in the enclaves takes place. As a consequence Ca gets released in plagioclase which in turn can now react upon K-feldspar to form myrmekite. Basically this process is very similar to the Ca-metasomatism on K-feldspar described above except for the Na-fluids acting as a trigger. An example is the Velay granite in the northeastern Massif Central in France. In the second variety Na- and Ca-bearing fluids truly act together. This leads via the replacement of primary K-feldspar (perthitic and non-perthitic microcline) to the formation of plagioclase (albite or oligoclase) and in certain places also to the formation of myrmekite. The myrmekite does not show wartlike tapering vermicules, but vermicules that are nearly constant in size because the host plagioclase containing the quartz vermicules has nearly a constant Na/Ca composition. 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. If only Na is present then no myrmekite will form. An example can be found in the Lyon Mountain granite gneiss north of Ausable Forks in New York. Myrmekite formed during progressive deformation During progressive deformation in mylonitic, ductile shear zones myrmekite is commonly concentrated in shortening quarters in the rim of sigmoidal K-feldspar crystals. Simpson and Wintsch (1989) explain the asymmetric distribution of myrmekite by a preferential proceeding of the K-feldspar breakdown reaction at sites of high differential stress (stress-concentration sites) during retrograde metamorphism. Internally the arrangement of the quartz vermicules in the myrmekites also shows a monoclinic symmetry, which independently can serve as an internal shear sense indicator. Asymmetric myrmekite is therefore a quarter structure. 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. Occurrence Myrmekite can appear in many different rock types and different geologic settings. Typically it occurs in granites and