Carbonate-Carbonate Immiscibility, Neighborite and Potassium Iron Sulphide in Oldoinyo Lengai Natrocarbonatite

Roger H. Mitchell
Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada P7B 5E1

Abstract: Porphyritic natrocarbonatite lavas erupted from the Oldoinyo Lengai volcano (Tanzania) on 17 October 1995 and 15–19 December 1995 differ from previously studied lavas in that they preserve textures indicative of groundmass carbonate-carbonate immiscibility. The immiscible fractions are considered to involve: a Na-K-Ca-CO2-Cl-rich, F-bearing fluid crystallizing gregoryite, sodian sylvite, potassium neighborite as well as a complex Ba-rich carbonate; and a Na-rich, Cl-poor carbonate liquid approximating to a nyerereite-gregoryite cotectic composition. Compositional data are given for potassium neighborite, this mineral being the first recognized occurrence of a fluorine-based perovskite group mineral in a magmatic environment. New compositional data are also given for a previously recognized potassium iron sulphide which indicate that this phase is probably a solid solution between the ternary sulphides, KFe3S4, K2Fe3S4, and KFe2S3. Textural and paragenetic data are interpreted to suggest that these recent lavas are more evolved than previously investigated Oldoinyo Lengai lavas and that natrocarbonatite is a highly evolved rather than a primitive magma.

Keywords: carbonatite • liquid immiscibility • neighborite • potassium iron sulphide • sylvite • Oldoinyo Lengai • Tanzania

Mineralogical Magazine; December 1997 v. 61; no. 409; p. 779-789; DOI: 10.1180/minmag.1997.061.409.03
© 1997, The Mineralogical Society
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