Mineralogical Magazine, June 1999, Vol. 63(3}, pp. 321~329 Silvialite, a new sulfate-dominant member of the scapolite group with an AI-Si composition near the 14/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 Ca4A16Si6024S04, is tetragonal, 141m, Z = 2, with a = 12.160(3), c = 7.560(]) A, v= I] 17.9(8) A3, c:a = 0.6217: 1, (J) = 1.583, I: = 1.558 (uniaxial negative), Dm = 2.75 g/cm3, Deale = 2.769 g/cm3 and H (Mohs) = 5.5. It is transparent and slightly yellow, has a good {IOO} 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.o6Ca2.86)(AI4.87Si7.13)024 [(S04)057(C03)0.4Il 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 -] 2 kbar under high Is02 and 10,. The name silvialite, currently used in literature to describe the sulfate analogue of meionite, was suggested by Brauns (1914). KEYWORDS: scapolite, meionite, si]vialite, new mineral, sulfate, X-ray data, electron-microprobe data, Queensland, Australia. Introduction MINERALSof the scapolite group have the general formula M4TI202~, with M = Na, Ca, minor K; T = AI, Si and A = CI, C03, S04' Until now, the group consisted of two valid species: marialite (Ma) Na4A13Si9024CI, and meionite (Me) Ca4A16Si6024C03. Early in this century, a significant S04 component was recognized in some samples of meionite, and Brauns (19]4) suggested the name silvialite for the hypothetical S04 analogue of meionite. However, fifty years passed before S04-dominant compositions were reported (Lovering and White, ] 964). Since then, samples with S>0.5 atoms per formula unit (a.p.f.u.) have been noted from a relatively large number of occurrences, but silivia]ite has not yet been accepted as a valid mineral name or defined as a distinct species. Here, we describe silvialite as a primary cumulate phase in mafic xenoliths from McBride Province, North Queensland. The mineral and mineral name have been approved by the Commission on New Minerals and Mineral Names of the International Minera]ogical Association. Ho]otype material is housed in the CCJ1999 The Mineralogical Society Systematic Reference Series of the National Mineral Collection at the Geological Survey of Canada, Ottawa, Ontario, under catalogue number NMC 68080. Silvia/ite in the Ma-Me series Among the rock-forming minerals, the scapolite group has long been of interest for its unusual structural and stoichiometric features. Phase transitions from 141m to P421n and then again to 141m are very rare for silicate minerals, but occur with increasing AI across the series from marialite to meionite (Fig. ]A). In an earlier study, we presented arguments that the structural changes across the series are not caused by substitutions at the M- or A -sites, but are due to three distinct patterns of (AI,Si) order in each part of the series (Teertstra and Sherriff, ] 996). The specific arrangements of Al in the framework must cause significant deviations from the plagioclase substi- tution, which maintain a nearly complete dominance of monovalent and divalent anions at the A-site of maria]ite and meionite (Fig. IB; Teertstra and Sherriff, 1997). In low-symmetry