Thermodynamic analysis of the secondary minerals' stability in altered carbonatites of the Gldoinyo Lengai volcano, Northern Tanzania

Perova E. N., Zaitsev A. N.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2016. V. 145. N 3. P. 1-13

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Language: Russian

Abstract

Carbonatites from the Oldoinyo Lengai volcano, northern Tanzania, are unstable under normal atmospheric conditions. During carbonatite interaction with water the major minerals gregoryite Na₂(CO₃), nyerereite Na₂Ca(CO₃)₂ and sylvite KCl are dissolved and replaced by secondary low-temperature minerals thermonatrite Na₂(CO₃)·H₂O, trona Na₃(CO₃)(HCO₃)·2H₂O, nahcolite Na(HCO₃), pirssonite Na₂Ca(CO₃)₂·2H₂O, calcite Ca(CO₃) and shortite Na₂Ca₂(CO₃)₃. Thermodynamic calculations show that the formation of secondary minerals within Oldoinyo Lengai carbonatites are controlled by pH of pore solution, H₂O and CO₂ fugacity and the ratio of Ca and Na activities in the system Na₂O—CaO—CO₂—H₂O.

Key words: gregoryite, nyerereite, pirssonite, calcite, carbonatites, low-temperature alteration, Oldoinyo Lengai volcano.