CRYSTAL-STRUCTURE REFINEMENT OF BOROMUSCOVITE POLYTYPES USING A COUPLED RIETVELD STATIC-STRUCTURE ENERGY-MINIMIZATION METHOD

Boromuscovite from a granitic pegmatite at Recice, Czech Republic, occurs only as very fine-grained aggregates, and is a mixture of 2M(1) and 1M polytypes. Crystal structures of both polytypes have been simultaneously refined in space groups C2/c and C2/m, respectively, using a coupled Rietveld - static-structure energy-minimization method. Intensity data were collected on a high-resolution powder diffractometer in transmission geometry, with the sample prepared such that preferred orientation is largely avoided. The refinement converged to R(wp) = 13.8% (R(exp) = = 7.9%), R(BRG) (2M(1)) = 3.8%, and R(BRG) (1M) = 3.8%. Boron is evenly distributed between the two polytypes, and distances correspond well with the B-content at the corresponding T-sites.