Abstract
AbstractSilvialite, ideally Ca4Al6Si6O24SO4, is tetragonal,I4/m, Z = 2, witha= 12.160(3),c= 7.560(1) Å,V= 1117.9(8) Å3,c:a= 0.6217:1, ω = 1.583, ε = 1.558 (uniaxial negative), Dm= 2.75 g/cm3, Dcalc= 2.769 g/cm3and H (Mohs) = 5.5. It is transparent and slightly yellow, has a good {100} cleavage, chonchoidal fracture, white streak and a vitreous lustre. It occurs in upper-mantle garnet-granulite xenoliths hosted by olivine nephelinite, from McBride Province, North Queensland, Australia. The empirical formula, derived from electron-microprobe analysis, is (Na1.06Ca2.86)(Al4.87Si7.13)O24[(SO4)0.57(CO3)0.41]. Crystal-structure refinement shows disordered carbonate and sulfate groups along the fourfold axis. Silvialite is a primary cumulate phase precipitated from alkali basalt at 900–1000°C and 8–12 kbar under highfSO2andfO2. The name silvialite, currently used in literature to describe the sulfate analogue of meionite, was suggested by Brauns (1914).
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