Silvialite, a New Sulfate-Dominant Member of the Scapolite Group with an Al-Si Composition Near the l4/m-p42/n Phase Transition

D. K. Teertstra, M. Schindler, B. L. Sherriff and F. C. Hawthorne
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2

Abstract: Silvialite, ideally Ca4Al6Si6O24SO4, is tetragonal, I4/m, Z = 2, with a = 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, Dcale 2.769 g/cm3 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 (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 high fSO2 andfO2. The name silvialite, currently used in literature to describe the sulfate analogue of meionite, was suggested by Brauns (1914).

Keywords: scapolite • meionite • silvialite • new mineral • sulfate • X-ray data • electron-microprobe data • Queensland • Australia

Mineralogical Magazine; June 1999 v. 63; no. 3; p. 321-329; DOI: 10.1180/002646199548547
© 1999, The Mineralogical Society
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