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Sylvite and fluorite microcrysts, and fluorite-nyerereite intergrowths from natrocarbonatite, Oldoinyo Lengai, Tanzania

Published online by Cambridge University Press:  05 July 2018

R. H. Mitchell
R. H. Mitchell*
Affiliation:
Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, Canada P7B 5E1
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Abstract

Natrocarbonatite lavas erupted from hornitos T37B and T49B at Oldoinyo Lengai (Tanzania) during 23–30 July, 2000 are unusual in containing sylvite and fluorite microcrysts together with fluorite-nyerereite intergrowths. The latter are relatively coarse grained and exhibit granular textures indicative of slow crystallization rates relative to those of their host subaerial lavas. Fluorite microcrysts are considered to be derived by the fragmentation of the fluorite-nyerereite clasts. Sylvite microcrysts contain inclusions of ferroan alabandite [(Mn0.67-0.71Fe0.33-0.29)S] and are poor in Na (1.9–7.7 wt.% Na; 6.1–23.4 mol.% NaCl). Intergrowth and microcrystal fluorite contains 1–3.5 wt.% Sr. Intergrowth nyerereite has a composition similar to that occurring as bona fide phenocrysts. The groundmass of lava erupted from hornito T37B contains nyerereite microphenocrysts (4–8 wt.% BaO) that are epitaxially mantled by barian nyerereite (12–20 wt.% BaO). The latter are compositionally and texturally distinct from the groundmass phase X, which is considered to be a burbankite-group mineral. The fluorite-nyerereite clasts are considered to be derived from the magma chamber underlying hornitos T37B and T49B, and thus representative of some of the products of crystallization of natrocarbonatite magma under hypabyssal conditions. The origins of the sylvite microcrysts cannot, as yet, be determined.

Keywords

carbonatitesylvitenyerereitefluoritealabanditeOldoinyo LengaiTanzania
Type
Research Article
Information
Mineralogical Magazine , Volume 70 , Issue 1 , February 2006 , pp. 103 - 114
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2006

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