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a rock-forming mineral of the silicate class; a member of the isomorphous series extending from akermanite to gehlenite. Melilite has the chemical composition (Ca, Na)2(Mg, Al) [(Si, Al)2O7] and contains admixtures of Mn2+ and Fe2+. Its crystal structure is characterized by lattices consisting of alternating groups of twinned tetrahedrons [(Si, Al)2O7] as well as of single tetrahedrons—MgO4 and AlO4. The lattices are linked by Ca or Na atoms surrounded by eight oxygen atoms.
Melilite crystallizes in the tetragonal system. It generally occurs in the form of granular aggregates, individual grains, and continuous cryptocrystalline accumulations; crystals are rare. Melilite has a hardness of 5-5.5 on Mohs’ scale and a density of 2,980-3,066 kg/m3. The luster is vitreous, and the color ranges from white to pale yellow, occasionally to greenish yellow and reddish brown. Melilite is usually found in certain ultrabasic alkaline volcanic rocks in association with olivine, nepheline, pyroxene, biotite, and other minerals. Occasionally it occurs in contact-metasomatic (tactile) limestones.