Lead-tellurium oxysalts from Otto Mountain near Baker, California: VII. Chromschieffelinite, Pb₁₀Te₆O₂₀(OH)₁₄(CrO₄)(H₂O)₅, the chromate analog of schieffelinite

Abstract

Chromschieffelinite, Pb₁₀Te₆O₂₀(OH)₁₄(CrO₄)(H₂O)₅, is a new tellurate from Otto Mountain near Baker, California, named as the chromate analog of schieffelinite, Pb₁₀Te₆O₂₀(OH)₁₄(SO₄)(H₂O)₅. The new mineral occurs in a single 1 mm vug in a quartz vein. Associated mineral species include: chalcopyrite, chrysocolla, galena, goethite, hematite, khinite, pyrite, and wulfenite. Chromschieffelinite is orthorhombic, space group C222₁, a = 9.6646(3), b = 19.4962(8), c = 10.5101(7) Å, V = 1980.33(17) ų, and Z = 2. Crystals are blocky to tabular on {010} with striations parallel to [001]. The forms observed are {010}, {210}, {120}, {150}, {180}, {212}, and {101}, and crystals reach 0.2 mm in maximum dimension. The color and streak are pale yellow and the luster is adamantine. The Mohs hardness is estimated at 2. The new mineral is brittle with irregular fracture and one perfect cleavage on {010}. The calculated density based on the ideal formula is 5.892 g/cm³. Chromschieffelinite is biaxial (-) with indices of refraction α = 1.930(5), β = 1.960(5), and γ = 1.975(5), measured in white light. The measured 2V is 68(2)°, the dispersion is strong, r < v, and the optical orientation is X = b, Y = c, Z = a. No pleochroism was observed. Electron microprobe analysis provided: PbO 59.42, TeO₃ 29.08, CrO₃ 1.86, H₂O 6.63 (structure), total 96.99 wt%; the empirical formula (based on 6 Te) is Pb_9.65Te₆O_19.96(OH)_14.04(CrO₄)_0.67(H₂O)_6.32. The strongest powder X-ray diffraction lines are [d_obs in Å (hkl) I]: 9.814 (020) 100, 3.575 (042,202) 41, 3.347 (222) 44, 3.262 (241,060,113) 53, 3.052 (311) 45, 2.9455 (152,133) 55, 2.0396 (115,353) 33, and 1.6500 (multiple) 33. The crystal structures of schieffelinite (R₁ = 0.0282) and chromschieffelinite (R₁ = 0.0277) contain isolated Te⁶⁺O₆ octahedra and Te₂⁶⁺O₁₁ corner-sharing dimers, which are linked into a three-dimensional framework via bonds to Pb²⁺ atoms. The framework has large channels along c, which contain disordered SO₄ or CrO₄ groups and H₂O. The lone-electron pair of each Pb²⁺ is stereochemically active, resulting in one-sided Pb-O coordination arrangements. The short Pb-O bonds of the Pb²⁺ coordinations are all to Te⁶⁺O₆ octahedra, resulting in strongly bonded layers parallel to {010}, which accounts for the perfect {010} cleavage.