Corundum - Al₂O₃

Named "corinvindum" in 1725 by John Woodward and derived from the Sanskrit, kuruvinda ("Ruby"). Richard Kirwan used the current spelling "corundum" in 1794. Known by many names in ancient times: adamant, sapphire, ruby, hyacinthos, asteria.

Corundum is a crystalline form of aluminium oxide (Al₂O₃). It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red and padparadscha if pink-orange. All other colors are called sapphire, e.g., "green sapphire" for a green specimen. Corundum occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic terranes. It also occurs in low silica igneous syenite and nepheline syenite intrusives. Other occurrences are as masses adjacent to ultramafic intrusives, associated with lamprophyre dikes and as large crystals in pegmatites.

Optical properties:

• Form Rarely euhedral, usually as small rounded crystals
• Color: Colourless
• Cleavage: None
• Relief: High
• Interference colors: Low (gray-yellow).

Bibliography

• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• 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.
• Vernon, R. H. (2018). A practical guide to rock microstructure. Cambridge university press.

(yellow interference colors)

Photo
Corundum crystals (with blue core) associated with biotite and andalusite (left side of the picture, colurless high relief). MME (Mafic microgranular enclaves) in the S.Andrea granite. Elba Island, Italy. PPL image, 2x (Field of view = 7mm)	Corundum crystals (with blue core) associated with biotite and andalusite (right side of the picture, colurless high relief). MME (Mafic microgranular enclaves) in the S.Andrea granite. Elba Island, Italy. PPL image, 2x (Field of view = 7mm)	Corundum crystals (with blue core) associated with biotite. MME (Mafic microgranular enclaves) in the S.Andrea granite. Elba Island, Italy. PPL image, 2x (Field of view = 7mm)
Corundum crystal (with blue core) associated with biotite. MME (Mafic microgranular enclaves) in the S.Andrea granite. Elba Island, Italy. PPL image, 10x (Field of view = 2mm)	Corundum crystal (with blue core) associated with biotite. MME (Mafic microgranular enclaves) in the S.Andrea granite. Elba Island, Italy. PPL image, 10x (Field of view = 2mm)	Corundum crystal (with blue core) associated with biotite. MME (Mafic microgranular enclaves) in the S.Andrea granite. Elba Island, Italy. XPL image, 10x (Field of view = 2mm)
Corundum crystal (with blue core) associated with biotite. MME (Mafic microgranular enclaves) in the S.Andrea granite. Elba Island, Italy. PPL image, 10x (Field of view = 2mm)	Corundum crystal (with blue core) associated with biotite. MME (Mafic microgranular enclaves) in the S.Andrea granite. Elba Island, Italy. PPL image, 10x (Field of view = 2mm)	Corundum crystal (with blue core) associated with biotite. MME (Mafic microgranular enclaves) in the S.Andrea granite. Elba Island, Italy. PPL image, 10x (Field of view = 2mm)
Corundum crystal (with blue core) associated with biotite and fibrous crystals of fibrolite-sillimanite. MME (Mafic microgranular enclaves) in the S.Andrea granite. Elba Island, Italy. PPL image, 2x (Field of view = 7mm)	Corundum crystal (with blue core) associated with biotite. MME (Mafic microgranular enclaves) in the S.Andrea granite. Elba Island, Italy. PPL image, 10x (Field of view = 2mm)	Corundum crystal (with blue core) associated with biotite. MME (Mafic microgranular enclaves) in the S.Andrea granite. Elba Island, Italy. XPL image, 10x (Field of view = 2mm)
Corundum crystal (with blue core) associated with biotite. MME (Mafic microgranular enclaves) in the S.Andrea granite. Elba Island, Italy. PPL image, 10x (Field of view = 2mm)	Corundum crystals (with blue core) associated with biotite. MME (Mafic microgranular enclaves) in the S.Andrea granite. Elba Island, Italy. PPL image, 2x (Field of view = 7mm)	Corundum crystal (with blue core) associated with biotite. MME (Mafic microgranular enclaves) in the S.Andrea granite. Elba Island, Italy. PPL image, 10x (Field of view = 2mm)
Corundum (high Relief, colorless crystals) in association with sapphirine (green), cordierite (colorless) and Sillimanite (beige color). PPL image, 10x (Field of view = 2mm)	Corundum (high Relief, colorless crystals) in association with sapphirine (green), cordierite (colorless) and Sillimanite (beige color). PPL image, 10x (Field of view = 2mm)	Corundum (high Relief, colorless crystals) in association with sapphirine (green), cordierite (colorless) and Sillimanite (beige color). PPL image, 10x (Field of view = 2mm)
Corundum (yellow interference colors) in association with sapphirine (deep-green), cordierite (gray) and Sillimanite (high interference colors). XPL image, 10x (Field of view = 2mm)	Corundum (high Relief, colorless crystals) in association with sapphirine (green), cordierite (colorless) and Sillimanite (beige color). PPL image, 2x (Field of view = 7mm)	Corundum (high Relief, colorless crystals) in association with sapphirine (green), cordierite (colorless) and Sillimanite (beige color). PPL image, 10x (Field of view = 2mm)
Corundum (yellow interference colors) in association with sapphirine (deep-green), cordierite (gray) and Sillimanite (high interference colors). XPL image, 10x (Field of view = 2mm)	Corundum (high Relief, colorless crystals) in association with sapphirine (green) and cordierite (colorless). PPL image, 10x (Field of view = 2mm)	Corundum (yellow interference colors) in association with sapphirine (deep-green) and cordierite (gray). XPL image, 10x (Field of view = 2mm)
Corundum crystals (high Relief, colorless). PPL image, 1x (Field of view = 9mm)	Corundum crystals (high Relief, colorless). PPL image, 1x (Field of view = 9mm)	Corundum crystals (high Relief, colorless). PPL image, 1x (Field of view = 9mm)
Corundum crystals (high Relief, colorless). PPL image, 2x (Field of view = 7mm)	Corundum crystals (I order yellow interference colors). XPL image, 2x (Field of view = 7mm)	Corundum crystals (high Relief, colorless). PPL image, 2x (Field of view = 7mm)
Corundum crystals (I order gray interference colors). XPL image, 2x (Field of view = 7mm)	Corundum crystals (high Relief, colorless). PPL image, 2x (Field of view = 7mm)	Corundum crystals (I order gray interference colors). XPL image, 2x (Field of view = 7mm)
Corundum crystals (high Relief, colorless). PPL image, 2x (Field of view = 7mm)	Corundum crystals (high Relief, colorless). PPL image, 2x (Field of view = 7mm)	Corundum crystals (high Relief, colorless). PPL image, 2x (Field of view = 7mm)
Corundum crystals (high Relief, colorless). PPL image, 10x (Field of view = 2mm)	Corundum crystals (high Relief, colorless). PPL image, 10x (Field of view = 2mm)	Corundum crystals (I order gray interference colors). XPL image, 10x (Field of view = 2mm)
Corundum crystals (high Relief, colorless). PPL image, 10x (Field of view = 2mm)	Corundum crystals (high Relief, colorless). PPL image, 10x (Field of view = 2mm)	Corundum crystals (high Relief, colorless). PPL image, 10x (Field of view = 2mm)

Metamorphic Minerals

Texture and Microstructure

Metamorphic Rocks

Corundum - Al2O3

Corundum - Al₂O₃