Crystal structure and revised chemical formula for burckhardtite, Pb-2(Fe3+Te6+)[AlSi3O8]O-6: a double-sheet silicate with intercalated phyllotellurate layers

摘要

The crystal structure of burckhardite from the type locality, Moctezuma, Sonora, Mexico, has been refined to R-1 = 0.0362 and wR(2) = 0.0370 for 215 reflections with l > 2 sigma(l). Burckhardtite is trigonal, space group P (3) over bar 1m, with the unit-cell parameters a = 5.2566(5) angstrom, c = 13.0221(10) angstrom, V = 311.62(5) angstrom(3) and Z = 1 for the ideal formula unit Pb-2(Fe3+Te6+)[AlSi3O8]O-6. There is no long-range order of (Fe3+, Te6+) or (Al3+, Si4+). New microprobe data were used to estimate site scattering factors, and Raman spectroscopic data showed no evidence of O - H stretching bands. Burckhardtite is not closely related to the micas, as supposed previously, but is a double-sheet silicate in which the aluminosilicate anion resembles that of minerals such as cymrite and kampfite. The [(Fe3+Te6+)O-6](3-) part of the structure is not bonded directly to the aluminosilicate layer, but forms a discrete anionic phyllotellurate layer that alternates with the [AlSi3O8](-) double sheets. Similar phyllotellurate layers are known from several synthetic phases. In burckhardtite, Pb2+ cations intercalate between phyllosilicate and phyllotellurate layers, forming a Pb-2[FeTeO6] module that is topologically similar to a slab of the structure of rosiaite, Pb[Sb2O6]. The crystal symmetry, structure, classification as a double-sheet silicate and chemical formula, including the determination of the 6+ valence of Te and absence of essential H2O, are all new findings for the mineral.