Pyroxenes are a group of dark-colored rock-forming minerals found in igneous and metamorphic rocks throughout the world. They form under conditions of high temperature and/or high pressure.
Augite, diopside, jadeite and spodumene are four of the best-known members of the pyroxene group; however, the pyroxene group has many other members.
Pyroxene
Pyroxene minerals are defined by their chemical composition and crystal structure. Their generalized chemical composition is described by the formula and comments below.
Spodumene often occurs in colorful gem-quality crystals. These beautiful blade-shaped crystals are highly prized by mineral collectors. When cut as gems, they have varietal names according to their color: kunzite (pink), hiddenite (green), and triphane (yellow).
Spodumene
Spodumene was once the most important ore of lithium metal – until the late 1900s, when subsurface brines with high concentrations of lithium were developed in Argentina, Chile, China and other locations. These brines can be pumped to the surface, allowed to evaporate, and lithium is easily processed from the evaporite material. Today, very little lithium is produce from spodumene.
Diopside
Diopside often occurs as a bright green, translucent to transparent gem that is cut into faceted stones, cabochons, and beads. Its green color is often bright enough that it is used as a substitute for emerald. A rare variety of diopside known as «star diopside» contains fine prismatic crystals of magnetite. When polished en cabochon, they display a silver four-ray star. The property of displaying a star is known as «asterism».
Diopside
Diopside has potential uses in ceramics, glass-making, biomaterials, nuclear waste immobilization, and fuel cell technology. Unfortunately, natural diopside is rarely found in deposits that simultaneously have a size, purity, and location that allows economic mining. This makes synthetic diopside cost-competitive with diopside produced by mining.
In Earth’s crust, pyroxenes are found in a wide range of igneous and metamorphic rocks. They are most abundant in the dark-colored igneous rocks, such as basalt and gabbro, that comprise most of the oceanic crust. Pyroxenes and plagioclase feldspar are the most abundand minerals in these rocks.
Pyroxenes are also important minerals in many peridotites. Rocks of the upper mantle are composed mainly of olivine and pyroxenes.
Extraterrestrial Occurrence of Pyroxenes
Pyroxene minerals are known beyond Earth. They are abundant in many stone meteorites and present in some stony-iron meteorites. They have also been identified in the rocks and regolith of the Moon and Mars.
When compared to terrestrial rocks, rocks of the Moon and Mars have simple mineral compositions. This is because they have not been significantly altered by chemical weathering. Their composition is dominated by plagioclase feldspar and pyroxene minerals.
Monoclinic and Orthorhombic Pyroxene
Pyroxene minerals crystallize in both the monoclinic and orthorhombic crystal systems. Monoclinic pyroxenes are known as «clinopyroxenes» and orthorhombic pyroxenes are known as «orthopyroxenes».
Pyroxenes can also be classified on the basis of their chemical composition. The classes include: magnesium-iron pyroxenes, calcium pyroxenes, calcium-sodium pyroxenes, sodium pyroxenes, and lithium pyroxenes.
Two lists of pyroxene minerals and their chemical compositions are presented below. One list contains clinopyroxenes, the other contains orthopyroxenes.
