2. REPLACEMENT OF PRIMARY PLAGIOCLASE BY SECONDARY K-FELDSPAR AND MYRMEKITE
Lorence G. Collins
email: lorencec@sysmatrix.net
November 21, 1996; revised February 17, 1997
It is recommended that you read the previous presentation before proceeding.
The following illustrations in this second presentation show four photomicrographs of cathodoluminescent images, two black-and-white scanning electron microscope images, and six photomicrographs of thin sections in which various stages of replacement of primary plagioclase by secondary microcline and myrmekite occur. Cathodoluminescent images were made by Karl Ramseyer in Switzerland.
Fig. 1, Fig. 2, Fig. 3, Fig. 4a, Fig. 4b, and Fig. 5
The colors in the photomicrographs (Figs. 1-3, 5) are not the fluorescent colors of K, Ca, or Na because these elements do not fluoresce with these colors. The colors represent fluorescence of trace elements associated with the K, Ca, and Na in the feldspars. Electron microprobe or scanning-electron analyses verify that K, Ca, and Na are dominant in the colored areas indicated.
The progressive changes from unaltered normal-zoned plagioclase in the diorite to reverse-zoned plagioclase shows the preparation prior to introduction of K in the interior of an altered plagioclase crystal. All plagioclase grains small or large are altered in this fashion in the deformed diorite in this stage. Moreover, at this stage the scanning-electron images (1,600x and 8,000x) indicate that some altered plagioclase crystals are virtual sieves with ample openings for fluids and elements to move in and out. The progression of the alteration shows that primarily Ca is subtracted, but some Al must also move out as Na tends to stay behind.
When K is introduced, the "holes" provide space for the expanded lattice of K-feldspar to grow inside the altered plagioclase crystal which still has a solid, silicate framework. Like a geodesic dome, the framework prevents collapse of the crystal even though adjacent solid grains in the deformed rock are pressing with high pressure on the altered plagioclase crystals.
When a particular altered crystal is replaced by K, then the K displaces most of the Ca and Na (but not all), and much of the Na atoms that are displaced move into other nearby less-altered plagioclase crystals (with holes) to cause them to recrystallize as a more sodic plagioclase. In the rocks at Temecula, some of the original zoned plagioclase crystals with calcic cores An37-39 and sodic rims An17-20 of the diorite are recrystallized as unzoned albite-twinned plagioclase An12-15; see Fig. 3 in the previous presentation
The myrmekite forms where residual Ca, Na, Al, and Si in the altered plagioclase are in the wrong proportions to recrystallize as plagioclase only. In most places as K comes in, avenues that permit Ca, Na, and Al to escape the plagioclase are available, and consequently, most plagioclase crystals are totally replaced by K-feldspar. But if avenues are not available for the Ca, Na, and Al to escape, then residual proportions of these elements relative to excess Si in the lattice cause the excess Si to recrystallize as either quartz vermicules in myrmekite or as quartz blebs in K-feldspar in ghost myrmekite. In ghost myrmekite most Na and Ca has been displaced by the K, but locally excess Si over Al from the original plagioclase lattice may still remain which cannot fit into the K-feldspar structure.
Myrmekite forms where displaced Ca is trapped between two centers of replacement (K-K or K-Na); e.g., either between two growing K-feldspar crystals or between a growing K-feldspar crystal and a recrystallizing sodic plagioclase crystal. The Ca requires two aluminum atoms in its structure whereas K and Na require only one. So where Ca occurs, excess silica remains to form quartz vermicules or blebs. The plagioclase lacking quartz vermicules is in optical continuity with plagioclase in the myrmekite because plagioclase in both places is recrystallizing from the same former altered crystal.
Because the displacement of Na and Ca by K is never perfect, residual islands remain that can later separate to form albitic perthite lamellae, and thereby, give the appearance that the K-feldspar crystallized from a melt because magmatic K-feldspar crystals are also perthitic. Moreover, in K-feldspar crystals formed by replacement, the albitic perthite lamellae may have a non-uniform distribution.
The preservation of replacement stages at Temecula (Figs. 1-5) is unusual and probably is not observed in most places because the stage showing reversed zoning in the Temecula transition rocks is rare. Undoubtedly, in other rocks, the replacement process normally proceeds rapidly, converting the plagioclase to microcline, and the "reversed zoning" and "holes" are in a nano-environment, moving ahead of the incoming K.
Ghost myrmekite ranges from tiny quartz blebs (barely visible) in microcline where the original plagioclase is relatively sodic, as in granite near Temecula (not shown), in which primary plagioclase averages An30, but bleb sizes are intermediate in Fig. 10 and Fig. 11 where the primary plagioclase is An45-60. Ghost myrmekite also occurs in microcline where the primary plagioclase averages An70-100, but it is no longer a "ghost" because the quartz blebs are so large that generally the K-feldspar-quartz intergrowth would be interpreted to be a graphic texture.
See the following reference for additional photos of myrmekite and ghost myrmekite.
Dr. Lorence G. Collins Department of Geological Sciences California State University Northridge 18111 Nordhoff Street Northridge, California 91330-8266 FAX 818-677-2820