Mineral Chemistry of a Peralkaline Combeite-lamprophyllite Nephelinite from Oldoinyo Lengai, Tanzania

J. B. Dawson and P. G. Hill
Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK

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

Mineralogical Magazine; April 1998 v. 62; no. 2; p. 179-196; DOI: 10.1180/002646198547567
© 1998, The Mineralogical Society
Mineralogical Society (www.minersoc.org)