of myrmekite. Unfortunately,
biotite-rich rocks are easily weathered and eroded, and in many places glaciers
and streams have removed these weaker rocks, and vegetation, soil, and other
deposits cover them. Thus, natural processes force a sample bias on the person
wanting to collect rocks in a given terrain. Only the resistant rocks in outcrop
can be collected unless diamond drilling of covered rocks is used to obtain core
samples. Because of this limitation, in many places only the final product of
K- and Si-metasomatism can be collected, forcing an unfortunate sample bias.
Moreover, in many places the replacements and recrystallization of the original
rock found in the resistant outcrop have removed nearly all evidence for
its original composition and for its former deformation that permitted the K-
Another bias in sampling occurs because modern
studies of granites deal with
their chemistry and isotopic compositions. Rocks needed for these investigations
must be fresh, and only a few samples are required. When the investigator
already believes ("knows") that the granite is entirely magmatic in origin,
there is no purpose in collecting lots of samples to prove its magmatic origin,
and samples that might be weathered or messy are avoided. Two thin sections may
be sufficient to determine the main minerals. Yet, it is the abundance of
samples in the wall rocks and transitions to the granite and their thin sections
(the geologic evidence) that may reveal the true history of the
Guidelines for sampling granitoids
On that basis, to avoid sampling bias certain rules
should be followed when
collecting samples of granitic rocks.
1. The sampling must be systematic, making sure
that a grid distribution of
samples is obtained wherever the outcrops make it possible.
2. Even though the granitoid may appear to be
uniform (magmatic in
appearance), two samples need to be taken at each collection site in a grid in
order to see the local and broad variations that occur. Where a granitoid is not
uniform, all rock types should be collected at a given grid site.
3. Where foliation is present, strike and dip must
be measured, and the
sample that is collected must be taken from the same place that the rock
attitude is determined. The structural attitude of the sample relative to
attitudes of rocks in surrounding or adjacent areas may determine the sample’s
mineral and chemical composition because the composition may result from
metasomatic fluids moving into low pressure sites that are controlled by the
4. Exotic structures and textures (enclaves,
schlieren, etc.) are commonly
attractive, and the investigator must be careful in the field not to bias the
sampling by making a collection consisting of 80-90 percent of the exotic rocks
and 10-20 percent from the common rock type. The sampling should be proportional
to the relative volumes of the different rock types.
5. Efforts must be made to find and collect from
transition rocks. For
example, (a) if the granitic rock contains K-feldspar megacrysts, then
transitions to portions of the granitic body must be looked for in which the
megacrysts are absent. (b) If the K-feldspar megacrysts are zoned, then
transitions to places where they lack zoning must be sought. (c) If pegmatite
dikes are found, then the dikes need to be walked out along strike to see if
they grade into zones of deformation and then to where the dikes and deformation
disappear. And (d), similarly, if migmatites occur, the granitic portions need
to be sampled progressively along strike until the granitic rocks disappear.
Samples of these different kinds of transition rocks are likely to provide clues
to the evolutionary history of the granitoid.
Because myrmekite cannot be seen in the field, when
it is discovered later in
thin section, one should go back in the field, perhaps several times, to collect
additional samples and narrow down the transitions to where myrmekite first
appears. Because perthitic intergrowths also cannot be seen in the field, if the
albite lamellae in the K-feldspar are not uniformly distributed and are
irregular in size, then additional sampling is needed. If the perthite lamellae
have uniform distribution and are nearly constant in size, then they are likely
formed by exsolution from orthoclase. If the lamellae are not uniform, this
perthite could be formed from replacement of plagioclase by K-feldspar. Return
trips may also show progressive Si-replacements, producing interior quartz blebs
in hornblende and biotite across the transitions. At any rate, without such a
guideline to seek transition areas, these many different things to look for
might be missed if the investigator has preconceived notions that the granitoid
is entirely magmatic and unchanged since emplacement. As Goethe said: "We see
what we know."
Some of these sampling rules may go against one’s
natural tendencies in the
field. In some places great numbers of samples do not seem to be necessary. I
have had to remind myself many times to follow these rules when my instincts
said: "What is the use of sampling here?" However, the unaided eye can not see
mineralogical and textural changes that are important. Moreover, I discovered
that it was only when I collected hundreds of samples systematically and made
thin sections of these samples that structural attitudes at outcrops could be
tied to metasomatism. It was only when I collected 900 samples of amphibolite
and interlayered biotite-orthopyroxene-plagioclase (An80) gneiss in
eight different layers from noses to limbs in isoclinal folds did the thin
sections and other methods of analysis reveal convincing evidence for
progressive losses of iron from the ferromagnesian silicates in deformed
amphibolite in the limbs (Collins, 1969). These losses explained how magnetite
concentrations were formed in sufficient quantities to be mined as iron ore.
