gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/002646198547567 Mineral Chemistry of a Peralkaline Combeite-lamprophyllite Nephelinite from Oldoinyo Lengai, Tanzania Dawson J. B. Hill P. G. Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK 4 1998 62 2 179 196 15 4 1997 26 8 1997 Copyright © 1998, The Mineralogical Society 1998 Abstract

A peralkaline nephelinite lava ([Na+K]/A1 2.15) from the active carbonatite volcano Oldoinyo Lengai, contains combeite, Ba lamprophyllite, a phase with affinities to delhayelite, CeSrNb perovskite, a CaNa phosphate high in Sr, Ba and K, and peralkaline glass; in addition to Fe-rich nepheline, aegirine-rich clinopyroxene and FeK-rich sodalite. The high alkali concentrations relative to almnina in the bulk rock could not have been achieved by fractionational crystallisation of the known Al-rich phenocryst phases (nepheline and sodalite) and some other process must be invoked.

 Keywords Oldoinyo Lengai Tanzania nephelinite natrocarbonatite combeite Ba lamprophyllite delhayelite CaNa phosphate peralkaline glasses