A black to brownish black octahedral crystal of
the spinel group exhibiting a submetallic to metallic luster,
with the chemical formula (Fe,Mg)(Cr,Al)2O4. It forms a primary
accessory mineral in mafic and ultramafic igneous rocks
and may form economically important layers, or pods, in layered
intrusions and ophiolitic complexes, and it may also
accumulate in detrital deposits derived from erosion of
igneous parent rocks. Chromite is isomorphous with magnesiochromite
(MgCr2O4), and it is the principal ore of chromium.
In some deposits, chromite and related minerals
(magnesiochromite, magnetite, and hematite) are so concentrated
that they make up the bulk of the rock, which may
then be referred to as a chromitite.
Chromite ores that are associated with continental layered
intrusions tend to form stratiform bodies, with several
centimeters to a meter-thick layers of chromite interlayered
with dunite, pyroxenite, and other types of peridotite. The
layers may be continuous for many tens if not hundreds of
meters. In contrast, chromite deposits associated with ophiolitic
complexes tend to form discontinuous pod-shaped bodies
in dunite or harzburgite. These podiform chromites
frequently show uniform geological characteristics, including:
(1) lensoidal geometry, distributed along foliation; (2) rootless
folds with chromite bands; (3) unique magmatic textures
and structures, such as dunite envelopes for the chromite
ores, and nodular and orbicular textures of chromites; and
(4) strong plastic deformation associated with harzburgites.
The origin of the podiform chromitites is attributed to meltrock
reaction or dynamic magmatism within melt channels in
the upper mantle. The oldest known podiform chromite
deposits are associated with China’s 2.5-billion-year-old
Dongwanzi ophiolite complex.
See also CHINA’S DONGWANZI OPHIOLITE; OPHIOLITE.
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