The range of possible chemical substitutions in pyroxene is constrained by the sizes of the available sites in the structure and the charge of the substituting cations. The chain silicate structure of the pyroxenes offers a good deal flexibility inside the incorporation of various cations and the names of the pyroxene minerals are ordinarily described by means of their chemical composition. At high temperatures, pyroxenes have more extensive fields of solid solution than they do at lower ones. The most common pyroxenes can be represented as part of the chemical system CaSiO3 (wollastonite, a pyroxenoid), MgSiO3 (enstatite), and FeSiO3 (ferrosilite). There are five principal combinations of exsolution pairs: (1) augite with enstatite lamellae, (2) augite with pigeonite lamellae, (3) augite with both pigeonite and enstatite lamellae, (4) pigeonite with augite lamellae, and (5) enstatite with augite lamellae. Pyroxene in basal section and isotropic laucite crystals in a basanite. Coupled substitutions involving Na+, Li+, or Al3+ for Mg2+in the enstatite structure yield pyroxenes that lie outside the quadrilateral compositional field. Pyroxenes crystallize in both the orthorhombic and monoclinic crystal systems. Complete substitution exists between enstatite (Mg2Si2O6) and ferrosilite (Fe2Si2O6), and complete solid solution of iron for magnesium exists between diopside (CaMgSi2O6) and hedenbergite (CaFeSi2O6). Hydroxyl groups in the amphibole structure decrease their thermal stability relative to the more refractory pyroxenes. 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. Consequently, as temperatures decrease, the pyroxene adjusts its composition in the solid state by exsolving a separate phase in the form of lamellae within the host pyroxene grain. Pyroxenes have the general formula XY(Si,Al)2O6, where X represents calcium, sodium, iron (II) or magnesium and more rarely zinc, manganese or lithium, and Y represents ions of smaller size, such as chromium, aluminium, iron (III), magnesium, cobalt, manganese, scandium, titanium, vanadiumor even iron (II). They are chemically analogous to the amphiboles except that, as discussed above, hydroxyls are absent in the pyroxene structure. Pyroxenes contain no essential water in the form of hydroxyls in their structure, whereas amphiboles are considered to be hydrous silicates. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. The chemical composition of minerals of the pyroxene group can be expressed by the general formula XYZ2O6, in which X= Na+, Ca2+, Mn2+, Fe2+, Mg2+, Li+; Y= Mn2+, Fe2+, Mg2+, Fe3+, Al3+, Cr3+, Ti4+; and Z= Si4+, Al3+. The pyroxenes differ compositionally from the amphiboles in two major respects. Although aluminium substitutes extensively for silicon in s… 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. Substitution of Li+ and Al3+ for 2 Mg2+ yields spodumene. Pyroxene quadrilateral nomenclature of the calcium, magnesium, iron pyroxenes. This leads to a miscibility gap between pigeonite and augite compositions. Cations in the X (M2) site can be coordinated with 6 to 8 oxygen atoms, depending on the cation size. 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. This leads to, The Tschermak substitution where a 3+ ion occupies the Y site and a T site leading to, Eulite, intermediate between Hypersthene and Ferrosilite, N. Morimoto, J. Fabries, A. K. Ferguson, I. V. Ginzburg, M. Ross, F. A. Seifeit and J.Zussman. Cations in Y (M1) web site are intently bound to 6 oxygens in octahedral coordination. Euhedral crystals will be Save 50% off a Britannica Premium subscription and gain access to exclusive content. A second important series of pyroxene minerals are the sodium-rich pyroxenes, corresponding to the 'pyroxene triangle' nomenclature. They are similar in colour, lustre, and hardness to the amphiboles but have slightly higher densities owing to the absence of hydroxyls. Pyroxenes that crystallize in the monoclinic system are known as clinopyroxenes and those that crystallize in the orthorhombic system are known as orthopyroxenes. 1989. The lamellae are exsolved along specific crystallographic directions, producing oriented intergrowths with parallel and herringbone texture. Magnesium, calcium and iron are by no means the only cations that can occupy the X and Y sites in the pyroxene structure. A wide range of other cations that can be accommodated in the different sites of pyroxene structures. Sodium pyroxenes with more than 20 mol.% calcium, magnesium or iron(II) components are known as omphacite and aegirine-augite, with 80% or more of these components the pyroxene falls in the quadrilateral. Increasing the calcium content prevents the formation of the orthorhombic phases and pigeonite ([Mg,Fe,Ca][Mg,Fe]Si2O6) only crystallises in the monoclinic system. Please select which sections you would like to print: Corrections? The coupled substitution of Na+ and Fe3+ for 2 Mg2+ produces the pyroxene aegirine (acmite). Omissions? 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. 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 formula XYT2O6. Cations in Y (M1) site are closely bound to 6 oxygens in octahedral coordination. As the calcium ion cannot occupy the Y site, pyroxenes with more than 50 mol.% calcium are not possible. High positive relief. Since no true pyroxenes exist with calcium contents greater than that of the diopside-hedenbergite join, the part of this system below this join is known as the pyroxene quadrilateral. In contrast, amphibole cleavage fragments (including hornblende minerals) exhibit a wedge-shaped cross-section, with cleavage faces meeting at 56o and 124o.
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