Melilite would be considered by most people to be a mineral. Melilite has an apparent formula of (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. Mineralogists do not accept melilite as a valid mineral in that it is not a unique chemical compound but is rather a solid solution of various silicates.
The term melilite is also used to define a group of minerals with the same basic structure, of general formula A2B(T2O7). The melilite group forms part of the broader sorosilicate (or pyrosilicate) group of silicates. 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.
A view down onto the sheets of the A2B(T2O7) melilite structure. The A sites are yellow spheres, the B sites are pale orange tetrahedra, and the T sites are blue tetrahedra. Oxygen atoms (not shown) are on the corners of the tetrahedra.
Minerals with the melilite structure often show a cleave parallel to the sheets (001) and may show weaker cleavage normal to this. Melilite structures are typically tetragonal.
Some of the minerals that are part of the melilite group are: Åkermanite Ca2Mg(Si2O7), Okayamalite Ca2B[BSiO7], Gehlenite Ca2Al[AlSiO7], 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)], Meliphanite (Ca,Na)2(Be,Al)[Si2O6(OH,F)]
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