Elbrusite-(Zr); a new uranian garnet from the Upper Chegem Caldera, Kabardino-Balkaria, Northern Caucasus, Russia

Elbrusite-(Zr) Ca (sub 3) (U (super 6+) Zr)(Fe (super 3+) (sub 2) Fe (super 2+) )O (sub 12) , a new uranian garnet (Ia<ovl>3</ovl>d, a nearly equal 12.55 Aa, V nearly equal 1977 Aa (super 3) , Z=8), within the complex solid solution elbrusite-kimzeyite-toturite Ca (sub 3) (U,Zr,Sn,Ti,Sb,Sc,Nb...) (sub 2) (Fe,Al,Si,Ti) (sub 3) O (sub 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% UO (sub 3) is (Ca (sub 3.040) Th (sub 0.018) Y (sub 0.001) ) (sub Sigma 3.059) (U (super 6+) (sub 0.658) Zr (sub 1.040) Sn (sub 0.230) Hf (sub 0.009) Mg (sub 0.004) ) (sub Sigma 1.941) (Fe (super 3+) (sub 1.575) Fe (super 2+) (sub 0.559) Al (sub 0.539) Ti (super 4+) (sub 0.199) Si (sub 0.099) Sn (sub 0.025) V (super 5+) (sub 0.004) ) (sub Sigma 3) O (sub 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 Fe (super 3+) -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 (super 3) The following broad bands are observed in the Raman spectra of elbrusite-(Zr): 730, 478, 273, 222, and 135 cm (super -1) . Elbrusite-(Zr) is radioactive and nearly completely metamict. The calculated cumulative dose (alpha -decay events/mg) of the studied garnets varies from 2.50X10 (super 14) [is equivalent to 0.04 displacement per atom (dpa)] for uranian kimzeyite (3.36 wt% UO (sub 3) ), up to 2.05X10 (super 15) (0.40 dpa) for elbrusite-(Zr) with 27.09 wt% UO (sub 3) .