Myrmekite
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.
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.
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.
Examples for this structure are found in California in the Mount Rubidoux leucogranite and in granodiorites in the Sierra Nevada.
Here the influx of Na-rich fluids in the temperature range 450 °C to 650 °C from the host leads to the replacement of alkali feldspar by myrmekite within the enclaves.
These vermicules are confined and scattered entirely within the interior of the plagioclase forming irregular spindles, arcuate patterns and ovals.
For this process to operate it is important that Ca is sufficiently present so that a fairly calcic plagioclase can be formed which in turn releases enough silica for the myrmekite vermicules.
An example can be found in the Lyon Mountain granite gneiss north of Ausable Forks in New York.
During progressive deformation in mylonitic, ductile shear zones myrmekite is commonly concentrated in shortening quarters in the rim of sigmoidal K-feldspar crystals.
Yet Lorence G. Collins does not agree with the assumption of the K-feldspar being primary magmatic and the myrmekite being formed due to deformation-induced Na-Ca-metasomatism.
His sampling beyond the shear zone revealed an undeformed, felsic biotite diorite whose primary plagioclase was being replaced from the inside out by K-feldspar due to K-metasomatism.
The deformations were therefore more or less continuous and had not only affected the shear zone but also the older plutonic country rocks, thence bringing about a metasomatic change in mineralogy.