This same extensive sampling also showed the progressive changes in the limbs in
the deformed biotite-orthopyroxene-plagioclase gneisses to convert them into
myrmekite-bearing granitic gneisses. Likewise, in other terranes it has been
only when great numbers of samples were collected from areas outside a granite
body through transitions into the granite that I could observe the progressive
stages of myrmekite formation. See index of articles in the Internet URL: http://www.csun.edu/~vcgeo005/index.html
On the basis of the above, new attention needs to
be applied to thin section
studies of more than a few samples and to the occurrence and absence of
myrmekite in granitic rocks. Although granitic plutons are emplaced by magmatic
processes, that does not mean that these rocks will necessarily remain unchanged
through eons following their solidification. The constant stirring in the
mantle, causing crustal plate motions, must deform at least some of the
crystallized plutons, enabling metasomatic fluids to move through them. It is
agreed that using only petrographic studies of thin sections is inadequate to
explain what happens in the plutons during their entire evolutionary history.
But so also is a study limited to chemical analyses inadequate.
Investigators need to combine experimental, chemical, and theoretical studies
with geologic evidence obtained in the field and from detailed thin section
analysis, concomitant with cathodoluminescence studies, if scientific errors in
interpretation are to be avoided.
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Myrmekite, ISSN 1526-5757, electronic Internet
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K-feldspar and myrmekite: Myrmekite, ISSN
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myrmekite-bearing granitic rocks formed by
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publication, no. 3, http://www.csun.edu/~vcgeo005/revised3.htm.
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ISSN 1526-5757, electronic Internet
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processes: Myrmekite, ISSN 1526-5757, electronic
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granite: Myrmekite, ISSN 1526-5757, electronic
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megacrystal quartz monzonite at Twentynine Palms,
- California, USA: Myrmekite, ISSN 1526-5757, electronic Internet
publication, no. 9, http://www.csun.edu/~vcgeo005/29palms.htm.
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northwest Ireland: Myrmekite, ISSN 1526-5757,
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and Al-Ti-Zr trends, Gold Butte, Nevada:
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Springs, California: Myrmekite, ISSN 1526-5757,
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a replacement pluton: Myrmekite, ISSN
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Mojave Desert, California, USA: Myrmekite,
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the Hall Canyon muscovite granite pluton,
- Panamint Mountains, California, USA: Myrmekite, ISSN 1526-5757, electronic
Internet publication, no. 15, http://www.csun.edu/~vcgeo005/hall.htm.
- Collins, L. G., 1997p, Sericitization in the Skidoo pluton, California: A
possible end-stage of large-scale
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publication, no. 16, http://www.csun.edu/~vcgeo005/skidoo.htm.
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aluminum during K- and Si-metasomatism: Myrmekite,
- ISSN 1526-5757, electronic Internet publication, no. 17, http://www.csun.edu/~vcgeo005/mobility.htm.
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mobility; implications for large-scale
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Myrmekite, ISSN 1526-5757, electronic Internet publication, no. 18, http://www.csun.edu/~vcgeo005/sphene.htm.
- Collins, L. G., 1997s, Contrasting characteristics of magmatic and
metasomatic granites and the myth that granite plutons
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publication, no. 19, http://www.csun.edu/~vcgeo005/myth.htm.
- Collins, L. G., 1997t, Failure of the exsolution silica-pump model for the
origin of myrmekite: Examination of
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Myrmekite, ISSN 1526-5757, electronic Internet publication, no. 20, http://www.csun.edu/~vcgeo005/pump.htm.
- Collins, L. G., 1997u, Three challenging outcrops in the Marlboro
Formation, Massachusetts, USA: Myrmekite, ISSN
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Scituate granite gneiss, Rhode Island,
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Internet publication, no. 22, http://www.csun.edu/~vcgeo005/augen.htm.
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Caledonian Orogen in Scotland: Myrmekite,
- ISSN 1526-5757, electronic Internet publication, no. 23, http://www.csun.edu/~vcgeo005/scotland.htm.
- Collins, L. G., 1997x, Magmatic resorption versus subsolidus metasomatism
– two different styles of K-feldspar replacement
- of plagioclase: Myrmekite, ISSN 1526-5757, electronic Internet
publication, no. 24, http://www.csun.edu/~vcgeo005/ajo.html.
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Relation to crustal structure of the Cretaceous
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- Collins, L. G., 1998a, The microcline-orthoclase controversy – can
microcline be primary?: Myrmekite, ISSN 1526-5757,
- electronic Internet publication, no. 26, http://www.csun.edu/~vcgeo005/primary.htm.
- Collins, L. G., 1998b, Metasomatic origin of the Cooma Complex in
southeastern Australia: Myrmekite, ISSN 1526-5757,
- electronic Internet publication, no. 27, http://www.csun.edu/~vcgeo005/cooma.htm.
- Collins, L. G., 1998c, Primary microcline and myrmekite formed during
progressive metamorphism and K-metasomatism
- of the Popple Hill gneiss, Grenville Lowlands, northwest New York, USA:
Myrmekite, ISSN 1526-5757, electronic Internet publication, no. 28, http://www.csun.edu/~vcgeo005/popple.htm.
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megacrystal granodiorite and the Sithonia
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I thank my wife, Barbara, whose many
excellent suggestions greatly
improved this article. I also express my appreciation to David Liggett,
who has helped me with the technical computer problems for getting this
article and all the other articles on line for this electronic
For more information contact Lorence Collins at: email@example.com
Lorence G. Collins
Department of Geological Sciences
California State University Northridge
18111 Nordhoff Street
Northridge, California 91330-8266