|Category||Oxide mineral – Hematite group|
|Chemical formula||Aluminium oxide, Al2O3|
|Color||Colorless, gray, brown; pink to pigeon-blood-red, orange, yellow, green, blue to cornflower 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)
Space group: R3c
|Twinning||Polysynthetic twinning common|
|Cleavage||None – parting in 3 directions|
|Fracture||Conchoidal to uneven|
|Mohs scale hardness||9|
|Luster||Adamantine to vitreous|
|Diaphaneity||Transparent, translucent to opaque|
|Optical properties||Uniaxial (–)|
|Refractive index||nω = 1.767–1.772
nε = 1.759–1.763
|Melting point||2044 °C|
|Alters to||May alter to mica on surfaces causing a decrease in hardness|
|Other characteristics||May fluoresce or phosphoresce under UV|
|Sapphire||Any color except red|
|Emery||Black granular corundum intimately mixed with magnetite, hematite, or hercynite|
Corundum is a crystalline form of aluminium oxide (Al2O3) with traces of iron, titanium and chromium. It is a rock-forming mineral. 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, while all other colors are called sapphire.
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 a lower average hardness near 8.0.
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. Because of its hardness and resistance to weathering, it commonly occurs as a detrital mineral in stream and beach sands. The largest documented single crystal of corundum measured about 65×40×40 cm.
Corundum for abrasives is mined in Zimbabwe, Russia, and India. Historically it was mined from deposits associated with dunites in North Carolina, USA and from a nepheline syenite in Craigmont, Ontario. Emery grade corundum is found on the Greek island of Naxos and near Peekskill, New York, USA. Abrasive corundum is synthetically manufactured from bauxite.
In 1837, Gaudin made the first synthetic rubies by fusing alumina at a high temperature with a small amount of chromium as a pigment. In 1847, Ebelmen made white sapphire 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 BaF2 and Al2O3 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.
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 from the UV to IR), and laser components.
CORUNDUM, a mineral composed of native alumina (Al203), remarkable for its hardness, and forming in its finer varieties a valuable gem-stone. Specimens were sent from India to England in the 18th century, and were described in 1798 by the Hon. C. Greville under the name of corundum - a word which he believed to be the native name of the stone (Hindi, kurund; Tamil, kurundam; Sanskrit, kuruvinda, " ruby"). The finely coloured, transparent varieties include such gem-stones as the ruby and sapphire, whilst the impure granular and massive forms are known as emery. The term corundum is often restricted to the remaining kinds, i.e. those crystallized and crystalline varieties which are not sufficiently transparent and brilliant for ornamental purposes, and which were known to the older mineralogists as "imperfect corundum." Such varieties were termed by J. Black, in con sequence of their hardness, adamantine spar, but this name is now usually restricted to a hair-brown corundum, remarkable for a pearly sheen on the basal plane.
Corundum crystallizes in the hexagonal system. In fig. 1, which is a form of ruby, the prism a is combined with a hexagonal pyramid n, a rhombohedron R, and the basal pinacoid C. in fig. 2, which represents a typical crystal of sapphire, the prism s is associated with the acute pyramids h, r, and a rhombohedron a. Other crystals show a tabular habit, consisting usually of the basal pinacoid with a rhombohedron, and it is notable that this habit is said to be characteristic of corundum which has consolidated from a fused magma. Corundum has no true FIG. I. FIG. 2.
cleavage, but presents parting planes due to the structure of the crystal, which have been studied by Prof. J. W. Judd.
Next to diamond, corundum is the hardest known mineral. Its hardness is generally given as 9, but there are slight variations in different stones, sapphire being rather harder than ruby, and ruby than common corundum. The colours are very varied, and it is probable that iron is responsible for many of the tints, though chromium is a possible agent in certain cases. The transparent varieties are often distinguished as "Oriental" stones. (See Ruby and Sapphire.) Corundum is used largely for watch-jewels, and for bearings in electrical apparatus.
The coloured corundums fit for gem-stones come chiefly from Ceylon, Burma, Siam and Montana. Coarse dull corundum is found in many localities, and usually has higher commercial value as an abrasive agent than emery, which is less pure. The coarse corundum, however, is often partially hydrated or otherwise altered, whereby its hardness is diminished. In India, where the native lapidaries use corundum-sticks and rubbers formed of the powdered mineral cemented with lac, it occurs in the Salem district, Madras, in Mysore and in Rewa. Large deposits of corundum exist in the United States, especially in N. Carolina and Georgia, where they are associated with peridotites, often near contact with gneiss. The mineral has been extensively worked, as at Corundum Hill, Macon county, N. C., near which, in 1871, were discovered numerous rubies, sapphires and pebbles of coarse corundum in the bed of a river. Corundum occurs also at many localities in Montana, where the crystals are often of gem quality. They are found mostly as loose crystals in gravel, but are known also in igenous rocks like andesite and lamprophyre. Prof. J. H. Pratt, who has studied the occurrence both in Montana and in N. Carolina, considers that the alumina was dissolved in a molten magma, from which it separated at an early period of consolidation, as illustrated by the experiments of J. Morozewicz. Corundum occurs also in Canada in an igneous rock, a nepheline-syenite, associated with Laurentian gneiss. Important deposits were discovered by the Geological Survey in 1896, in Hastings county, Ontario; and corundum is now worked there and in Renfrew county. New South Wales, Queensland and Victoria are other localities for corundum. The mineral is found also in the Urals and the Ilmen Mountains, in the Alps (in dolomite), in the basalts of the Rhine, and indeed as a subordinate rock-constituent corundum seems to enjoy a wide distribution, being found even in the British Isles.
See Joseph Hyde Pratt, "Corundum and its Occurrence and Distribution in the United States," Bulletin U.S. Geol. Surv., No. 269 (1906); T. H. Holland, Economic Geology of India (2nd ed.), part i. (1898). (F. W. R.*)