The Pyroxene Mineral Group Pyroxenes are the most significant and abundant group of rock-forming ferromagnesian silicates. 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.
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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).
The classification and nomenclature of the pyroxenes have been largely based on their crystal chemistry. In practice the chemical content of the pyroxene formula unit calculated to six oxygens, or to four cations (Vieten and Hamm, 1978), is essential for the classification.
Structure and Classification of Pyroxene The pyroxenes differ compositionally from the amphiboles in two major respects. Pyroxenes contain no essential water in the form of hydroxyls in their structure, whereas amphiboles are considered to be hydrous silicates.
Pyroxenes are classified as ferromagnesian minerals in allusion to their high content of magnesium and iron. Their conditions of formation are almost exclusively restricted to environments of high temperature, high pressure, or both.
Pyroxenes are single-chain silicates with the general formula XYSi2 O 6, where X and Y are either both divalent cations (mainly Ca, Fe, Mg), or mono- (Na, Li) and trivalent cations (Al, Fe) respectively.
The result of unmixing in the Ca-Mg-Fe-pyroxenes is commonly visble in thin section as set of thin exsolution lamellae. The miscibility of Ca-poor (pigeonite or orthopyroxene) and Ca-rich (augite or clinopyroxene) reduces with lowering temperature, and each may exolve the other. In the case of augite lamellae exsolving from pigeonite (or.
Pyroxenes differ from amphiboles because their structures involve single not double chains of silicate tetrahedra (pyroxenes alter to amphiboles under retrograde hydrous conditions) pyroxenes are anhydrous (no structural OH or H2O).
Pyroxenes are classified based on the occupancy of the M1 and M2 site. In orthopyroxenes the M2 site is usually octahedral and commonly contains Fe and Mg.
Meteoritic pyroxenes, while analogous in many respects to those from terrestrial rocks, show some unique features. Among these are the presence of minerals close to MgSiO 3 in composition, the common occurence of minerals belonging to the clinoenstatite-clinohypersthene series, the rarity of calcium-rich and aluminium-rich pyroxenes, and the absence of jadeitic varieties.
Pyroxenes and Amphiboles: The pyroxenes and amphiboles are two groups of ferromagnesian minerals (heavy group) the structure of which consists of long chains of linked silica tetrahedra. The pyroxenes consist of a single chain (2 oxygen shared in each tetrahedron) whereas amphiboles consist of a double chain (alternately 2 and 3 oxygen atoms shared successive tetrahedra).
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Pyroxenes are one of the most common silicate minerals found in mantle and crustal rocks. This chapter is a summarised presentation of the classification, structure, composition and most significant natural occurrences of these minerals, with examples of their use in the geosciences.
Crystal chemistry of silicate pyroxenes Reviews in Mineralogy and Geochemistry. Crystal chemistry of martian minerals from Bradbury Landing through Naukluft Plateau, Gale crater, Mars American Mineralogist. NEW MINERALS AND NOMENCLATURE MODIFICATIONS APPROVED IN 2017 European Journal of Mineralogy.
Enstatite: Enstatite is the most common silicate under the Orthopyroxene group in the larger classification of Pyroxene minerals (which are rock-forming silicates).Orthopyroxenes form a chemical series composed of the magnesium-rich enstatite, and the iron-containing bronzite and hypersthene. It has an orthorhombic crystal system and appears rarely as stubby, prismatic crystals, but more.
Pyroxene definition is - any of a group of igneous-rock-forming silicate minerals that contain calcium, sodium, magnesium, iron, or aluminum, usually occur in short prismatic crystals or massive form, are often laminated, and vary in color from white to dark green or black.
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Pyroxenes are minerals widely represented in volcanic, metamorphic and plutonic rocks. In spite of being major rock forming minerals, little interest was paid to this mineral group outside Earth Sciences, until the discovery of ferroic behavior in NaFeSi 2 O 6 aegirine pyroxene ().Subsequent investigation on several phases with pyroxene structure showed a fancy of different magnetic structures.
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Experimental determination of the phase boundaries in relevant systems, and the detailed determination of crystal structures, are together yielding increased understanding of the relationships observed among the naturally occurring pyroxenes. The relatively rigid configuration of a pyroxene chain is an important factor determining the structures which result when different cations are major.
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