Samples of natrocarbonatite showing various degrees of alteration are preserved in an older carbonatite platform that existed before and until the 1917 explosive eruption at Oldoinyo Lengai volcano, in Tanzania. The carbonatite samples contain two mineral assemblages: the first consists of relict primary magmatic minerals: nyerereite, fluorite, apatite-(CaF), barite, magnetite and khanneshite. The second consist of low-temperature subsolidus minerals: pirssonite, calcite, shortite, nahcolite, trona, thermonatrite, jacobsite and barytocalcite. Observed relationships between key primary and subsolidus minerals show the following sequence of formation: nyerereite → pirssonite → calcite → shortite. Three major processes, mineral dissolution, metasomatic replacement and crystallization from solution, alter the natrocarbonatite to pirssonite carbonatite, shortite carbonatite and calcite carbonatite. Nyerereite - pirssonite - calcite relationships depend on variations in P(H₂O) and P(CO₂).