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Kathy Feick
The pyroxene group is a large group of single chain silicate minerals with the general formula ABSi2O6. The group gets its name from the Greek words for Fire and Silver. It is believed that they were named this way because of their presence in volcanic lavas, where they are sometimes seen as crystals embedded in volcanic glass. This is because they are early-forming members that crystallized before the lava erupted.
Occurrence:
Pyroxenes are common in meteorites and in extrusive igneous rocks, especially basalt. Pyroxenes are the most significant and abundant group of rock-forming ferromagnesium silicates. They are found in almost every variety of igneous rock. They also form under conditions of regional and contact metamorphism. Their conditions of formation are almost exclusively restricted to environments of high temperature, high pressure, or both. Pyroxenes are believed to be significant components of the Earth’s mantle.
Because of their susceptibility to chemical and mechanical weathering, pyroxenes are a rare constituent of sedimentary rocks.
Pyroxene image source: http://csmres.jmu.edu/geollab/fichter/RockMin/pyrox-70.html
Identification:
Pyroxenes typically occur as stubby prismatic crystals. They are generally dark green to black in colour, though they can range from apple-green and lilac to colourless, depending on their chemical composition. Pyroxenes have a hardness between 5 and 7. Finally, a key diagnostic feature is that pyroxenes have two cleavages at roughly 90°.
Minerals in this group:
Clinopyroxenes (monoclinic)
- Aegirine (sodium iron silicate)
- Augite (calcium sodium magnesium iron aluminum silicate)
- Clinoenstatite (magnesium silicate)
- Diopside (calcium magnesium silicate)
- Essenite (calcium iron aluminum silicate)
- Hedenbergite (calcium iron silicate)
- Hypersthene (magnesium iron silicate)
- Jadeite (sodium aluminum silicate)
- Jervisite (sodium calcium iron scandium magnesium silicate)
- Johannsenite (calcium manganese silicate)
- Kanoite (manganese magnesium silicate)
- Kosmochlor (sodium chromium silicate)
- Namansilite (sodium manganese silicate)
- Natalyite (sodium vanadium chromium silicate)
- Omphacite (calcium sodium magnesium iron aluminum silicate)
- Petedunnite (calcium zinc manganese iron magnesium silicate)
- Pigeonite (calcium magnesium iron silicate)
- Spodumene (lithium aluminum silicate)
Orthopyroxenes (orthorhombic)
- Donpeacorite (manganese Magnesium silicate)
- Enstatite (magnesium silicate)
- Ferrosilite (iron magnesium silicate)
- Nchwaningite (hydrated Manganese silicate)