Hoperanchite, (NH₄)₂(S₂O₃), a new mineral from an active vent in a burning bituminous shale

Abstract

Hoperanchite (IMA2024-017), (NH₄)₂(S₂O₃), is a newly approved mineral species from an active vent in a burning bituminous shale at Hope Ranch, Santa Barbara County, California, U.S.A. Hoperanchite occurs as tabular crystals up to about 0.3 mm in diameter. Tablets are flattened on {001} and exhibit the forms {100}, {010}, {0-10}, {110}, and {-1-10}. The mineral is colorless and transparent with vitreous luster and white streak. The Mohs hardness is ~2½. The mineral has brittle tenacity, irregular fracture, and one good cleavage on {001}. The measured density is 1.68(2) g·cm⁻³. The mineral dissolves instantly in room temperature H₂O. The mineral is optically biaxial (+), α = 1.602(2), β = 1.616(2), γ = 1.634(2) (white light); 2V_meas = 84(2)°; orientation Y = b, X ^ a = 20° in obtuse β; nonpleochroic. Electron microprobe analysis provided the empirical formula N_1.89H_7.40S_1.97O₃. Hoperanchite is monoclinic, C2, a = 10.2313(5), b = 6.4998(3), c = 8.8098(6) Å, β = 94.611(7)°, V = 583.97(6) ų, and Z = 4. The crystal structure (R₁ = 0.0325 for 1176 I > 2σ_I reflections) is the same as that of synthetic (NH₄)₂(S₂O₃). Hoperanchite is the first thiosulfate mineral that does not contain essential Pb.

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