An ephemeral pentasodium phosphate carbonate from natrocarbonatite lapilli, Oldoinyo Lengai, Tanzania

摘要

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 deg 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 (Na_(5-4.5)PO_4(CO_3,F,Cl) that is not similar to that of nahpoite (Na_2HPO_4), dorfmanite [Na_2(PO_3OH).2H_2O] or natrophosphate [Na_7(PO_4)_2F.19H_2O]. 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.