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Te-rich raspite, Pb(W 0.56 Te 0.44 )O 4 , 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 Te 6+  substitution for W 6+ . It displays monoclinic symmetry with space group P2 1 /a and unit-cell parameters a = 13.621(3), b = 5.019(1), c = 5.586(1) Å, b = 107.979(5)°, and V = 363.2(2) Å 3 . Its structure consists of distorted MO 6  (M = W + Te) octahedra sharing edges to form zigzag chains running parallel to [010]. These octahedral chains are linked together by seven-coordinated Pb 2+  cations. In addition, a refinement of the regular raspite structure with measured chemistry Pb 1.00 W 1.00 O 4 , P2 1 /a symmetry, and unit-cell parameters a = 13.5773(8), b = 4.9806(3), c = 5.5670(3) Å, b = 107.658(3)°, and V = 358.72(4) Å 3  is presented. Compared with regular raspite (PbWO 4 ), the partial substitution of the small radius Te 6+  for larger W 6+  results in a decrease in the MO 6  octahedral distortion, with a concomitant increase in the MO 6  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.

Te-rich raspite, Pb(W 0.56 Te 0.44 )O 4 , from Tombstone, Arizona, U.S.A.: The first natural example of Te 6+ substitution for W 6+