Diopside

Diopside is a mineral from the pyroxene group, with a monoclinic crystal structure and the chemical formula MgCaSi2O6. It can form a complete solid solution series with hedenbergite and augite, and partial solid solutions with orthopyroxene and pigeonite. Its crystals are typically a dull green, though colors vary, and they belong to the monoclinic prismatic class. Like other pyroxenes, it has two distinct prismatic cleavages at angles of 87° and 93°. It ranks six on the Mohs hardness scale, has a Vickers hardness of 7.7 GPa under a 0.98 N load, and a specific gravity ranging from 3.25 to 3.55. Diopside occurs in ultramafic igneous rocks like kimberlite and peridotite, and diopside-rich augite is common in mafic rocks such as olivine basalt and andesite. It is also found in metamorphic settings, particularly in contact metamorphosed skarns derived from high-silica dolomites. The mineral is significant in the Earth’s mantle and appears frequently in peridotite xenoliths brought up by kimberlite and alkali basalt eruptions. Through hydrothermal alteration and magmatic differentiation, diopside can act as a precursor to chrysotile (white asbestos); it may react with hydrous magnesium and chlorine solutions when heated to form chrysotile. Some vermiculite deposits, notably those near Libby, Montana, contain chrysotile and other asbestos forms that originated from diopside. At high temperatures, a miscibility gap exists between diopside and pigeonite, and at lower temperatures, between diopside and orthopyroxene. Chrome diopside, with the formula Ca(Mg,Cr)Si2O6, is a common component of peridotite xenoliths. Scattered grains near kimberlite pipes serve as a prospecting indicator for diamonds. It has been reported in Canada, South Africa, Russia, Brazil, and many other locations. In the United States, chromian diopside occurs in the serpentinite belt of northern California, in kimberlite in the Colorado-Wyoming State Line district and the Iron Mountain district o