Diopside

  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 and a specific gravity of 3.25 to 3.55. It is transparent to translucent with indices of refraction of n_α=1.663–1.699, n_β=1.671–1.705, and n_γ=1.693–1.728. The optic angle is 58° to 63°.

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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.

Diopside is a precursor of chrysotile (white asbestos) by hydrothermal alteration and magmatic differentiation;^[1] it can be artificially converted into chrysotile by heating at 600°C for three days.^[2] Some vermiculite deposits, most notably those in Libby, Montana, are contaminated with chrysotile (as well as other forms of asbestos) that formed from diopside.^[3]

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.

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.

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 and a wide variety of other locations.

Diopside was first described about 1800 and derives its name from the Greek dis, "twise", and òpsè, "face" in reference to the two ways of orienting the vertical prism.

References

• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, pp 403-404, ISBN 0-471-80580-7
• Mindat: Diopside
• Mindat: Chromian diopside, with locales
• Webmineral
• Mineral galleries
• Handbook of Mineralogy, Mineral Data Publishing, 2001, Diopside PDF version
• http://gemstone.org/gem-by-gem/english/diopside.html
• Greek-English-Greek dictionary