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.

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

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

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

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