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 5El 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-COz-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 coteetic 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. Ks carbonatite, liquid immiscibility, neighborite, potassium iron sulphide, sylvite, Oldoinyo Lengai, Tanzania. Introduction Tim natroearbonatite lavas erupted from the volcano Oldoinyo Lengai (Tanzania) are the only known examples of active carbonatite magmatism. The unusual composition and mineralogy of the lavas relative to common calcite carbonatite are well documented (Dawson et al., 1995, 1996; Peterson, 1990; Church and Jones, 1995), although their origin and evolution remains controversial (Church and Jones, 1995; Gittins, 1989; LeBas, 1989; Twyman and Gittius, 1987). This paper does not attempt to discuss the problems of the genesis of natrocarbonatite but focuses on the petrology of the final stages of crystallization of the magma and demonstrates that lavas erupted in the crater of Oldoinyo Lengai on the 17th October 1995 and from 15-19th December 1995 have significant textural and mineralogical differences to previously studied lavas. Mineralogical Magazine, December 1997, Vol. 61, pp. Copyright the Mineralogical Society Sampling and analytical methods The majority of the material studied was collected from a small, rapidly flowing, phenocryst-rich lava emanating from the breached north side of homito T37 on 17 October 1995. Samples were collected from the crust of the flowing lava and from phenocryst-poor material squeezed up from the interior of the flow. All samples were immediately sealed in plastic with silica gel to prevent hydration. The temperature of the interior of the lava flow, as measured by thermocouple, was 538~ (J. Keller, pers. comm.). Polished thin sections of the lava were prepared approximately two weeks after their collection using kerosene to prevent the dissolution of water-soluble phases. Vetsch (1995) reports that this homito was still active during December 1995 and that the temperature of the flow was 510~ Vetsch (1995) also recorded intermittent effusive activity at hornito T36 (vents A-D) during 779-789