Corundum is a crystalline form of aluminium oxide (Al
2O 3) with traces of iron, titanium and chromium.
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 Tamil word Kuruvindam or Sanskrit word Kuruvinda
meaning ruby. Because of corundum's hardness, 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.
Geology and occurrence
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. It commonly occurs as a detrital mineral in stream and beach
sands because of its hardness and resistance to weathering. The largest documented single
crystal of corundum measured about 65×40×40 cm, and weighed 152 kg. The record has since
been surpassed by certain synthetic boules. 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. Emery
grade corundum is found on the Greek island of Naxos and near Peekskill, New York, USA.
Abrasive corundum is synthetically manufactured from bauxite. 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. 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. 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
2O 3 with a little chromium at temperatures above
2,000 °C. 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, scratch-resistant optics, scratch-resistant watch crystals, instrument
windows for satellites and spacecraft, and laser components.
References