Elbrusite-(Zr)-A new uranian garnet from the Upper Chegem caldera, Kabardino-Balkaria, Northern Caucasus, Russia

Elbrusite-(Zr) Ca-3(U6+Zr)(Fe-2(3+) Fe2+)O-12, a new uranian garnet (la (3) over bard, a approximate to 12.55 angstrom, V approximate to 1977 angstrom(3), Z = 8), within the complex solid solution elbrusite-kimzeyite-toturite Ca-3(U,Zr,Sn,Ti,Sb,Sc,Nb ... )(2)(Fe,Al,Si,Ti)(3)O-12 was discovered in spurrite zones in skarn xenoliths of the Upper Chegem caldera. The empirical formula of holotype elbrusite-(Zr) with 25.14 wt% UO3 is (Ca3.040Th0.018Y0.001)(Sigma 3.059)(U0.6586+Zr1.040Sn0.230 Hf0.009Mg0.004)(Sigma 1.941) (Fe1.5753+Fe0.5592+Al0.539 Ti0.1994+Si0.099Sn0.025V0.0045+)Sigma O-3(12). Associated minerals are spurrite, rondorfite, wadalite, kimzeyite, perovskite, lakargiite, ellestadite-(OH), hillebrandite, afwillite, hydrocalumite, ettringite group minerals, and hydrogrossular. Elbrusite-(Zr) forms grains up to 10-15 mu m in size with dominant {110} and minor {211} forms. It often occurs as zones and spots within Fe3+-dominant kimzeyite crystals up to 20-30 mu m in size. The mineral is dark-brown to black with a brown streak. The density calculated on the basis of the empirical formula is 4.801 g/cm(3) The following broad bands are observed in the Raman spectra of elbrusite-(Zr): 730, 478, 273, 222, and 135 cm(-1). Elbrusite-(Zr) is radioactive and nearly completely metamict. The calculated cumulative dose (alpha-decay events/mg) of the studied garnets varies from 2.50 x 10(14) [is equivalent to 0.04 displacement per atom (dpa)] for uranian kimzeyite (3.36 wt% UO3), up to 2.05 x 10(15) (0.40 dpa) for elbrusite-(Zr) with 27.09 wt% UO3.