Myrmekite

Myrmekite describes a , 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.

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.

During K-metasomatism of plagioclase several different types of myrmekite can appear:

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).

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.

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