Melilite

Melilite refers to a mineral of the melilite group. Minerals of the group are solid solutions of several endmembers, the most important of which are gehlenite and akermanite. A generalized formula for common melilite is (CaNa)₂(AlMgFe²⁺)[(AlSi)SiO₇]. Discovered in 1793 near Rome, it has a yellowish, greenish brown colour. The name derives from the Greek words for honey and stone.

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Minerals of the melilite group are sorosilicates. They have the same basic structure, of general formula A₂B(T₂O₇). The melilite structure consist of pairs of fused TO₄, where T may be Si, Al, B, in bow-tie form. Sharing one corner, the formula of the pair is T₂O₇. These bow-ties are linked together into sheets by the B cations. The sheets are held together by the A cations, most commonly Ca, Na. Note that Al may sit on either the T or the B site.  

Minerals with the melilite structure may show a cleavage parallel to the (001) crystallographic directions and may show weaker cleavage normal to this, in the {110} directions. Melilite is tetragonal.

The important endmembers of common melilite are Åkermanite Ca₂Mg(Si₂O₇) and Gehlenite Ca₂Al[AlSiO₇]. Many melilites also contain appreciable iron and sodium.

Some other compositions with the melilite structure include , Okayamalite Ca₂B[BSiO₇], Gugiaite Ca₂Be[Si₂O₇], Hardystonite Ca₂Zn[Si₂O₇], Barylite BaBe₂[Si₂O₇], Andremeyerite BaFe₂⁺²[Si₂O₇]. Some structures formed by replacing one oxygen by F or OH: Leucophanite (Ca,Na)₂(Be,Al)[Si₂O₆(F,OH)], Jeffreyite (Ca,Na)₂(Be,Al)[Si₂O₆(O,OH)], and Meliphanite (Ca,Na)₂(Be,Al)[Si₂O₆(OH,F)]

Occurrences

Melilite with compositions dominated by the endmembers akermanite and gehlenite is widely distributed but uncommon. It occurs in metamorphic and igneous rocks and in meteorites.

Typical metamorphic occurrences are in high-temperature metamorphosed impure limestones. For instance, melilite occurs in some high-temperature skarns.

Melilite also occurs in unusual silica-undersaturated igneous rocks. Some of these rocks appear to have formed by reaction of magmas with limestone. Other igneous rocks containing melilite crystallize from magma derived from the Earth's mantle and apparently uncontaminated by the Earth's crust. The presence of melilite is an essential constituent in some rare igneous rocks, such as olivine melilitite. Extremely rare igneous rocks contain as much as 70% melilite, together with minerals such as pyroxene and perovskite.

Melilite is a constituent of some calcium- and aluminum-rich inclusions (CAIs) in chondritic meteorites. Isotope ratios of magnesium and some other elements in these inclusions are of great importance in deducing processes that formed our solar system.

References

Deer, W. A., Howie, R. A., and Zussman, J., 1986, Rock-forming minerals. Volume 1B: Disilicates and Ring Silicates. 2nd Edition. John Wiley & Sons, Inc., New York