Chemistry:Melilite

Short description: Sorosilicate mineral

Melilite
Orthite and melilite (blue) with quartz, from a thin section in crossed polarized light.
General
Category	Sorosilicates
Formula (repeating unit)	(Ca,Na) 2(Al,Mg,Fe2+ )[(Al,Si)SiO 7]
Strunz classification	9.BB.**
Crystal system	Tetragonal
Space group	P421m (no. 113)
Identification
Color	Yellowish, greenish brown
Crystal habit	Massive – granular
Cleavage	Distinct on {001}, weak on {110}
Fracture	Uneven
Mohs scale hardness	5–5.5
|re|er}}	Vitreous – greasy
Streak	white
Diaphaneity	Translucent
Specific gravity	2.9–3.0
Optical properties	Uniaxial (−)
Refractive index	nω = 1.632 – 1.669 nε = 1.626 – 1.658
Birefringence	δ = 0.006 – 0.011
References

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.

Crossed polarized light image of melilite in Moiliili lava flow, Moiliili Quarry, Honolulu, O‘ahu, Hawai‘i surrounded by matrix of granular nepheline and volcanic glass. This rock (nepheline-melilite basalt) was erupted during rejuvenated volcanism on O‘ahu.

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)₂(Al,Mg,Fe²⁺)[(Al,Si)SiO₇]. 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".The name refers to a group of minerals (melilite group) with chemically similar composition, nearly always minerals in åkermanite-gehlenite series.

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 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 Ca
2Mg(Si
2O
7) and gehlenite Ca
2Al[AlSiO
7]. Many melilites also contain appreciable iron and sodium.

Some other compositions with the melilite structure include: alumoåkermanite (Ca,Na)
2(Al,Mg,Fe2+
)(Si
2O
7), okayamalite Ca
2B[BSiO
7], gugiaite Ca
2Be[Si
2O
7], hardystonite Ca
2Zn[Si
2O
7], barylite BaBe
2[Si
2O
7], andremeyerite BaFe
2+2
[Si
2O
7]. Some structures formed by replacing one oxygen by F or OH: leucophanite (Ca,Na)
2(Be,Al)[Si
2O
6(F,OH)], jeffreyite (Ca,Na)
2(Be,Al)[Si
2O
6(O,OH)], and meliphanite (Ca,Na)
2(Be,Al)[Si
2O
6(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 Ba
2Co(Ge
2O
7) shows giant directional dichroism (different absorption for counter-propagating light beams) and hosts magnetically switchable chirality.

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–aluminium-rich inclusions in chondritic meteorites. Isotope ratios of magnesium and some other elements in these inclusions are of great importance in deducing processes that formed the Solar System.

Melilitite is a volcanic rock composed of over 90% melilite, with small amounts of olivine, clinopyroxene, and perovskite. The intrusive equivalent is melilitolite. When pyroxene is absent from the rock, and the accessory minerals are olivine, magnetite, leucite, kalsilite, nepheline, and perovskite, the rock is sometimes called katungite. However, more modern classification schemes avoid the term katungite and describe the rock instead as (for example) kalsilite-leucite-olivine melilitite, depending on the most abundant accessory minerals. Alnöite is a melilitite devoid of glass or feldspathoids with melilite in its groundmass.

See also

• Classification of minerals
• List of minerals

References

• Deer, W.A., Howie, R.A., and Zussman, J. (1986) Disilicates and Ring Silicates, Rock-forming minerals 1B, 2nd Ed., New York : John Wiley & Sons, ISBN:0-582-46521-4

External links

• Zinner, E. "Using Aluminum-26 as a Clock for Early Solar System Events." PSR Discoveries. September 2002 Accessed December, 2007
• Optical properties of multiferroics