Crystal structure and revised chemical formula for burckhardtite, Pb₂(Fe³⁺Te⁶⁺)[AlSi₃O₈]O₆: A double-sheet silicate with intercalated phyllotellurate layers

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 I > 2σ(I). Burckhardtite is trigonal, space group P31m, with the unit-cell parameters a = 5.2566(5) Å, c = 13.0221(10) Å, V = 311.62(5) ų and Z = 1 for the ideal formula unit Pb₂(Fe³⁺Te⁶⁺)[AlSi₃O₈]O₆. 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⁺)O₆]^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 Pb₂[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.