Melilite - (Ca,Na)2(Mg, Fe, Al, Si)3O7

Melilite was discovered in 1793 near Rome, it has a yellowish, greenish brown color. The name derives from the Greek words meli "honey" and lithos "stone"in allusion to the color. The melilite structure is characterized by Si2O7 groups of twin SiO4 tetrahedra united by a single bridging oxygen. Both Al3+ and Mg2+ ions, however, are in tetrahedral coordination, and all tetrahedral are bound in sheet to yield the properties of sheet silicate. Melilites form a solid solution series from pure Gehlenite Ca2Al2Si3O7] to pure Akermanite [Ca2MgSi2O7]; although pure members are very rare. Melilite occurs in unusual silica-undersaturated igneous rocks. Some of these rocks appear to have formed by reaction of magmas with limestone. The presence of melilite is an essential constituent in some rare igneous rocks, such as olivine melilitite (venanzite).

Optical Properties:
• Relief: low
• Color: colorless
• Interference colors: weak, but anomalous, with berlin blue colors.
• cleavage: Basal (001) poor on (110)
• Form: commonly appears as subhedral to euhedral tabular crystals. Section are roughly rectangular with the short dimension parallel to c-axis.

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Orange-brown melilite crystals, colorless nepheline needles, black-metallic magnetite. Emmelberg, Eifel Mountains, Germany





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Orange-brown melilite crystals, colorless nepheline needles, black-metallic magnetite. Emmelberg, Eifel Mountains, Germany



Bibliography



• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc


Photo
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Colorless melilite crystal in a Venanzite, San Venanzo (Umbria, Italy). PPL image , 2x (Field of view = 7mm)
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Melilite (with anomalous, berlin blue colors) in a Venanzite, San Venanzo (Umbria, Italy). XPL image , 2x (Field of view = 7mm)
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Melilite (with anomalous, berlin blue colors) in a Venanzite, San Venanzo (Umbria, Italy). XPL image , 2x (Field of view = 7mm)
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Melilite (with anomalous, berlin blue colors) and leucite in a melilitolite, San Venanzo (Umbria, Italy). XPL image , 2x (Field of view = 7mm)
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Melilite (with anomalous, berlin blue colors) in a Venanzite, San Venanzo (Umbria, Italy). XPL image , 2x (Field of view = 7mm)
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Melilite (with anomalous, berlin blue colors) in a Venanzite, San Venanzo (Umbria, Italy). XPL image , 10x (Field of view = 2mm)
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Melilite (with anomalous, berlin blue colors) in a Venanzite, San Venanzo (Umbria, Italy). XPL image , 10x (Field of view = 2mm)
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Melilite (with anomalous, berlin blue colors) in a Venanzite, San Venanzo (Umbria, Italy). XPL image , 10x (Field of view = 2mm)
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Melilite (with anomalous, berlin blue colors) in a Venanzite, San Venanzo (Umbria, Italy). XPL image , 10x (Field of view = 2mm)
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Melilite (with anomalous, berlin blue colors) in a Venanzite, San Venanzo (Umbria, Italy). XPL image , 10x (Field of view = 2mm)
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Melilite (with anomalous, berlin blue colors) in a Venanzite, San Venanzo (Umbria, Italy). XPL image , 10x (Field of view = 2mm)
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Melilite (with anomalous, berlin blue colors) in a Venanzite, San Venanzo (Umbria, Italy). XPL image , 10x (Field of view = 2mm)
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Melilite (with anomalous, berlin blue colors) in a Venanzite, San Venanzo (Umbria, Italy). XPL image , 10x (Field of view = 2mm)
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Melilite (with anomalous, berlin blue colors) in a Venanzite, San Venanzo (Umbria, Italy). XPL image , 10x (Field of view = 2mm)
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Melilite (with anomalous, berlin blue colors) in a Venanzite, San Venanzo (Umbria, Italy). XPL image , 10x (Field of view = 2mm)
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Melilite (with anomalous, berlin blue colors) in a Venanzite, San Venanzo (Umbria, Italy). XPL image , 10x (Field of view = 2mm)
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Melilite (with anomalous, berlin blue colors) in a Venanzite, San Venanzo (Umbria, Italy). XPL image , 10x (Field of view = 2mm)
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Melilite (with anomalous, berlin blue colors) in a Venanzite, San Venanzo (Umbria, Italy). XPL image , 10x (Field of view = 2mm)
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Melilite (with anomalous, berlin blue colors), leucite crystals and secondary calcite in a Venanzite, San Venanzo (Umbria, Italy). XPL image , 10x (Field of view = 2mm)
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Melilite (with anomalous, berlin blue colors), leucite crystals and secondary calcite in a Venanzite, San Venanzo (Umbria, Italy). XPL image , 10x (Field of view = 2mm)