Silvialite, a new sulfate-dominant member of the scapolite group with an Al-Si composition near the I4/m−P42/n phase transition

Silvialite, ideally Ca Al Si SO , is tetragonal, I4/m, Z = 2, with a = 12.160(3), c = 7.560(1) Å, V = 1117.9(8) Å , c:a = 0.6217:1, ω= 1.583, Ɛ= 1.558 (uniaxial negative), D = 2.75 g/cm , D = 2.769 g/cm 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 Ca )(Al Si )O [(SO )0.57(CO ]. 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 and f . The name silvialite, currently used in literature to describe the sulfate analogue of meionite, was suggested by Brauns (1914).