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