ON THE OCCURRENCE OF METAMORPHIC 2M1 PHENGITES - SOME THOUGHTS ON POLYTYPISM AND CRYSTALLIZATION CONDITIONS OF 3T PHENGITES

Potassic white micas occur in metamorphic rocks as polytype 2M1 and so metimes 3T. Metamorphic phengites typically crystallize under high-pre ssure/low-temperature conditions (occurrence type I), but they are als o expected to be common in rocks of low metamorphic grade which have p articular bulk compositions, specifically low in Al2O3 (occurrence typ e II). Occurrence type I is well known, and polytype 3T is common ther ein; in contrast occurrence type II is only sparsely documented, and n o data exist concerning polytypism therein. The question has been rais ed as to whether the crystallization of 3T polytype is controlled by p ressure or by mica chemistry. The present paper reports the occurrence of metamorphic phengites not related to high-pressure metamorphism bu t clearly due to rock bulk compositional constraints (Kfs-Chl +/- Bt-b earing mineral assemblage). These phengites are very similar in compos ition to those occurring in high pressure terrains. All of them are 2M 1, suggesting that 3T stacking arrangement largely depends on factors other than mica chemistry. The factors which may control the mica poly typism are discussed. A high-pressure/low-temperature environment seem s to be the most necessary condition for the crystallization of 3T phe ngites. These 3T micas are expected to invert to 2M1 during the later tectonometamorphic evolution, due to the effect of the pressure releas e concomitant with exhumation and recovery of the isotherms. However a fully comprehensive crystallochemical understanding of how the physic al conditions facilitate the formation of the 3T polytype remains elus ive.