melilita

Melilite is a group of calcium aluminum silicate minerals within the nesosilicate class. It forms a solid-solution series with the end-members gehlenite, Ca2Al2SiO7, and åkermanite, Ca2MgSi2O7. Members share a common structural framework and differ mainly in the relative amounts of aluminum and magnesium.

The minerals are typically described as calcium- and aluminum-rich silicates with varying magnesium substitution. The group is named for its occurrence in calcium-rich rocks and its role as a major or characteristic constituent in certain igneous and metamorphic environments.

Melilite occurs mainly in calcium-rich igneous rocks of alkaline or ultramafic affinity, especially melilitites and related rocks. It can also appear in high-temperature metamorphic settings, particularly in calcareous or calc-silicate rocks that have undergone contact metamorphism. In such rocks, melilite commonly occurs with nepheline, perovskite, wollastonite, diopside, and other calcium-rich minerals.

In terms of physical properties, melilite minerals are usually colorless to white, pale yellow, brown, or gray. Crystals are typically observed as tabular or blocky grains, and they commonly exhibit a vitreous to resinous luster. The Mohs hardness is about 5.5 to 6, and the mineral can show a pale streak. Melilite can display chemical zoning along the solid-solution series between gehlenite and åkermanite.

Because melilite crystallizes early in certain alkaline magmas and is a defining mineral in melilitic rocks, its presence provides petrological information about magma composition and crystallization history.