Silvialite, a new sulfate-dominant member of the scapolite group with an Al-Si composition near the 14/m–P4₂/nphase transition

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Silvialite, ideally Ca 4 Al 6 Si 6 O 24 SO 4 , is tetragonal, I 4/ m , Z = 2, with a = 12.160(3), c = 7.560(1) Å, V = 1117.9(8) Å 3 , c:a = 0.6217:1, ω = 1.583, ε = 1.558 (uniaxial negative), D m = 2.75 g/cm 3 , D calc = 2.769 g/cm 3 and 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 (Na 1.06 Ca 2.86 )(Al 4.87 Si 7.13 )O 24 [(SO 4 ) 0.57 (CO 3 ) 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 high f SO 2 and f O 2 . The name silvialite, currently used in literature to describe the sulfate analogue of meionite, was suggested by Brauns (1914).