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

Petrol., chem., and polytype data are presented for dioctahedral potassic micas from K-feldspar-bearing metamorphic and igneous rocks of acidic compn. 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 compn. to dioctahedral potassic mica; and (2) this muscovite crystallizes as 2M1, notwithstanding its phengitic compn. The samples (157 in no.) are from widespread provenances. We conclude that the growth of 3T polytype of muscovite is not a function of mica compn. This is consistent with the recent crystallog. knowledge on polytypism, cation ordering, elastic properties, and structural deformational mechanisms of muscovite, which address the stabilization of 3T with pressure.