The pyroxenes (commonly abbreviated to Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) or magnesium (Mg) and more rarely zinc, manganese or lithium, and Y represents ions of smaller size, such as chromium (Cr), aluminium (Al), magnesium (Mg), cobalt (Co), manganese (Mn), scandium (Sc), titanium (Ti), vanadium (V) or even iron (Fe II or Fe III). Although aluminium substitutes extensively for silicon in silicates such as feldspars and amphiboles, the substitution occurs only to a limited extent in most pyroxenes. They share a common structure consisting of single chains of silica tetrahedra. Pyroxenes that crystallize in the monoclinic system are known as clinopyroxenes and those that crystallize in the orthorhombic system are known as orthopyroxenes. The name pyroxene is derived from the Ancient Greek words for 'fire' ( ) and 'stranger' ( ). Pyroxenes were so named because of their presence in volcanic lavas, where they are sometimes found as crystals embedded in volcanic glass; it was assumed they were impurities in the glass, hence the name meaning 'fire-strangers'. However, they are simply early-forming minerals that crystallized before the lava erupted. The upper mantle of Earth is composed mainly of olivine and pyroxene minerals. Pyroxene and feldspar are the major minerals in basalt, andesite, and gabbro rocks. Structure Pyroxenes are the most common single-chain silicate minerals. (The only other important group of single-chain silicates, the pyroxenoids, are much less common.) Their structure consists of parallel chains of negatively-charged silica tetrahedra bonded together by metal cations. In other words, each silicon ion in a pyroxene crystal is surrounded by four oxygen ions forming a tetrahedron around the relatively small silicon ion. Each silicon ion shares two oxygen ions with neighboring silicon ions in the chain. The tetrahedra in the chain all face in the same direction, so that two oxygen ions are located on one face of the chain for every oxygen ion on the other face of the chain. The oxygen ions on the narrower face are described as apical oxygen ions. Pairs of chains are bound together on their apical sides by Y cations, with each Y cation surrounded by six oxygen ions. The resulting pairs of single chains have sometimes been likened to I-beams. The I-beams interlock, with additional X cations bonding the outer