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 akermanite to Na-Al-melilite (alumoakermanite) is documented and reflects the whole peralkaline trend of Oldoinyo Lengai. The data presented allow the volcano's 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)A1 (Si2O7) end-member ternary compositional diagram. (C) 2010 Elsevier B.V. All rights reserved.