Diopside

Hand Specimen identification: Association, form, two cleavages at 86° to each other, and color identify diopside . It may be confused with other pyroxenes or with hornblende, but the latter has two cleavages at 60° to each other. Diopside may contain substantial Fe, Al, or other impurities; exact composition cannot be determined without additional analytical data. Diopside is often white to green or pale green; augite tends to be darker green or black. Plates 6.2 and 6.3 contain color photos of diopside.

Chemical Composition : CaMgSi2O6

Hardness : 51 2 to 61 2

Specific Gravity3.2 to 3.5

Cleavage/Fracture : two perfect prismatic {110}/ uneven

Luster/Transparency : vitreous/transparent to translucent

Color : light green, variable

Streak : white, gray

Optical Properties : Diopside is colorless in thin section when Fe-free. With increasing iron content, it may become pleochroic in greens or browns. Higher birefringence and clined extinction distinguish clinopyrox-ene from orthopyroxene. Grains are prismatic or blocky, depending on orientation. Extinction angle, optic sign, and 2V help the different clinopyroxenes, but telling them apart may be diffi-cult. Biaxial 1+ 2, a = 1.665, b = 1.672, g = 1.695,d = 0.030, 2V = 56° to 62°.

Crystallography : Monoclinic, a = 9.7, b = 8.9, c = 5.25, b = 105.83°, Z = 4; space group C2/c; point group 2/m.

Habit : Prismatic crystals often have a square or octahedral cross section. Diopside may also be massive or finely disseminated. Polysynthetic twins are common but may not be visible without a microscope.

Structure and composition : The structure of diopside is similar to that of other pyroxenes. Chains of SiO4 tetrahedrarun parallel to the c-axis, with octahedral Ca and Mg connecting opposing chains to each other. Ca and Mg occupy different structural sites; Ca: Mg ratios are always … 1:1. A complete solid solution series exists between diopside, hedenbergite, CaFeSi O , and johannsenite, CaMn1Si2O62. Small amounts2of6Al, Mn, Na, Ti, and Cr may be present.

Occurrence and associations : Diopside is a very common pyroxene. It is found in mafic and ultramafic igneous rocks, associated with plagioclase, hornblende, and olivine. It is found in marbles associated with calcite, quartz or forsterite, tremolite, scapolite, and garnet. It is also found in medium- and high-grade metamorphosed mafic rocks.

Varieties : Chrome diopside is a chromium-rich variety known for its vivid green color.

Related Minerals : All pyroxenes are closely related. Hedenbergite, CaFeSi O , is the iron end member of the diopside 2 6 1 21 2 series. Augite, Ca,Mg,Fe,Na Mg,Fe,Al Si2O6, is a ted pyroxene with slightly different structure. Pigeonite is a high-temperature, subcalcic pyroxene with compositions that approach those of Fe-Mg diopside.

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Hand Specimen identification: Association, form, two cleavages at 86° to 87°, and color identify pyroxene, …

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