Spurrite

Abstract: Elbrusite-(Zr) Ca 3 (U 6+ Zr)(Fe 3+ 2 Fe 2+ )O 12 , a new uranian garnet ( Ia 3 d , a ≈ 12.55 A, V ≈ 1977 A 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% UO 3 is (Ca 3.040 Th 0.018 Y 0.001 ) ∑3.059 (U 6+ 0.658 Zr 1.040 Sn 0.230 Hf 0.009 Mg 0.004 ) ∑1.941 (Fe 3+ 1.575 Fe 2+ 0.559 Al 0.539 Ti 4+ 0.199 Si 0.099 Sn 0.025 V 5+ 0.004 ) ∑3 O 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 μm in size with dominant {110} and minor {211} forms. It often occurs as zones and spots within Fe 3+ -dominant kimzeyite crystals up to 20–30 μm 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 (α-decay events/mg) of the studied garnets varies from 2.50 × 10 14 [is equivalent to 0.04 displacement per atom (dpa)] for uranian kimzeyite (3.36 wt% UO 3 ), up to 2.05 × 10 15 (0.40 dpa) for elbrusite-(Zr) with 27.09 wt% UO 3 .

Abstract: Mixtures of crystalline CaCO_3, Ca_2SiO_4, CaSiO_3, quartz, and spurrite were reacted between 7 and 27 kbar. The results, combined with other published data, provide a PT projection for the system CaO-SiO_2-CO_2 from 1 bar to 30 kbar, and a series of isobaric liquidus diagrams giving the changes in composition of eutectic and peritectic liquids in the univariant reactions as a function of pressure. The assemblage calcite + quartz dissociates producing wollastonite + CO_2 at pressures below an invariant point at 18.5 kbar, 1,325°C; at this point, the univariant dissociation reaction meets the fusion curve for wollastonite + CO_2 = quartz + liquid; at higher pressures, calcite + quartz melts incongruently to liquid + CO_2, and there is in addition a eutectic reaction between calcite, wollastonite, and quartz. The thermal barrier on the liquidus associated with the congruent melting of lamite in the system CaO-SiO_2 is eliminated by solution of a few percent CO_2 at pressures greater than about 1 kbar; the CO_2 causes expansion of the liquidus fields for calcite and wollastonite until they meet and exclude both spurrite and lamite from the CO_2-saturated liquidus field boundary. The liquidus diagrams show limiting conditions for coprecipitation of calcite and wollastonite in carbonatite magmas. Liquids produced by partial melting of siliceous limestones (±wollastonite) at pressures above about 15 kbar have compositions near 50% CaCO_3, 50% CaSiO_3. There is a good prospect that some subducted pelagic limestone might escape dissociation and melting and be carried to considerable depths for long-term storage of carbon in the mantle either as aragonite (reacting to dolomite or magnesite), or as diamond if the carbonate is reduced.