Silvialite, a new sulfate-dominant member of the scapolite group with an Al-Si composition near the 14/m-P4(2)/n phase transition

Silvialite, ideally Ca4Al6Si6O24SO4, is tetragonal, I4/m, Z = 2, with a = 1 2.160(3), c = 7.560(1) Angstrom, V = 1117.9(8) Angstrom(3), c:a = 0.6217:1, omega = 1.583, epsilon = 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 yel low, has a good {100} cleavage, chonchoidal fracture, white streak and a vi treous 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)O-24 [(SO4)(0.57)(CO3)(0.41)] Crystal-stnlcture refi nement shows disordered carbonate and sulfate groups along the fourfold axi s. Silvialite is a primary cumulate phase precipitated from alkali basalt a t 900-1000 degrees C and 8-12 kbar under high f(SO2) and f(O2). The name si lvialite, currently used in literature to describe the sulfate analogue of meionite, was suggested by Brauns (1914).