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

JIAN-JIE LIANG, FRANK C. HAWTHORNE, MILAN NOVÁK* and PETR CERNÝ
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2

LUISA OTTOLINI
CNR Centro di Studio per la Cristallochimica e la Cristallografia, Via Abbiategrasso 209, 27100 Pavia, Italy

* Permanent address: Department of Mineralogy and Petrology, Moravian Museum, Zelný trh 6, 659 37 Brno, Czech Republic.


Abstract

Boromuscovite from the Little Three mine granitic pegmatite, California, occurs only as very fine- grained aggregates, and is a mixture of 2M1 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 Rwp = 13.8% (Rexp = 7.9%), RBRG (2M1) = 3.8%, and RBRG (1M) = 3.8%. Boron is evenly distributed between the two polytypes, and distances correspond well with the B-content at the corresponding T-sites.


Keywords: boromuscovite, Rietveld structure refinement, chemical analysis, polytype, energy minimization.