AN EPHEMERAL PENTASODIUM PHOSPHATE CARBONATE FROM NATROCARBONATITE LAPILLI, OLDOINYO LENGAI, TANZANIA

Abstract

Lapilli formed by a Strombolian eruption are associated with the formation of a large lava flow of natrocarbonatite on or about 21-22 July, 2000 at Oldoinyo Lengai volcano, Tanzania. Fresh lapilli consist of vesicular natrocarbonatite similar to that occurring in rapidly quenched lavas. The lapilli were altered at low temperature (<50°C) by degassing to aggregates of sodian sylvite, potassian halite, trona, thermonatrite and a novel F-bearing sodium phosphate-carbonate. The latter is considered to be a new mineral as it has a composition (Na5-4.5 PO4 (CO3,F,Cl) that is not similar to that of nahpoite (Na2HPO4), dorfmanite [Na2 (PO3OH).2H2O] or natrophosphate [Na7(PO4) 2F.19H2O]. However, in common with these minerals, it is ephemeral and undergoes rapid decomposition under normal atmospheric conditions. The sodium phosphate-carbonate and associated halide-sodium carbonate assemblages are considered to be a part of a previously unrecognized hyperagpaitic assemblage forming as sublimates at Oldoinyo Lengai. © 2006 The Mineralogical Society.