Te-rich raspite, Pb(W_0.56Te_0.44)O₄, from Tombstone, Arizona, U.S.A. The first natural example of Te⁶⁺ substitution for W⁶⁺

Te-rich raspite, Pb(W_0.56Te_0.44)O₄, from the Grand Central mine, Tombstone, Arizona, U.S.A., was studied with single-crystal X-ray diffraction, Raman spectroscopy, and electron microprobe analysis. The mineral represents the first natural example of Te₆₊ substitution for W₆₊. It displays monoclinic symmetry with space group P2₁/a and unit-cell parameters a = 13.621(3), b = 5.019(1), c = 5.586(1) Å, β = 107.979(5)°, and V = 363.2(2) ų. Its structure consists of distorted MO₆ (M = W + Te) octahedra sharing edges to form zigzag chains running parallel to [010]. These octahedral chains are linked together by seven-coordinated Pb²⁺ cations. In addition, a refinement of the regular raspite structure with measured chemistry Pb_1.00W_1.00O₄, P2₁/a symmetry, and unit-cell parameters a = 13.5773(8), b = 4.9806(3), c = 5.5670(3) Å, β = 107.658(3)°, and V = 358.72(4) ų is presented. Compared with regular raspite (PbWO₄), the partial substitution of the small radius Te⁶⁺ for larger W⁶⁺ results in a decrease in the MO₆ octahedral distortion, with a concomitant increase in the MO₆ octahedral volume and the average Pb-O bond length. In addition, as should be expected for mixed occupancy compounds, most Raman bands for the mixed Te-rich raspite are broader than the corresponding ones for the end-member regular raspite. High-temperature annealing experiments reveal that Te-rich raspite transforms irreversibly to the stolzite structure at 590(10) °C, which is considerably higher than the reported transformation temperature of 395(5) °C for regular raspite.