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}, {01¯0}, {110}, and {1¯ 1¯0}. The mineral is colorless and transparent with a 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 H2O at room temperature. The mineral is optically biaxial (+), α = 1.602(2), β = 1.616(2), γ = 1.634(2) (white light); 2Vmeas = 84(2)°; orientation Y = b, X^a=20° in obtuse β; nonpleochroic. Electron microprobe analysis provided the empirical formula N1.89H7.40S1.97O3. 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) Å3, and Z = 4. The crystal structure (R1 = 0.0325 for 1176 I > 2σI reflections) is the same as that of synthetic (NH4)2(S2O3). Hoperanchite is the first thiosulfate mineral that does not contain essential Pb.