Original paper

Koksharovite, CaMg2Fe3+4(VO4)6, and grigorievite, Cu3Fe3+ 2Al2(VO4)6, two new howardevansite-group minerals from volcanic exhalations

Pekov, Igor V.; Zubkova, Natalia V.; Yapaskurt, Vasiliy O.; Kartashov, Pavel M.; Polekhovsky, Yury S.; Murashko, Mikhail N.; Pushcharovsky, Dmitry Y.

Abstract

Two new howardevansite-group minerals were discovered in the exhalations of fumaroles related to two volcanoes in Kamchatka, Russia. Koksharovite, CaMg2Fe3 + 4(VO4)6, is found at the Bezymyannyi volcano in association with bannermanite. Grigorievite, Cu3Fe3+ 2Al2(VO4)6, associated with bannermanite, ziesite, hematite, etc., was found at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano. Koksharovite occurs as equant to prismatic crystals up to 30 × 70 μm. It is translucent, yellowish-brown to reddish-brown with an adamantine lustre. Grigorievite forms prismatic to tabular crystals up to 40 × 100 μm. It is opaque, black with a semi-metallic lustre. Both minerals are brittle. The VHN hardness is 368 and 489 kg mm–2, the calculated density (Dcalc) 3.39 and 3.67 g cm–3 for koksharovite and grigorievite, respectively. In reflected light, koksharovite is light grey, grigorievite is grey; both minerals are weakly anisotropic. Reflectance values [koksharovite//grigorievite: Rmax–Rmin, % (λ, nm)] are: 15.3–14.4//16.8–16.4 (470), 14.1–13.2//15.9–15.5 (546), 13.8–13.0//15.3–14.9 (589), 13.4–12.7//14.8–14.1 (650). Chemical data (wt%, electron-microprobe analysis; first value is for koksharovite, second for grigorievite) are: Na2O 0.76, 0.00; K2O 0.05, 0.00; MgO 9.43, 2.78; CaO 3.57, 0.95; MnO 0.46, 0.04; CuO 0.00, 17.70; NiO 0.11, 0.00; ZnO 0.00, 0.14; Al2O3 3.04, 11.76; Fe2O3 23.88, 10.10; TiO2 2.42, 1.47; SiO2 0.20, 0.00; P2O5 0.98, 0.13; V2O5 53.86, 54.97; total 98.76, 100.04. The empirical formulae, based on 24 O atoms per formula unit, are: Na0.24K0.01Ca0.63Mg2.30Mn0.06Ni0.01Al0.59Fe3+ 2.94Ti0.30Si0.03P0.14V5.83O24 (koksharovite); Ca0.17Mg0.69Mn0.01Cu2.23Zn0.02Al2.31Fe3+ 1.27Ti0.18P0.02V6.05O24 (grigorievite). Both minerals are triclinic, space group P-1, Z = 1. Unit-cell dimensions are: a 8.1758(7), b 9.8292(9), c 6.6940(6) Å, α 105.041(8), β 102.040(8), γ 106.025(8) , V 476.02(8) Å3 for koksharovite; a 8.0217(5), b 9.6858(10), c 6.5475(9) Å, α 103.645(10), β 102.369(8), γ 106.281(8) , V 452.60(8) Å3 for grigorievite. The strongest reflections of the X-ray powder patterns [d,Å (I)(hkl)] are: 7.47(28)(100), 3.75(44)(200, –201), 3.26(27)(0–12), 3.17(100)(–130), 3.09(94)(–221, 210), 2.039(28)(–3–21, –340) for koksharovite; 7.36(24)(100), 4.718(29)(–1–11, –111), 3.671(26)(200, –201), 3.141(100)(–130), 3.044(92)(210), 2.811(26)(–1–22) for grigorievite. The crystal structure of both minerals [single-crystal X-ray diffraction data, R = 0.079 (koksharovite) and 0.055 (grigorievite)] is based on the pseudo-frameworks built by VO4 tetrahedra, MIO6 octahedra [species-defining MI = Fe3+, Al] and MIIO5 polyhedra [MII = (Cu, Mg) in grigorievite and (Mg, Fe) in koksharovite]. The Ca (koksharovite) and Cu(2) sites (grigorievite) are located in tunnels. The new minerals (IMA 2012–092 and 2012–047, respectiveley) are named in honour of outstanding Russian mineralogists, Nikolay Ivanovich Koksharov (1818–1892) and Dmitry Pavlovich Grigoriev (1909–2003).

Keywords

bezymyannyi volcanocrystal structurefumarole sublimategrigorievitehowardevansite groupkamchatkakoksharovitenew mineraltolbachik volcanovanadate