World Library  
Flag as Inappropriate
Email this Article

Corundum

Article Id: WHEBN0000005974
Reproduction Date:

Title: Corundum  
Author: World Heritage Encyclopedia
Language: English
Subject: Pleochroism, Aluminium oxide, Sapphire, Chrysoberyl, Mohs scale of mineral hardness
Collection:
Publisher: World Heritage Encyclopedia
Publication
Date:
 

Corundum

Corundum
General
Category Oxide mineral – Hematite group
Formula
(repeating unit)
Aluminium oxide, Al
2
O
3
Strunz classification 04.CB.05
Dana classification 4.3.1.1
Crystal symmetry Trigonal (32/m)
Unit cell a = 4.75 Å, c = 12.982 Å; Z=6
Identification
Color Colorless, gray, brown; pink to red, orange, yellow, green, blue, violet; may be color zoned, asteriated mainly grey and brown
Crystal habit Steep bipyramidal, tabular, prismatic, rhombohedral crystals, massive or granular
Crystal system Trigonal (Hexagonal Scalenohedral)
Symbol (32/m)
Space group: R3c
Twinning Polysynthetic twinning common
Cleavage None – parting in 3 directions
Fracture Conchoidal to uneven
Tenacity Brittle
Mohs scale hardness 9 (defining mineral)[1]
Luster Adamantine to vitreous
Streak White
Diaphaneity Transparent, translucent to opaque
Specific gravity 3.95–4.10
Optical properties Uniaxial (–)
Refractive index nω = 1.767–1.772
nε = 1.759–1.763
Pleochroism None
Melting point 2044 °C
Fusibility Infusible
Solubility Insoluble
Alters to May alter to mica on surfaces causing a decrease in hardness
Other characteristics May fluoresce or phosphoresce under UV light
References [2][3][4][5]
Major varieties
Sapphire Any color except red
Ruby Red
Emery Black granular corundum intimately mixed with magnetite, hematite, or hercynite

Corundum is a crystalline form of aluminium oxide (Al
2
O
3
) typically containing traces of iron, titanium, vanadium and chromium.[2][3] It is a rock-forming mineral. It is one of the naturally 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.

The name "corundum" is derived from the Tamil word Kuruvindam or Sanskrit word Kuruvinda meaning ruby.[6]

Because of corundum's hardness (pure corundum is defined to have 9.0 Mohs), it can scratch almost every other mineral. It is commonly used as an abrasive on everything from sandpaper to large machines used in machining metals, plastics, and wood. Some emery is a mix of corundum and other substances, and the mix is less abrasive, with an average Mohs hardness of 8.0.

In addition to its hardness, corundum is unusual for its density of 4.02 g/cm3, which is very high for a transparent mineral composed of the low-atomic mass elements aluminium and oxygen.[7]

Geology and occurrence

Corundum from Brazil, size about 2 cm × 3 cm (0.8 in × 1 in).

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.[5] It commonly occurs as a detrital mineral in stream and beach sands because of its hardness and resistance to weathering.[5] The largest documented single crystal of corundum measured about 65×40×40 cm (26×16×16 in), and weighed 152 kg (335 lb).[8] The record has since been surpassed by certain synthetic boules.[9]

Corundum for abrasives is mined in Zimbabwe, Russia, Sri Lanka and India. Historically it was mined from deposits associated with dunites in North Carolina, USA and from a nepheline syenite in Craigmont, Ontario.[5] Emery grade corundum is found on the Greek island of Naxos and near Peekskill, New York, USA. Abrasive corundum is synthetically manufactured from bauxite.[5] Four corundum axes dating back to 2500 BCE from the Liangzhou culture have been discovered in China. The surfaces of the axes are remarkably smoothly polished.[10]

Synthetic corundum

In 1837, Marc Antoine Gaudin made the first synthetic rubies by fusing alumina at a high temperature with a small amount of chromium as a pigment.[11] In 1847, Ebelmen made white synthetic sapphires by fusing alumina in boric acid. In 1877 Frenic and Freil made crystal corundum from which small stones could be cut. Frimy and Auguste Verneuil manufactured artificial ruby by fusing BaF
2
and Al
2
O
3
with a little chromium at temperatures above 2,000 °C (3,632 °F). In 1903, Verneuil announced he could produce synthetic rubies on a commercial scale using this flame fusion process.[12]

The Verneuil process allows the production of flawless single-crystal sapphires, rubies and other corundum gems of much larger size than normally found in nature. It is also possible to grow gem-quality synthetic corundum by flux-growth and hydrothermal synthesis. Because of the simplicity of the methods involved in corundum synthesis, large quantities of these crystals have become available on the market causing a significant reduction of price in recent years. Apart from ornamental uses, synthetic corundum is also used to produce mechanical parts (tubes, rods, bearings, and other machined parts), scratch-resistant optics, scratch-resistant watch crystals, instrument windows for satellites and spacecraft (because of its transparency in the ultraviolet to infrared range), and laser components.

Structure and physical properties

Crystal structure of corundum
Molar volume vs. pressure at room temperature.

Corundum crystallizes with trigonal symmetry in the space group R3c and has the lattice parameters a = 4.75 Å and c = 12.982 Å at standard conditions. The unit cell contains six formula units.

In the lattice of corundum, the oxygen atoms form a slightly distorted hexagonal close packing in which two thirds of the gaps between the octahedra are occupied by aluminum ions.

References

  1. ^
  2. ^ a b
  3. ^ a b Corundum. Mindat.org
  4. ^ Corundum. Webmineral
  5. ^ a b c d e Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, pp. 300–302 ISBN 0-471-80580-7
  6. ^
  7. ^ The Mineral Corundum. galleries.com
  8. ^
  9. ^ Rubicon Technology Grows 200kg “Super Boule”, LED Inside, April 21, 2009
  10. ^
  11. ^
  12. ^
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 



Copyright © World Library Foundation. All rights reserved. eBooks from World eBook Library are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.