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melilite A member of the melilite group of minerals, that has the general formula (Mg,Al) (Ca,Na)2 (Si,Al)2 O7. There is a solid solution series between gehlenite (aluminium-rich) and åkermanite (magnesium rich); melilite occupies an intermediate position, with some substitution of sodium for calcium, and of aluminium for silicon or magnesium. Sp. gr. 2.9; hardness 5.5; white or greenish-white; granular or in tabular crystals; occurs in basic lavas, metamorphosed limestones, and furnace slags.