page_title: Melilite
hierarchical_section_title: Melilite
caption_attribution_description: Deutsch: Orthit und Melilith (bläulich) mit Quarz, gekreuzte Polarisatoren, vergl Image:Orthit_11.jpg English: Orthite amd melilite (bluish) with quartz, crossed light, compare Image:Orthit_11.jpg
context_page_description: 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[SiO₇]. Discovered in 1793 near Rome, it has a yellowish, greenish-brown color. The name derives from the Greek words meli "honey" and lithos "stone".
  Minerals of the melilite group are sorosilicates. They have the same basic structure, of general formula A₂B. 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 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 Ca₂Mg and
  gehlenite Ca₂Al[AlSiO₇]. Many melilites also contain appreciable iron and sodium.
context_section_description: 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".
  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 Ca₂Mg(Si₂O₇) and 
  gehlenite Ca₂Al[AlSiO₇]. Many melilites also contain appreciable iron and sodium.
  Some other compositions with the melilite structure include:  alumoåkermanite (Ca,Na)₂(Al,Mg,Fe²⁺)(Si₂O₇), 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)]
  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 Ba₂Co(Ge₂O₇) shows giant directional dichroism (different absorption for counter-propagating light beams) and hosts magnetically switchable chirality.
caption: Orthite and melilite (blue) with quartz, from a thin section in crossed polarized light.
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