Natromolybdite, Na₂MoO₄·2H₂O, a new mineral from fumarole deposits of the Tolbachik volcano, Kamchatka, Russia

Abstract The new mineral natromolybdite, ideally Na 2 MoO 4 ·2H 2 O, was found in the Arsenatnaya fumarole, Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. The associated minerals are halite, sylvite, aphthitalite, belomarinaite, powellite, hematite, sanidine, tilasite, johillerite, bradaczekite, badalovite, arsmirandite, wrightite, arsenatrotitanite, dmisokolovite, litidionite, rutile and cristobalite. Natromolybdite occurs as rectangular, octagonal or rhomb-like lamellar to thin-tabular crystals up to 40 μm across and up to 3 μm thick, in near-parallel, pile-like or rose-like aggregates and crystal crusts up to 0.2 mm across. It is transparent and colourless, with vitreous lustre. D calc is 2.573 g/cm 3 . The synthetic analogue of natromolybdite is optically biaxial (+), α = 1.575(2), β = 1.576(2), γ = 1.598(3) and 2V meas = 20(10)°. The chemical composition (wt.%, electron microprobe, H 2 O is calculated by stoichiometry) is: Na 2 O 25.51, K 2 O 0.66, SO 3 1.04, MoO 3 58.21, H 2 O calc 15.05, total 100.47. The empirical formula, calculated based on O = 6 apfu, is (Na 1.971 K 0.034 ) Σ2.005 (Mo 0.968 S 0.031 ) Σ0.999 O 4 ·2H 2 O. Natromolybdite is orthorhombic, space group Pbca, a = 8.483(1), b = 10.577(2), c = 13.842(2) Å, V = 1242.0(2) Å 3 and Z = 8. The nine strongest reflections of the powder XRD pattern [ d ,Å( I )( hkl )] are: 6.92(100)(002), 4.243(20)(200), 4.206(32)(022), 3.618(31)(202), 3.310(31)(220), 3.169(49)(131), 3.067(21)(114), 2.987(30)(222) and 2.681(15)(204, 311). Natromolybdite (IMA-accepted symbol Nmyb) is named for the chemical composition. It is a natural analogue of a well-studied synthetic sodium molybdate dihydrate.