A complete solid solution series ranging from gehlenite, Ca₂Al₂SiO₇, to akermanite, Ca₂MgSi₂O₇, often containing appreciable Na and Fe. Melilites are tetragonal sorosilicates and form short prismatic crystals with poor basal cleavage. The Mohs hardness is 5–6, and the density increases progressively from 2.94 for akermanite to 3.05 for gehlenite. The luster is vitreous to resinous, and the color is white, yellow, greenish, reddish, or brown. Akermanite-rich varieties occur in thermally metamorphosed siliceous limestones and dolomites, but more gehlenite-rich ones result if Al is present, as in Durango, Mexico, and Monzoni, Italy. Melilites are found instead of plagioclase in silica-deficient, feldspathoid-bearing basalts, as at Capo di Bove, Italy; in the Kaiserstuhl, Germany; in the Hawaiian Islands; and at Vesuvius, where SiO₂ was removed from the magma by reaction with carbonate rocks (see illus.). In carbonaceous chondrite meteorites, melilites ranging from pure gehlenite to 80 mole % akermanite occur with hibonite, spinel, perovskite, and pyroxenes in inclusions thought to be ancient condensates from the gas of the solar nebula.