Cupromolybdite is a new fumarolic mineral from the New Tolbachik scoria cones of Tolbachik volcano, Kamchatka Peninsula, Russia. Cupromolybdite forms prismatic crystals up to 150 μm in length or acicular crystals with a thickness and length of 1–5 μm and 100 μm, respectively. The acicular crystals of cupromolybdite are typically combined in radiating aggregates. The mineral is associated with piypite, fedotovite, vergasovaite, hematite, magnetite, aphthitalite, langbeinite, palmierite, As-bearing orthoclase, lammerite, klyuchevskite, alumoklyuchevskite, euchlorine, lyonsite, pseudolyonsite, averievite, rutile and native gold. Cupromolybdite is honey-yellow or brown, and its acicular crystals are bright yellow. The mineral has yellow or light brown streak and an adamantine lustre. Cupromolybdite is brittle, and its fracture ranges from uneven to splintery. The calculated density is 4.512 g/cm3. The reflectance values (R1 and R2, %) in air for the four COM wavelengths are 14.95, 16.3 (470 nm), 13.4, 14.85 (546 nm), 12.73, 14.16 (589 nm), and 12.15, 13.5 (650 nm). The chemical composition (the mean of 7 electron microprobe analyses) of cupromolybdite is: CuO 43.03, FeO 0.08, ZnO 0.53, MoO3 54.48, SO3 0.71, V2O5 0.28, SiO2 0.04, total 99.15 wt%, corresponding, on the basis of 9 O atoms, to (Cu2.83Zn0.03Fe0.01)∑2.87(Mo1.98S0.05V0.02)∑2.05O9. The simplified formula of cupromolybdite is Cu3O(MoO4)2 = Cu3Mo2O9. Cupromolybdite is orthorhombic, space group Pnma, a = 7.6638(1), b = 6.8670(1), c = 14.5554(2) Å, V = 766.01(2) Å 3 and Z = 4. The strongest powder-diffraction lines [d in Å (I) (hkl)] are: 7.312 (67) (002); 3.518 (55) (113); 3.436 (100) (020); 3.301 (99) (210, 104); 3.065 (79) (121); 2.556 (62) (220); 2.506 (66) (301, 024). The crystal structure was solved from single-crystal data and refined to R = 0.0314. Cupromolybdite is a natural analogue of the synthetic compound Cu3Mo2O9 and the S-free analogue of vergasovaite, Cu3O[(Mo,S)O4][SO4]. The crystal structure of cupromolybdite contains strings of corner-shared Cu(1)O6 octahedra elongated in the [010] direction. In cupromolybdite, the Cu(2)O5 square pyramids and the Cu(3)O6 distorted octahedra are linked to the Cu(1)O6 octahedra of the strings by edges and to the MoO4 and (Mo,S)O4 tetrahedra by vertices. A minor admixture of S is concentrated in one of two independent tetrahedral positions. Cupromolybdite is named for its chemical composition.

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