Everything about Corundum totally explained
Corundum (from
Tamil kurundam குருந்தம் or
kuruvindam குருவிந்தம்) is a
crystalline form of
aluminium oxide and one of the
rock-forming
minerals. It is naturally clear, 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. A pinkish-orange sapphire is called
padparadscha.
Due to corundum's hardness (pure corundum is defined to have 9.0
Mohs), it can scratch almost every other mineral, leaving behind a streak of white on the 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
Mohs hardness near 8.0.
In addition to its hardness, corundum is unusual for its high
density of 4.02 g/cm³, which is very high for a transparent mineral composed of the low
atomic mass elements
aluminium and
oxygen.
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.
Corundum for abrasives is mined in
Zimbabwe,
Russia, and
India. Historically it was mined from deposits associated with
dunites in
North Carolina and from a
nepheline syenite in
Craigmont, Ontario.
Emery grade corundum is found on the
Greek island of
Naxos and near
Peekskill, New York. Abrasive corundum is synthetically manufactured from
bauxite.
Synthetic corundum
In
1837 Gaudin made the first synthetic rubies by fusing
alumina at a high temperature with a little chromium as a pigment. In
1847 Edelman 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 BaF
2 and Al
2O
3 with a little Chromium at temperatures above 2000°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
sapphire,
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
watch crystals and windows for optical equipment, spacecraft as well as for
lasers.
Further Information
Get more info on 'Corundum'.
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