Table of Contents
corundum gneiss with sapphire Corundum from Sackett mine, Chester, Hampden Co., Massachusetts, United States Corundum from Jagdalik, Kabul Prov., Afghanistan
Corundum is the hardest mineral After diamond on Earth. The name corundum comes from the Sanskrit kuruvinda, meaning “ruby” the name given to red corundum. Ruby and sapphire are gem varieties of corundum. An aluminum oxide, corundum is commonly white, gray, or brown, but gem colors include red ruby and blue, green, yellow, orange, violet, and pink sapphire. Colorless forms also occur. Ruby forms a continuous color succession with pink sapphire; only stones of the darker hues are considered to be ruby. Corundum crystals are generally hexagonal, either tabular, tapering barrel-shaped, or dipyramidal. Corundum can also be massive or granular. It forms in syenites certain pegmatites and in high-grade metamorphic rocks. It is concentrated in placer deposits.
Name: Probably from the Sanskrit kurivinda, for ruby, through the Tamil kurundam
Mineral Group: Hematite group.
Chemical Properties of Corundum
Chemical Classification | Oxide |
Chemical Composition | Al2O3 |
Physical Properties of Corundum
Color | Typically gray to brown. Colorless when pure, but trace amounts of various metals produce almost any color. Chromium produces red (ruby) and combinations of iron and titanium produce blue (sapphire). |
Streak | Colorless (harder than the streak plate) |
Luster | Adamantine to vitreous |
Diaphaneity | Transparent, translucent to opaque |
Cleavage | None. Corundum does display parting perpendicular to the c-axis. |
Mohs Hardness | 9 |
Specific Gravity | 3.9 to 4.1 (very high for a nonmetallic mineral) |
Diagnostic Properties | Hardness, high specific gravity, hexagonal crystals sometimes tapering to a pyramid, parting, luster, conchoidal fracture |
Crystal System | Hexagonal |
Tenacity | Brittle |
Parting | Rhombohedral and basal parting {0001}, sometimes perfect but interrupted; also on {1011} due to exsolution (Boehmite), observed on large blocks (Georgia, USA). |
Fracture | Irregular/Uneven, Conchoidal |
Density | 3.98 – 4.1 g/cm3 (Measured) 3.997 g/cm3 (Calculated) |
Optical Properties of Corundum
Type | Anisotropic |
Color / Pleochroism | Not Visible |
Twinning | Polysynthetic twinning common |
Optic Sign | Uniaxial (-) |
Birefringence | Low, first-order greys and whites. |
Relief | High |
Occurrence
Characteristic of Al-rich, Si-poor geological environments; in syenite and monzonite, and some quartz-free pegmatites; primary or a reaction product in eclogitic xenoliths in kimberlites. In regional or contact, high-grade metamorphic aluminous rocks, and some advanced argillic and potassic hydrothermal alteration assemblages; detrital in placers.
Uses Area
- Used as gemstone.
- It is used as abrasive because of its hardness.
- It is used for polishing and sanding of optical glasses.
- It is also used in refractories due to its high melting point (2,040 ° C or 3,700 ° F).
- Artificial corundum can be produced as a special product with slow deposition and controlled growth on a coil in an oxyhydrogen flame, as for jewelry use. This procedure is known as the Verneuil process
Association
Andesine, oligoclase, nepheline, scapolite (syenites); spinel, rutile, chondrodite, “hornblende”, phlogopite, calcite (metamorphosed limestones); kyanite, sillimanite, dumortierite, chlorite (schists); pyrope-rich garnet, spinel, phlogopite, omphacitic clinopyroxene, kyanite, rutile, graphite, diamond (eclogitic xenoliths).
Distribution
Numerous localities.
- In the USA, from Chester, Hampden Co., Massachusetts; the Cortland district, Westchester Co., New York; at Franklin, Sussex Co., New Jersey; large crystals from Hogback Mountain, Jackson Co., and Buck Creek, Clay Co., North Carolina; and from the Laurel Creek mine, Rabun Co., Georgia.
- At Bancroft and Haliburton, Ontario, Canada.
- On Naxos and Samos Islands, Greece.
- Large crystals from around the Soutpansberg, Transvaal, South Africa.
- Red gems from: the Mogok district, Myanmar (Burma).
- In the Ratnapura district, Sri Lanka.
- Around Mysore, India.
- In the Jegdalek marble, near Sorobi, Laghman Province, Afghanistan.
- At Merkestein, near Longido, and the Morogoro district, Tanzania. From Ampanihy, Madagascar.
- Blue, green, and yellow gems from: Chanthaburi and Trat, Thailand. Around Bottambang and Pailin, Cambodia.
- From the Zanskar district, Kashmir, India.
- In the Umba Valley, Tanzania.
- From around Andranondambo and Antsiermene, Madagascar.
- At Anakie, Queensland, Australia.
- From Yogo Gulch, 25 km southwest of Utica, Fergus Co., Montana, USA.
References
- Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
- Handbookofmineralogy.org. (2019). Handbook of Mineralogy. [online] Available at: http://www.handbookofmineralogy.org [Accessed 4 Mar. 2019].
- Mindat.org. (2019). Corundum: Mineral information, data and localities.. [online] Available at: https://www.mindat.org/min-727.html [Accessed 4 Mar. 2019].