Melilite-group minerals at Oldoinyo Lengai, Tanzania

Oldoinyo Lengai and the volcanic centres of the Lake Natron–Engaruka province contain melilite as a widespread mineral. Extraordinarily Na–Al-rich melilites (up to 6 wt.% Na2O and 9 wt.% Al2O3) from recent explosive eruptions are among the most Na-rich ever reported. Their unusual mineral composition leads to optical properties with vivid birefringence colours of 2nd order. The continuous variation in mineral composition from common åkermanite to Na–Al–melilite (alumoåkermanite) is documented and reflects the whole peralkaline trend of Oldoinyo Lengai. The data presented allow the volcanoʹs evolution from primitive olivine melilitites to highly evolved and peralkaline combeite–wollastonite nephelinites to be traced. Melilite compositions of Oldoinyo Lengai extend the magmatic field in the Ca2Fe(Si2O7)–Ca2Mg(Si2O7)–(CaNa)Al(Si2O7) end-member ternary compositional diagram.