Original paper

On the occurrence of metamorphic 2M1 phengites: some thoughts on polytypism and crystallization conditions of 3T phengites

Sassi, Francesco P.; Guidotti, Charles V.; Rieder, Milan; Pieri, Renzo De

European Journal of Mineralogy Volume 6 Number 1 (1994), p. 151 - 160

35 references

published: Feb 4, 1994
manuscript accepted: Nov 17, 1993
manuscript received: Aug 10, 1993

DOI: 10.1127/ejm/6/1/0151

BibTeX file

ArtNo. ESP147050601015, Price: 29.00 €

Download preview PDF Buy as PDF


Abstract Potassic white micas occur in metamorphic rocks as polytype 2M1 and sometimes 3T. Metamorphic phengites typically crystallize under high-pressure/low-temperature conditions (occurrence type I), but they are also expected to be common in rocks of low metamorphic grade which have particular bulk compositions, specifically low in Al2O3 (occurrence type II). Occurrence type I is well known, and polytype 3T is common therein; in contrast occurrence type II is only sparsely documented, and no data exist concerning polytypism therein. The question has been raised as to whether the crystallization of 3T polytype is controlled by pressure or by mica chemistry. The present paper reports the occurrence of metamorphic phengites not related to high-pressure metamorphism but clearly due to rock bulk compositional constraints (Kfs-Chl ┬▒ Bt-bearing mineral assemblage). These phengites are very similar in composition to those occurring in high pressure terrains. All of them are 2M1, suggesting that 3 T stacking arrangement largely depends on factors other than mica chemistry. The factors which may control the mica polytypism are discussed. A high-pressure/low-temperature environment seems to be the most necessary condition for the crystallization of 3 T phengites. These 3 T micas are expected to invert to 2M1 during the later tectonometamorphic evolution, due to the effect of the pressure release concomitant with exhumation and recovery of the isotherms. However a fully comprehensive crystallochemical understanding of how the physical conditions facilitate the formation of the 3 T polytype remains elusive


muscovitephengitepolytypismmetamorphic occurrences