Pyroxene ternary diagram series#

The pyroxenes differ compositionally from the amphiboles in two major respects. 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. The lamellae are exsolved along specific crystallographic directions, producing oriented intergrowths with parallel and herringbone texture. 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. Kosmochlor has chromium (Cr) in place of iron or aluminum in a sodic pyroxene.ДЄt high temperatures, pyroxenes have more extensive fields of solid solution than they do at lower ones. Johannsenite involves the substitution of manganese for iron in hedenbergite. Other less common pyroxenes with compositions outside the pyroxene quadrilateral include johannsenite, and kosmochlor (ureyite). The substitution of Al 3+ for Mg 2+ and Al 3+ for Si 4+ yields the ideal tschermakite component MgAlSiAlO 6. Substitution of Li + and Al 3+ for 2Mg 2+ yields spodumene. The coupled substitution of Na + and Fe 3+ for 2Mg 2+ produces the pyroxene aegirine (acmite). The coupled substitution of Na+ and Al 3+ for 2Mg 2+ in enstatite produces the pyroxene jadeite. Monoclinic pigeonite encompasses a field of magnesium-iron solid solution with a slightly higher calcium content than the orthorhombic enstatite-orthoferrosilite series.Д¬oupled substitutions involving Na +, Li +, or Al 3+ for Mg 2+ in the enstatite structure yield pyroxenes that lie outside the quadrilateral compositional field. Augites with substantial aluminum or sodium cannot be strictly represented in the quadrilateral plane.

Pyroxene ternary diagram series#

Compositionally, augite is related to members of the diopside-hedenbergite series with limited substitution of Na+ for Ca 2+, Al 3+ for Mg 2+ and Fe 2+, and Al 3+ for Si 4+ in the Z (tetrahedral) site. Augite, subcalcic augite, and pigeonite lie within the interior of the pyroxene quadrilateral. 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.ДЇig.1: Ternary Composition Diagram for Pyroxenes.Д¬omplete substitution exists between enstatite (Mg 2Si 2O 6) and ferrosilite (Fe 2Si 2O 6), and complete solid solution of iron for magnesium exists between diopside (CaMgSi 2O 6) and hedenbergite (CaFeSi 2O 6). The most common pyroxenes can be represented as part of the chemical system CaSiO 3 (wollastonite, a pyroxenoid), MgSiO 3 (enstatite), and FeSiO 3 (ferrosilite). For example, the pair consisting of Na + and Al 3+ substitutes for 2Mg 2+. When a substituting ion differs in charge, electrical neutrality is maintained by coupled substitutions. Most pyroxenes have only limited substitution of aluminum for silicon in the Z (tetrahedral) site. Extensive atomic substitution occurs between the ideal end-member compositions. The X-cation sites in general are larger than the Y-cation sites. 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. TThe chemical composition of minerals of the pyroxene group can be expressed by the general formula: The name pyroxene is derived from the Greek pyro, meaning 'fire', and xenos, meaning 'stranger', and was given by HaГјy to the greenish crystals found in many lavas which he considered to have been accidentally included there. They are found in almost every variety of igneous rock and occur in rocks of widely different compositions formed under conditions of regional and contact metamorphism. Pyroxenes are the most significant and abundant group of rock-forming ferromagnesian silicates.