Abstract
The polymorphism (2M1 or 3T modification) of phengites coexisting with Jadeite+quartz+K-feldspar in gneisses of granitic composition from the internal part of the Sesia Zone correlates with their Mg/(Al+Fe+Mg) ratio. For values ≲0.015 the 2M1 modification, for values ≳0.025 the 3T modification is formed. For Mg/(Al+Fe+Mg) values between these limits both polymorphs coexist as distinct grains; less than 1% of a large number of separated grains show an intergrowth of both modifications. In these cases the predominant phase boundaries are (hkO) planes, rational at least with respect to the 2M1 lattice; boundaries parallel (001) occur as subordinate “steps” in (hkO) boundaries. Extensive intergrowth parallel (001) is not observed. As far as revealed by optical microscopy, the stacking sequence once established in a nucleus is retained during growth of the crystal.
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