What is "melilite"

Melilite \Mel"i*lite\ (m[e^]l"[i^]*l[imac]t), n. [Gr. me`li honey + -lite; cf. F. m['e]lilithe.] (Min.) A mineral occurring in small yellow crystals, found in the lavas (melilite basalt) of Vesuvius, and elsewhere. [Written also mellilite.]

n. (context mineralogy English) any mineral consisting of a solid solution of gehlenite (Ca₂Al₂SiO₇) and akermanite (Ca₂MgSi₂O₇)

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 åkermanite. A generalized formula for common melilite is ( Ca, Na)( Al, Mg, Fe)[( Al, Si) Si O]. Discovered in 1793 near Rome, it has a yellowish, greenish brown color. The name derives from the Greek words meli (μέλι) "honey" and lithos (λίθους) "stone".

Minerals of the melilite group are sorosilicates. They have the same basic structure, of general formula AB(TO). The melilite structure consist of pairs of fused TO, where T may be Si, Al, B, in bow-tie form. Sharing one corner, the formula of the pair is TO. These bow-ties are linked together into sheets by the B cations. The sheets are held together by the A cations, most commonly calcium and sodium. Aluminium 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 perpendicular to this, in the {110} directions. Melilite is tetragonal.

The important endmembers of common melilite are åkermanite CaMg(SiO) and gehlenite CaAl[AlSiO]. Many melilites also contain appreciable iron and sodium.

Some other compositions with the melilite structure include: alumoåkermanite (Ca,Na)(Al,Mg,Fe)(SiO), okayamalite CaB[BSiO], gugiaite CaBe[SiO], hardystonite CaZn[SiO], barylite BaBe[SiO], andremeyerite BaFe[SiO]. Some structures formed by replacing one oxygen by F or OH: leucophanite (Ca,Na)(Be,Al)[SiO(F,OH)], jeffreyite (Ca,Na)(Be,Al)[SiO(O,OH)], and meliphanite (Ca,Na)(Be,Al)[SiO(OH,F)]

New members of this mineral group were artificially grown and became intensively studied due to their multiferroic property, i. e. they simultaneously show ferroelectric and magnetic ordering at low temperatures. This gives rise to peculiar optical properties, for example BaCo(GeO) shows giant directional dichroism (different absorption for counter-propagating light beams) and hosts magnetically switchable chirality.