Abstract: A calcite-rich block in nephelinitic agglomerate at Oldoinyo Lengai comprises Sr-calcite, apatite, fluorite, a Ba-Mn oxide akin to romanéchite, and a distinctive Mn-Fe spinel. Formerly reported as sövite, on textural, mineralogical and stable isotope grounds it is now re-interpreted as a highly-altered natrocarbonatite lava. An implication of this study is that natrocarbonatite is not just a modern phenomenon, but has been extruded from Oldoinyo Lengai for at least around 1250 years.