DIOPSIDE[GEM]
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Diopside is a monoclinic
pyroxene
mineral
with composition MgCaSi2O6. It forms complete solid
solution series with hedenbergite (FeCaSi2O6) and augite, and partial solid solutions with orthopyroxene and pigeonite.
It forms variably colored, but typically dull green crystals in the monoclinic
prismatic class. It has two distinct prismatic cleavages at 87 and 93° typical
of the pyroxene series. It has a Mohs hard
detailed
description
Diopside is a monoclinic pyroxene mineral with composition MgCaSi2O6.
It forms complete solid solution series with hedenbergite (FeCaSi2O6) and
augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but
typically dull green crystals in the monoclinic prismatic class. It has two distinct
prismatic cleavages at 87 and 93° typical of the pyroxene series. It has a Mohs hardness of six, a Vickers hardness of 7.7 GPa at a load of 0.98 N[4],
and a specific
gravity of 3.25 to 3.55.
It is transparent to translucent with indices of refraction of nα=1.663–1.699, nβ=1.671–1.705,
and nγ=1.693–1.728. The optic angle is 58° to 63°.
Formation
Diopside is found
in ultramafic (kimberlite and peridotite) igneous rocks, and diopside-rich augite is common
in mafic rocks, such as olivine basalt and andesite. Diopside is also found in a variety
of metamorphic rocks, such as in contact
metamorphosed skarns developed from high silica dolomites. It is an important mineral in the Earth's mantle and is common in peridotite xenoliths erupted in kimberlite and alkali basalt.
Mineralogy and occurrence
Diopside is a
precursor of chrysotile (white asbestos) by hydrothermal alteration and
magmatic differentiation;[5] it can react with hydrous solutions
of magnesium and chlorine to yield chrysotile by heating at
600°C for three days.[6] Some vermiculite deposits, most notably those in Libby, Montana, are contaminated with chrysotile (as
well as other forms of asbestos) that formed from diopside.[7]
At relatively
high temperatures, there is a miscibility gap between diopside and pigeonite, and at lower temperatures, between
diopside and orthopyroxene. The calcium/(calcium+magnesium+iron) ratio in diopside that formed with one of
these other two pyroxenes is particularly sensitive to temperature above 900°C,
and compositions of diopside in peridotite xenoliths have been important in
reconstructions of temperatures in the Earth's mantle.
Chrome diopside
((Ca,Na,Mg,Fe,Cr)2(Si,Al)2O6) is a common
constituent of peridotite xenoliths, and dispersed grains are
found near kimberlite pipes, and as such are a prospecting
indicator for diamonds. Occurrences are reported in Canada, South Africa, Russia, Brazil, and a wide variety of other locations.
As a gem
Gemstone quality diopside is found in two
forms: the black star diopside and the chrome diopside (which
includes chromium giving it a rich green colour). At
5.5–6.5 on the Mohs scale, chrome diopside is relatively soft
to scratch. Mohs scale of hardness does not measure tensile strength or
resistance to fracture.
Etymology and history
Diopside derives
its name from the Greek dis, "twice", and òpsè,
"face" in reference to the two ways of orienting the vertical prism.
Diopside was
first described about 1800.
Potential uses
Diopside based
ceramics and glass-ceramics have potential applications in various
technological areas. A diopside based glass-ceramic named 'silceram' was
produced by scientists from Imperial College, UK during 1980s from blast
furnace slag and other waste products. The as produced glass-ceramic is a
potential structural material. Similarly, diopside based ceramics and
glass-ceramics have potential applications in the field of biomaterials,
nuclear waste immobilization and sealing materials in solid oxide fuel cells.
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