"White" micas: indicators of P, T, X conditions in the metamorphic realm

Annibale Mottana (a)
(a) Accademia Nazionale delle Scienze, detta dei XL, Via Spallanzani 7, 00185, Rome, Italy. Corresponding e mail: annibale.mottana@uniroma3.it

"White" micas have been extensively studied to understand the relationships between their crystal chemistry and genesis, and hence, to relate their crystal chemical and structural aspects (polytype, ordering state etc.) to petrologically important intensive parameters (P and T) and activities of H2O and other volatiles of a given rock. Sassi and coworkers pioneered their studies on "petrologic mineralogy" using an innovative approach: these micas and the rocks containing them are chemical systems that need to be studied in their context, and not in isolation. In particular they correlated metamorphic P and (Mg+Fe) content to the b0 parameter. They also established a correlation between polytypes and the petrogenetic conditions in force during crystallization. (i) HP conditions favor cation ordering, stabilizing the 3T polytype. (ii) During decompression and heating related to exhumation of HP terrains, the first-formed, HP-related, 3T phengites recrystallize as 2M1 and trend to muscovite composition. A detailed crystal structure determination would only give a more complete picture of the influence of crystallization conditions on the phengite crystal chemistry, while EMP chemical analysis, unless complemented by SIMP on light atoms, would often give misleading indications as certain atoms can be determined only with large uncertainty. A small number of well-characterized examples of such "white" micas gives support to the above-given conclusions.

celadonite, metapelite, muscovite, paragonite, phengite.