Strontian melilite in a nephelinite lava from Etinde, Cameroon

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Abstract
Strontium-rich (up to 16 wt% SrO) melilite occurs as microphenocrysts in a nosean-leucite-nephelinite lava from Etinde, Cameroon. Electron microprobe analyses show that the melilite is very strongly zoned with Sr, Fe, and Na enrichment towards the crystal rims. Sodium melilite and sodium ferrimelilite are the dominant end-member molecules and together account for up to 76 mole%. The crystals are optically negative and strongly birefringent with birefringence ranging from 0.014 in the cores to 0.032 at the crystal rims. Refractive indices measured at the rim of one crystal gave ω = 1.656, ɛ = 1.628. The cell dimensions of strontian melilite are a = 7.765Å, c = 5.054Å.