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

Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada P7B 5E1

^* E-mail: rmitchel{at}lakeheadu.ca

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 (Na_5–4.5PO₄(CO_3,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.

KEYWORDS: sodium phosphate-carbonate, natrocarbonatite, sublimate, trona, thermonatrite, sylvite, Oldoinyo Lengai, Tanzania