Polytypism in micas from Black Hill and apparent polytypism in micas from Alto Paranaibo.

Micas are commonly found minerals of igneous and metamorphic rocks which form in a wide range of chemical and physical conditions. This particular feature can be understood on the ground of the compositional variation and different cation arrangement connected with the intensive parameters change during the crystallization process. Recent studies of cation site partitioning in Tirich 1M polytype from Black Hill (Australia) using a multi-technical approach [1] showed the occurrence of Fe, as Fe2+ and Fe3+, and Ti, as Ti3+ and Ti4+. While Titanium populates both octahedral and tetrahedral site, Fe is located only at the octahedral position. In addition Mg and Fe distribution was shown to be shared out in approximately equal amounts across M1 and M2 site. The present study investigates the 2M1 polytype, coexisting with 1M form, from Black Hill. Structural analysis showed similar M1 and M2 cation distribution in both polytypes with some noteworthy differences regarding occupancy of the octahedral cations. More specifically, while Mg and Fe randomly populate both M1 and M2 site in the 1M polytype, these cations are distributed in orderly manner in the 2M1 polytype, resulting in symmetry change from C2/m (1M) to C2/c (2M1) as well as c parameter doubling. This work also investigates an apparent polytype, observed in the trioctahedral micas from Alto Paranaibo igneous province (Southeastern Brazil). Crystal chemistry of 1M polytype from this locality was investigated by [2]. By making use of the multitechnical approach, [3] recently reanalysed one of the sample quoted in [2] and labelled Ma1. A number of interesting new results were found: Na is lodged at octahedral site, Ti at both octahedral and tetrahedral sites and all Fe3+ at tetrahedral site and Fe2+ populating the same octahedral site. Further investigations showed that the Ma1 can also form apparent polytypes, i.e. twinnings of 1M polytype which simulate diffraction of polytype patterns with long periodicity (c ≈ 30 Å). Structure refinement of the twinned crystal (R = 3.9%, carried out on the family reflections) indicated that the twin law is [310], the twin index is 3 and the volume element ratio is roughly 0.85 : 0.09 : 0.06. The same conclusions were reached by [4] when studying ferric phlogopites from Ruiz Peak (Northern Chile).