Pyroxene means fire stranger, a name born from a geological misunderstanding
Early mineralogists found crystals sealed inside volcanic glass and assumed they were intruders, impurities in the melt. They named them pyroxenes, from Greek words for fire and stranger. In fact the crystals had formed first, before the lava ever erupted, and they turn out to be one of the building blocks of Earth's mantle.
Pyroxenes are a family of rock-forming minerals common in igneous and metamorphic rocks. Together with olivine they make up most of Earth's upper mantle, and together with feldspar they dominate basalt, andesite and gabbro. They are the most abundant of the single-chain silicates; the only other significant group of that kind, the pyroxenoids, is far rarer.
Their architecture explains a lot. Each silicon ion sits inside a tetrahedron of four oxygen ions and shares two of them with its neighbours, forming long chains. Because all the tetrahedra point the same way, pairs of chains lock together along their narrow faces around smaller metal ions, creating units mineralogists have compared to I-beams. Larger metal ions then bind these beams to one another. Those outer bonds are fairly weak, which gives pyroxene crystals their characteristic cleavage.
The general formula has two metal sites, labelled X and Y. The larger site can hold calcium, sodium, iron or magnesium, and occasionally zinc, manganese or lithium; the smaller one can take chromium, aluminium, titanium, vanadium and others. Such flexibility means pyroxenes are named mainly by chemistry. Crystal shape adds a second label: monoclinic forms are clinopyroxenes, orthorhombic ones orthopyroxenes. As of 1989, the International Mineralogical Association recognised twenty names and had retired 105 older ones.
Calcium content drives many of the distinctions. Iron-magnesium pyroxenes such as hypersthene hold only a little calcium. More calcium blocks the orthorhombic forms, and compositions between roughly 15 and 25 percent calcium are unstable, splitting into two separate crystals, which leaves a gap between pigeonite and augite. Because calcium cannot occupy the smaller site, no pyroxene can exceed 50 percent calcium. Wollastonite has the formula of that hypothetical end member but a different structure, so it counts as a pyroxenoid instead.
Source: Pyroxene
