Thepyroxenes (commonly abbreviatedPx) are a group of important rock-forminginosilicateminerals found in manyigneous andmetamorphicrocks. Pyroxenes have the general formulaXY(Si,Al)2O6,[1] where X represents ions ofcalcium (Ca),sodium (Na),iron (Fe(II)) ormagnesium (Mg) and more rarelyzinc,manganese orlithium, and Y represents ions of smaller size, such aschromium (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 asfeldspars andamphiboles, the substitution occurs only to a limited extent in most pyroxenes. They share a common structure consisting of single chains of silicatetrahedra. Pyroxenes that crystallize in themonoclinic system are known asclinopyroxenes and those that crystallize in theorthorhombic system are known asorthopyroxenes.
The namepyroxene is derived from theAncient Greek words for 'fire' (πυρ,pur) and 'stranger' (ξένος,xénos). Pyroxenes were so named due to their presence in volcaniclavas, where they are sometimes found ascrystals embedded involcanic glass; it was assumed they were impurities in the glass, hence the name meaning "fire stranger". However, they are simply early-forming minerals that crystallized before the lava erupted.
Theupper mantle of Earth is composed mainly ofolivine and pyroxene minerals. Pyroxene andfeldspar are the major minerals inbasalt,andesite, andgabbro rocks.[2][3]
Pyroxenes are the most common single-chain silicate minerals. (The only other important group of single-chain silicates, thepyroxenoids, 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.[4]
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 toI-beams. The I-beams interlock, with additional X cations bonding the outer faces of the I-beams to neighboring I-beams and providing the remaining charge balance. This binding is relatively weak and gives pyroxenes their characteristiccleavage.[4]
![A single chain of silicon tetrahedra viewed in the [100] direction](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aIno_a.jpg)
![A single chain of silica tetrahedra viewed in the [010] direction](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aIno_b.jpg)
The chain silicate structure of the pyroxenes offers much flexibility in the incorporation of variouscations and the names of the pyroxene minerals are primarily defined by their chemical composition. Pyroxene minerals are named according to the chemical species occupying the X (or M2) site, the Y (or M1) site, and the tetrahedral T site. Cations in Y (M1) site are closely bound to 6 oxygens in octahedral coordination. Cations in the X (M2) site can be coordinated with 6 to 8 oxygen atoms, depending on the cation size. As of 1989[update], twenty mineral names are recognised by the International Mineralogical Association's Commission on New Minerals and Mineral Names and 105 previously used names have been discarded.[5]
A 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 formulaXYT2O6. The names of the common calcium–iron–magnesium pyroxenes are defined in the 'pyroxene quadrilateral'. Theenstatite-ferrosilite series ([Mg,Fe]SiO3) includes the common rock-forming mineralhypersthene, contains up to 5 mol.% calcium and exists in three polymorphs,orthorhombic orthoenstatite and protoenstatite andmonoclinic clinoenstatite (and the ferrosilite equivalents). Increasing the calcium content prevents the formation of the orthorhombic phases andpigeonite ([Mg,Fe,Ca][Mg,Fe]Si2O6) only crystallises in the monoclinic system. 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. This leads to amiscibility gap between pigeonite andaugite compositions. There is an arbitrary separation between augite and thediopside-hedenbergite (CaMgSi2O6 − CaFeSi2O6) solid solution. The divide is taken at >45 mol.% Ca. As the calcium ion cannot occupy the Y site, pyroxenes with more than 50 mol.% calcium are not possible. A related mineralwollastonite has the formula of the hypothetical calcium end member (Ca2Si2O6) but important structural differences mean that it is instead classified as a pyroxenoid.
Magnesium, calcium and iron are by no means the only cations that can occupy the X and Y sites in the pyroxene structure. A second important series of pyroxene minerals are the sodium-rich pyroxenes, corresponding to the 'pyroxene triangle' nomenclature. 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. Injadeite andaegirine this is added by the inclusion of a +3 cation (aluminium and iron(III) respectively) on the Y site. Sodium pyroxenes with more than 20 mol.% calcium, magnesium or iron(II) components are known asomphacite andaegirine-augite. With 80% or more of these components the pyroxene is classified using the quadrilateral diagram.
A wide range of other cations that can be accommodated in the different sites of pyroxene structures.
| T | Si | Al | Fe3+ | ||||||||||||||
| Y | Al | Fe3+ | Ti4+ | Cr | V | Ti3+ | Zr | Sc | Zn | Mg | Fe2+ | Mn | |||||
| X | Mg | Fe2+ | Mn | Li | Ca | Na |
In assigning ions to sites, the basic rule is to work from left to right in this table, first assigning all silicon to the T site and then filling the site with the remaining aluminium and finally iron(III); extra aluminium or iron can be accommodated in the Y site and bulkier ions on the X site.
Not all the resulting mechanisms to achieve charge neutrality follow the sodium example above, and there are several alternative schemes:
In nature, more than one substitution may be found in the same mineral.
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