Diopside is a pyroxene mineral with a chemical composition of MgCaSi2O6. It occurs in igneous and metamorphic rocks at many locations around the world.
Gem-quality crystals of diopside are faceted into attractive gemstones that are occasionally seen in commercial jewelry. Granular diopside can be easily cut and polished. When it has an attractive color, it is sometimes used as an ornamental stone.
Perhaps the most important use of diopside is its value as an indicator mineral in the search for diamonds. Trail-to-lode prospecting using diopside and other indicator minerals has found diamond deposits in Canada, the United States, Africa, and other locations.
Diopside has potential uses in the glass and ceramics industries, but the mineral usually occurs in accumulations that are too small or impure for effective mining.

Chromium Diopside

Geologic Occurrence of Diopside

The most common occurrence of diopside at Earth's surface is as a primary mineral in olivine-rich basalts and andesites. In these rocks it can be present in quantities of a few weight percent.
Diopside also forms during contact metamorphism of limestones and dolomites. Most of the crystalline diopside used to cut faceted gems and the granular diopside used as ornamental stone occurs in these carbonate deposits.

Diopside is much more abundant in Earth's mantle than at the surface. Evidence for this is diopside as a common mineral in ophiolites, and diopside as a common mineral in kimberlites and peridotites that were formed during deep-source volcanic eruptions.

Physical Properties

Physical Properties of Diopside

Chemical Classification Silicate.
Color Grayish white, light blue to purple, light green to vivid green, brown, black.
Streak White to light green.
Luster Vitreous, sugary, earthy.
Diaphaneity Opaque, translucent, transparent.
Cleavage Two distinct directions, at 87 and 93 degrees; imperfect; prismatic.
Mohs Hardness 5.5 to 6.5
Specific Gravity 3.2 to 3.5
Diagnostic Properties Cleavage, monoclinic crystal form.
Chemical Composition MgCaSi2O6
Crystal System Monoclinic.
Uses Gemstone, diamond indicator mineral, potential industrial use in ceramics.

Diopside as a Diamond Indicator Mineral

Most diamonds found at or near Earth's surface were delivered from the mantle during deep-source volcanic eruptions. These diamonds occur in vertical igneous structures known as pipes, which are often composed of kimberlite or peridotite.
These pipes are difficult to locate. Their surface exposure is usually covered with soil and vegetation, and it might be only a few acres in size. The pipes are often found by searching soils and sediments for mineral grains that are characteristic of the pipe but absent in local surface materials. Small particles of chromium-rich diopside are bright green in color, are often abundant in the pipes, and are easy to recognize in surface materials.

Geologists use these green diopside fragments to locate the pipes. They know that the fragments are liberated as the pipe weathers, then scattered by the actions of mass wasting, streams, and glaciers. When diopside fragments are discovered, the geologist knows that they originated up-slope, up-stream, or up-ice from the location in which they were found.
A trail of diopside fragments can lead the geologist to the pipe from which they were weathered. This activity, known as "trail-to-lode" prospecting, finds many diamond pipes and an even larger number of pipes without diamonds.

Sources: Geology.com

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