Myrmekite is 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 is formed under metasomatic conditions, usually in conjunction with tectonic deformations. It has to be clearly separated from micrographic and granophyric intergrowths, which are magmatic. Etymology The word myrmekite is derived from the Ancient Greek μὑρμηχἰα (wart) or μὑρμηξ (ant) and was used by Jakob Sederholm in 1899 for the first time to describe these structures. Myrmekite formed during K-metasomatism During K-metasomatism of plagioclase several different types of myrmekite can appear: rim myrmekite wartlike myrmekite ghost myrmekite Rim myrmekite This is the initial stage of K-metasomatism in cataclastically-deformed magmatic plutonic rocks. The breakage happens primarily along grain boundary seals and the K-metasomatism may locally replace rims of zoned plagioclase crystals to form interstitial alkali feldspar and rim myrmekite (see illustration). Wartlike myrmekite When tectonic strains increase and the cataclasis becomes more intense interior breakage in the crystals ensues and albite-twinned plagioclase crystals are bent. The K-metasomatism therefore can reach deeper into the crystals and increase its effects. Nearly complete to complete replacement of plagioclase takes place and leads to the formation of wartlike myrmekite in places where the replacement was incomplete. The illustration shows tartan-twinned microcline having completely replaced plagioclase. The places with incomplete replacement are taken up by wartlike myrmekite. Gradations occur from rocks containing exclusively rim myrmekite to those containing both rim myrmekite and wartlike myrmekite and finally to those containing exclusively wartlike myrmekite. A very important observation is that the maximum coarseness (tubular diameter) of the quartz vermicules shows a strong correlation with the Ca content of the plagioclase in the original, unreplaced, non-myrmekite-bearing magmatic rock. The coarsest vermicules occur in the metasomatized rock where the original plagioclase was the most calcic. An example for the formation of wartlike myrmekite can be found at the Twentynine Palms, California quartz monzonite which issued from an older, yet undated diorite. Ghost myrmekite This is the third type of quartz-feldspar intergrowth in metasomatic granitoids. Again this process depends on tectonically deformed crystals. In this particular case an irregular subtraction of Ca, Na and Al from deformed plagioclase happens which causes an imbalance in the relative amounts of residual Al and Si. More Si