Chromite,
a brownish black cubic mineral belonging to the spinel group, is the only ore
mineral from which metallic chromium and chromium compounds are obtained. It
has the chemical formula FeCr2O4, and a theoretical
composition of 32.0% FeO and 68.0% Cr2O3. In its natural
form it occurs in solid solution with other minerals in the spinel group, so
chrome contents are less than the theoretical 68%. The most important use of
chrome is in production of high-strength alloys and alloys which are heat,
abrasion, corrosion and oxidation resistant. Approximately half of all
chromite production goes into the manufacture of stainless steel. Chromium
chemicals are used for many purposes; pigments, photography and plating being
but a few. Chromite is used as a refractory in the production of steel,
copper, glass and cement.
Chromite occurs as a primary accessory mineral in basic and
ultrabasic igneous rocks. Economic deposits form by crystallisation of
chromite from a cooling magma, resulting in large stratiform deposits or
smaller pod-like deposits. The most significant occur in large, layered,
igneous intrusions in shield areas older than 1.9 billion years. The largest
and best known of these is the Bushveld Complex (Republic of South Africa),
which extends over 12 000 km2 and contains 60% of the world's known
chromite reserves. South Africa and Kazakhstan are the world's major
producers, accounting for two-thirds of the world's 12 Mt annual production.
The world annual production of chromite in 2000 was 14 400 Mt.
Australia's only significant chromite deposit, Coobina, is a stratiform
deposit in the Pilbara region of Western Australia, with probable reserves of
124 000 t at 39.3% Cr2O3. The mine produced 14 610 t of
ore for refractory brick manufacture during the 1950s. Since re-starting
operations in 1998, 227 000 t of chromite ore have been produced for export to
China.
South Australian Chromite
In South Australia, chromite was intersected in drillholes
south and southwest of Mount Christie in a zone of intermediate to ultramafic
rocks of Palaeoproterozoic age. The best intersection was 1.5 m at 9.8% Cr2O3.
Microprobe analysis showed the chromite to contain ~48% Cr2O3
and 30% FeO, which may only be suitable as chemical-grade chromite. Chemical
composition is typical of chromite found in layered ultramafic complexes.
Chromite has been identified within Mesoproterozoic Giles
Complex rocks in the far northwest of the State. It occurs as rare veins up to
10 mm thick within layers of pyroxenite in the Tomkinson Ranges (Coats, 1956)
and as grains in ultrabasic rocks of the Wingelinna Hills intrusions. A
representative sample gave an assay of 16.9% Cr2O3.
