Te-rich raspite, Pb(W0.56Te0.44)O4, from Tombstone, Arizona, U.S.A.: The first natural example of Te6+ substitution for W6+

Abstract

Te-rich raspite, Pb(W0.56Te 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 Te 6+ substitution for W 6+ . 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) A, b = 107.979(5)°, and V = 363.2(2) A 3 . 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 Pb 2+ cations. In addition, a refinement of the regular raspite structure with measured chemistry Pb1.00W1.00O4, P21/a symmetry, and unit-cell parameters a = 13.5773(8), b = 4.9806(3), c = 5.5670(3) A, b = 107.658(3)°, and V = 358.72(4) A 3 is presented. Compared with regular raspite (PbWO4), the partial substitution of the small radius Te 6+ for larger W 6+ 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.