MONTBRAYITE, (Au,Ag,Sb,Pb,Bi)23(Te,Sb,Pb,Bi)38, FROM THE ROBB-MONTBRAY MINE, MONTBRAY, QUEBEC: CRYSTAL STRUCTURE AND REVISION OF THE CHEMICAL FORMULA

The crystal structure of the mineral montbrayite, a rare gold telluride, was solved using intensity data collected from a crystal of the co-type material from the Robb-Montbray mine, Montbray, Abitibi County, Quebec, Canada. The study revealed that the structure is triclinic, space group P (1) over bar, with cell parameters: a 10.8045(6), b 12.1470(6), c 13.4480(7) angstrom, alpha 108.091(5), beta 104.362(5), gamma 97.471(5)degrees, and V 1583.65(15) angstrom(3). The refinement of an anisotropic model led to an R index of 6.36% for 5145 independent reflections with F-o > 4 sigma(F-o), GooF 1.023. There are 19 Te sites and 12 Au sites in the crystal structure of montbrayite. The Au sites host variable amounts of Sb, whereas all the Te sites but one host only Te. All the Au atoms can be considered to be in octahedral coordination forming more regular AuTe6 or more distorted AuTe4+2 polyhedra, with the exception of one Au position which links five Te atoms and closes its coordination sphere with a contact with itself. Crystal-chemical features of the Au and Te atoms are discussed in relation to other gold and silver tellurides. Electron microprobe analyses of the crystal used for the structural study led to the formula Au21.60Sb0.68Te37.41Bi1.31, calculated on the basis of 61 atoms per formula unit. On the basis of information gained from the structural and chemical characterization, the crystal-chemical formula was revised, yielding (Au,Ag,Sb,Bi,Pb)(23)(Te,Sb,Bi,Pb)(38) (Z = 1) instead of (Au,Sb)(2)Te-3 (Z = 12) as previously reported. The redefinition of montbrayite has been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (voting proposal 17-F).