Pyroxene Group

Most important group of rock-forming ferromagnesian silicates whose major end members are enstatiteA mineral that is composed of Mg-rich pyroxene, MgSiO3. It is the magnesium endmember of the pyroxene silicate mineral series - enstatite (MgSiO3) to ferrosilite (FeSiO3). (MgSiO₃), ferrosiliteA mineral that is composed of Fe-rich pyroxene, FeSiO3. It is the iron endmember of the pyroxene silicate mineral series – enstatite (MgSiO3) to ferrosilite (FeSiO3). (FeSiO₃), and, compositionally, wollastonite (CaSiO₃). Note that wollastonite is considered a pyroxenoid and is the most common of the pyroxenoidsSingle-chain silicates like pyroxene, but the tetrahedra composing chains are rotated and twisted. Octahedrally coordinated cations occur between chains as in pyroxenes. The interval of repetition is different for each pyroxenoid (below). The twisting results in lower symmetry than pyroxenes (all pyroxenoids are triclinic) and a splintery cleavage and sometimes. Since pyroxenes found in nature or planetary settings rarely exceed 50 mol% Ca, the most useful part of the diagram is the “pyroxeneA class of silicate (SiO3) minerals that form a solid solution between iron and magnesium and can contain up to 50% calcium. Pyroxenes are important rock forming minerals and critical to understanding igneous processes. For more detailed information, please read the Pyroxene Group article found in the Meteoritics & Classification category. quadrilateral” where compositions are displayed in the lower half of the Mg₂Si₂O₆   Fe₂Si₂O₆   CaSiO₃ ternary. Solid solutions are dominated by Ca²⁺, Fe²⁺ and Mg²⁺. Enstatite and ferrosilite form almost complete solid solutionCompositional variation resulting from the substitution of one ion or ionic compound for another ion or ionic compound in an isostructural material. This results in a mineral structure with specific atomic sites occupied by two or more ions or ionic groups in variable proportions. Solid solutions can be complete (with (Mg²⁺ substitutes for Fe²⁺ up to about 90 mol%). Ferrosillite is rarely found in nature, because under most geological conditions it breaks down: Fe₂Si₂O₆ (ferrosilite) → Fe₂SiO₄ (fayalitePure* iron end-member (Fe2SiO4) of the olivine solid solution series and an important mineral in meteorites. When iron (Fe) is completely substituted by magnesium, it yields the the pure Mg-olivine end-member, forsterite (Mg2SiO4). The various Fe and Mg substitutions between these two end-members are described based on their forsteritic (Fo)) + SiO₂ (quartzComposed of SiO2, quartz is one of the silica group minerals most common in Earth's crust, but never found in meteorites as inclusions visible to the naked eye. Quartz in meteorites has been found in very small quantities in eucrites, other calcium-rich achondrites, and in the highly reduced E chondrites.). Meteoriticists usually state the composition of pyroxene present in a meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and based on its ferrosilite content which corresponds to the percentage of iron in the pyroxene. So, Fe_9.7 means the iron content in the pyroxene is 9.7%.  

Pyroxenes are divided into two crystallographic groups: Orthorhombic pyroxene called orthopyroxeneOrthorhombic, low-Ca pyroxene common in chondrites. Its compositional range runs from all Mg-rich enstatite, MgSiO3 to Fe-rich ferrosilite, FeSiO3. These end-members form an almost complete solid solution where Mg substitutes for Fe up to about 90 mol. % and Ca substitutes no more than ~5 mol. % (higher Ca contents occur (opx), and monoclinic pyroxene called clinopyroxene (cpx). Under certain conditions, pyroxenes can invert from a monoclinic crystal structureMutual arrangement of atoms, molecules or ions that are packed together in a crystal lattice to form a crystal. to an orthorhombic structure. For example, Mg-rich pigeonite that slowly cools from a high temperature will exsolve some of its calcium as augite lamellae thus lowering its calcium content and will then transform (invert) from monoclinic to an orthorhombic enstatite structure. When these augite lamellae are present, they indicate an igneous melt that cooled slowly. Note that in the past, intermediate pyroxene compositions had individual names (e.g., hypersthene, bronzite), but modern nomenclature rulings have disallowed these. Now, general names are used with modifiers to indicate unusual attributes (e.g. chromian diopside and titian augite).      

