Myrmekite

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Myrmekite describes a vermicular, or wormy, intergrowth of quartz in plagioclase. The intergrowths are microscopic in scale, typically with maximum dimensions less than 1 millimeter. The plagioclase is sodium-rich, usually albite or oligoclase. These quartz-plagioclase intergrowths are associated with and commonly in contact with potassium feldspar.

Myrmekite typically occurs in granite and similar igneous rocks and in metamorphic gneiss similar to granite in composition.

These characteristic intergrowths have been explained in a variety of ways. A probable explanation is that the texture was created as plagioclase formed by exsolution from alkali feldspar during cooling, under conditions in which silicon was mobile in the rock. The process does not require that magma be present during myrmekite formation.

[edit] References

Castle, R. O., and Lindsley, D. H., 1993, An exsolution silica-pump model for the origin of myrmekite. Contributions to Mineralogy and Petrology, v. 115, pages 58-65.