In jadeite and aegirine this is added by the inclusion of a +3 cation (aluminium and iron(III) respectively) on the Y site. The inclusion of sodium, which has a charge of +1, into the pyroxene implies the need for a mechanism to make up the "missing" positive charge.
Ternary diagram pyroxene series#
A second important series of pyroxene minerals are the sodium-rich pyroxenes, corresponding to the 'pyroxene triangle' nomenclature. Magnesium, calcium and iron are by no means the only cations that can occupy the X and Y sites in the pyroxene structure. A related mineral wollastonite has the formula of the hypothetical calcium end member but important structural differences mean that it is instead classified as a pyroxenoid. As the calcium ion cannot occupy the Y site, pyroxenes with more than 50 mol.% calcium are not possible. There is an arbitrary separation between augite and the diopside-hedenbergite ( CaMgSi 2O 6 – CaFeSi 2O 6) solid solution. This leads to a miscibility gap between pigeonite and augite compositions. There is not complete solid solution in calcium content and Mg-Fe-Ca pyroxenes with calcium contents between about 15 and 25 mol.% are not stable with respect to a pair of exolved crystals. Increasing the calcium content prevents the formation of the orthorhombic phases and pigeonite ( Si 2O 6) only crystallises in the monoclinic system. The enstatite-ferrosilite series ( SiO 3) includes the common rock-forming mineral Hypersthene, contains up to 5 mol.% calcium and exists in three polymorphs, orthorhombic orthoenstatite and protoenstatite and monoclinic clinoenstatite (and the ferrosilite equivalents). The names of the common calcium–iron–magnesium pyroxenes are defined in the 'pyroxene quadrilateral'. Pyroxene triangle nomenclature of the sodium pyroxenesĪ typical pyroxene has mostly silicon in the tetrahedral site and predominately ions with a charge of +2 in both the X and Y sites, giving the approximate formula XYT 2O 6. Pyroxene and feldspar are the major minerals in basalt, andesite, and gabbro rocks. The upper mantle of Earth is composed mainly of olivine and pyroxene minerals. However, they are simply early-forming minerals that crystallized before the lava erupted. 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'. The name pyroxene is derived from the Ancient Greek words for 'fire' ( pyr πυρ) and 'stranger' ( ksénos ξένος). Pyroxenes that crystallize in the monoclinic system are known as clinopyroxenes and those that crystallize in the orthorhombic system are known as orthopyroxenes. They share a common structure consisting of single chains of silica tetrahedra. Although aluminium substitutes extensively for silicon in silicates such as feldspars and amphiboles, the substitution occurs only to a limited extent in most pyroxenes. Pyroxenes have the general formula XY(Si,Al) 2O 6, 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). The pyroxenes (commonly abbreviated to Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxene ( diopside) crystals from Afghanistan