Sodic (Na-rich) pyroxenes are classified using a ternary diagram with quadrilateral components (Ca, Mg, Fe) at the apex and jadeite (NaAlSi₂O₆) and aegirine (NaFe³⁺Si₂O₆) at the bottom two corners. Though the sodic pyroxenes are occasionally found within meteorites, they are limited to the left side of the diagram and typically the result of a high shock event. “Jadeite occurs as the shocked product of albite feldsparAn alumino-silicate mineral containing a solid solution of calcium, sodium and potassium. Over half the Earth’s crust is composed of feldspars and due to their abundance, feldspars are used in the classification of igneous rocks. A more complete explanation can be found on the feldspar group page. in shocked meteorites, and is one of the most common high-pressure polymorphs in shock-melt veins of meteorites. The characteristic textures of jadeite in shocked ordinary chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such show that some of jadeite crystals were formed from originally albite feldspar by a solid state transformation and some were crystallized from a shock-induced albite melt” ¹. Jadeite was discovered in the shock-melt veins of the Chelyabinsk meteorite ².     Omphacite has been found as an accessory mineralConstituent mineral present in small quantity and not taken into account in identifying or classifying a rock.   in the following meteorites:

1. NWA 801 (CR2): Within clasts that formed under high-pressure conditions. ³
2. Château-Renard (L6): Within shock-melt veins. ⁴
3. Zagami (Martian shergottiteIgneous stony meteorite with a Martian origin consisting mainly of plagioclase (or a shocked glass of plagioclase composition) and pyroxene. They are the most abundant type of SNC meteorites and the type member is the Shergotty meteorite, which fell in India in 1865. Shergottites are igneous rocks of volcanic or): Within shock-melt veins. ⁵

Additional Information:

Terrestrial pyroxenes are found in maficOne of the two broad categories of silicate minerals, the other being felsic, based on its magnesium (Mg) and/or iron (Fe) content. Mafic indicates silicate minerals that are predominantly comprised of Mg and/or Fe.The term is derived from those major constituents: Magnesium + Ferrum (Latin for iron) + ic (having and ultramaficTerm used for silicate minerals with cations predominantly Mg and/or Fe. Mafic minerals are dominated by plagioclase and pyroxene, and also contain smaller amounts of olivine. igneous rocks and high-temperature granulite facies metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. rocks. The pyroxene, diopside also occurs in amphibolite facies calc-silicates and marbles; whereas, pigeonite is mainly restricted to meteorites and terrestrial volcanicIgneous rock that forms from cooling magma on the surface of a planet or asteroid. rocks. Na-rich pyroxenes occur in high pressure eclogite-facies metamorphism of mafic igneous rocks. Pyroxene transforms into a garnetMineral generally found in terrestrial metamorphic rocks, although igneous examples are not uncommon. Garnet is a significant reservoir of Al in the Earth's upper mantle. The garnet structure consists of isolated SiO4 tetrahedra bound to two cation sites. The A site holds relatively large divalent cations (Ca, Mg, Fe, Mn); the structure at high pressure to form majorite, (Mg^VIII)₃(Si^VI)₂(Si^IV)₃O₁₂; note that Si is octahedrally coordinated. This probably occurs at the transition zone (at ~400+ km depth). Pyroxenes are single chain silicates called inosilicates with a structure consisting of linked (SiO₄)^{4 –} tetrahedra. The structure of pyroxene should not be confused with the formula. Since one of the oxygens of one tetrahedra is shared with another tetrahedra, the formula for a minimum repeating unit in the chain consist of two tetrahedra is XY(Si₂O₆), where X and Y are cations. Pyroxenes have two distinct octahedral sites. There is a smaller relatively regular M1 site coordinated by oxygens of two opposing chains, yielding at tetrahedral-octahedral-tetrahedral (T-O-T) strip running parallel to the c-axis. The second is a larger irregular M2 site that serves to cross-link T-O-T strips.       The T-O-T strips, sometimes called “I-beams”, have very strong bonds that resist breaking and produce the typical near 90° {110} cleavage of pyroxenes (dashed lines below).                 Pyroxenes have the general formula of XYZ₂O₆, where X is the regular 6- to 8-fold M2 site, Y the distorted 6-fold M1 site, and Z is the tetrahedral site. M2 cations are normally larger than M1 cations. In the diopside-hedenbergite series, the M2 site is filled by Ca²⁺ (1.12 Å) and M1 site by randomly distributed Fe²⁺ (0.78 Å) and Mg²⁺ (0.72 Å). Site occupancies are as follows: on the tetrahedral Z site – Si⁴⁺, Al³⁺, Fe³⁺ (rare); on the octahedral Y site (M1) – Al³⁺, Fe³⁺, Ti⁴⁺, Cr³⁺, Mg²⁺, Fe²⁺, Mn²⁺; and on the octahredral X site (M2) – Mg²⁺, Fe²⁺, Mn²⁺, Ca²⁺, Li⁺, and Na⁺. Augite is closely related to diopside-hedenbergite series. Pigeonite has higher Ca contents than the orthorhombic enstatite-ferrosilite series. Non-quadrilateral components are accommodated by coupled substitutions. Three are important in augite, the most common terrestrial pyroxene: R²⁺(M2) + R²⁺(M1) → Na⁺(M2) + ½ Fe²⁺(M1) + ½ Ti⁴⁺(M1) yielding titanian augite; R²⁺(M1) + R⁴⁺(T) → R³⁺(M1) + R³⁺(T) yielding aluminous augite; and R²⁺(M2) + R²⁺(M1) → Na⁺(M2) + Fe³⁺(M1) yields aegirine augite.