1966 Ash Eruption of the Carbonatite Volcano Oldoinyo Lengai: Mineralogy of Lapilli and Mixing of Silicate and Carbonate Magmas

J. B. Dawson, J. V. Smith and I. M. Steele
Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW
Department of the Geophysical Sciences, University of Chicago, Illinos 60637, U.S.A.

Abstract: Lapilli from the August 1966 eruption of the carbonatite volcano Oldoinyo Lengai consist of carbonate-cemented aggregates of (i) mono- and poly-mineralic fragments of ijolitic rocks, (ii) single grains and clusters of euhedral nepheline, Ti-andradite, and Ti-magnetite, and (iii) corroded pyroxene and wollastonite grains surrounded by coronas containing combeite, melilite, Ca-silicates (possibly larnite and rankinite), and rounded bodies of submicrometre intergrowths with complex bulk compositions dominated by Na,K,Ca-phosphate-carbonate and alkali-iron-sulphide-carbonate. The (ii,iii) materials, together with abundant Na-carbonate, sylvite and fluorite occurring as cement and shells in the lapilli, are attributed to mixing and incomplete reaction of ijolite and carbonatite magmas during the explosive eruption. The rounded submicrometre intergrowths are interpreted as the quench products of two types of immiscible liquids whose properties should be studied by controlled synthesis.

Keywords: carbonatite • silicate • carbonate • magmas • Oldoinyo Lengai • ash

Mineralogical Magazine; March 1992 v. 56; no. 382; p. 1-16; DOI: 10.1180/minmag.1992.056.382.01
© 1992, The Mineralogical Society
Mineralogical Society (www.minersoc.org)