What drives the distribution in nature of 3T vs. 2M1 polytype in muscovites and phengites? A general assessment based on new data from metamorphic and igneous granitoid rocks.

Petrologic, chemical, and polytype data are presented for dioctahedral potassic micas from K-feldspar-bearing metamorphic and igneous rocks of acidic composition unaffected by high-pressure (HP) conditions. The paper aims to demonstrate that: (1) under non-HP conditions, in both metamorphic and igneous plutonic environments, a given bulk-rock compositional constraint imposes a more or less marked phengitic composition to dioctahedral potassic mica; and (2) this muscovite crystallizes as 2M1, notwithstanding its phengitic composition. The samples (157 in number) are from widespread provenances. We conclude that the growth of 3T polytype of muscovite is not a function of mica composition. This is consistent with the recent crystallographic knowledge on polytypism, cation ordering, elastic properties, and structural deformational mechanisms of muscovite, which address the stabilization of 3T with pressure.