Tombstoneite, a new mineral from Tombstone, Arizona, USA, with a pinwheel-like Te⁶⁺O₃(Te⁴⁺O₃)₃ cluster

Abstract

The new mineral tombstoneite (IMA2021-053), (Ca₀.₅Pb₀.₅)Pb₃Cu²⁺₆Te⁶⁺₂O₆(Te⁴⁺O₃)₆(Se⁴⁺O₃)₂(SO₄)₂⋅3H₂O, occurs at the Grand Central mine in the Tombstone district, Cochise County, Arizona, USA, in cavities in quartz matrix in association with jarosite and rodalquilarite. Tombstoneite crystals are green pseudohexagonal tablets, up to 100 μm across and 20 μm thick. The mineral has a pale green streak and adamantine lustre. It is brittle with irregular fracture and a Mohs hardness of ~2½. It has one perfect cleavage on {001}. The calculated density is 5.680 g cm⁻³. Optically, the mineral is uniaxial (–) and exhibits pleochroism: O = green, E = light yellow green; O > E. The Raman spectrum exhibits bands consistent with Te⁶⁺O₆, Te⁴⁺O₃, Se⁴⁺O₃ and SO₄. Electron microprobe analysis provided the empirical formula (Ca₀.₅₁Pb₀.₄₉)_(Σ1.00)Pb₃.₀₀Cu²⁺₅.₈₅Te⁶⁺₂.₀₀O₆(Te⁴⁺₁.₀₀O₃)₆(Se⁴⁺₀.₆₉Te⁴⁺₀.₂₄S₀.₀₇O₃)₂(S₁.₀₀O₄)₂⋅3H₂O. Tombstoneite is trigonal, P321, a = 9.1377(9), c = 12.2797(9) Å, V = 887.96(18) ų and Z = 1. The structure of tombstoneite (R₁ = 0.0432 for 1205 I > 2σI) contains thick heteropolyhedral layers comprising Te⁶⁺O₆ octahedra, Jahn-Teller distorted Cu²⁺O₅ pyramids, Te⁴⁺O₃ pyramids and Se⁴⁺O₃ pyramids. Pb²⁺ cations without stereoactive 6s² lone-pair electrons are hosted in pockets in the heteropolyhedral layer. Pb²⁺ cations, possibly with stereoactive 6s² lone-pair electrons, are located in the interlayer region along with SO₄ tetrahedra and H₂O groups. Within the heteropolyhedral layer, the Te⁶⁺O₆ octahedra and the Te⁴⁺O₃ pyramids form finite Te⁶⁺O₃(Te⁴⁺O₃)₃ clusters with a pinwheel-like configuration. This is the first known finite complex including both Te⁴⁺ and Te⁶⁺ polyhedra in any natural or synthetic tellurium oxysalt structure.

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