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Silvialite, a new sulfate-dominant member of the scapolite group with an Al-Si composition near the 14/mP42/n phase transition

Published online by Cambridge University Press:  05 July 2018

D. K. Teertstra
Affiliation:
Departmenl of Geological Sciences, University of Manitoba, Winnipeg, Maniloba, Canada R3T 2N2
M. Schindler
Affiliation:
Departmenl of Geological Sciences, University of Manitoba, Winnipeg, Maniloba, Canada R3T 2N2
B. L. Sherriff
Affiliation:
Departmenl of Geological Sciences, University of Manitoba, Winnipeg, Maniloba, Canada R3T 2N2
F. C. Hawthorne
Affiliation:
Departmenl of Geological Sciences, University of Manitoba, Winnipeg, Maniloba, 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, Dcalc = 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 and fO2. The name silvialite, currently used in literature to describe the sulfate analogue of meionite, was suggested by Brauns (1914).

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1999

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