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Pyroxene Group

Ternary Composition Diagram for Pyroxenes. Image Source: Mendy Ouzillou (SkyFall Meteorites)

Most important group of rock-forming ferromagnesian silicates whose major end members are enstatite (MgSiO3), ferrosilite (FeSiO3), and, compositionally, wollastonite (CaSiO3). Note that wollastonite is considered a pyroxenoid and is the most common of the pyroxenoids.

Since pyroxenes found in nature or planetary settings rarely exceed 50 mol% Ca, the most useful part of the diagram is the “pyroxene 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 solution (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 (fayalite) + SiO2 (quartz).

Meteoriticists usually state the composition of pyroxene present in a meteorite based on its ferrosilite content which corresponds to the percentage of iron in the pyroxene. So, Fe9.7 means the iron content in the pyroxene is 9.7%.

Pyroxenes are divided into two crystallographic groups: Orthorhombic pyroxene called orthopyroxene (opx), and monoclinic pyroxene called clinopyroxene (cpx). 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 feldspar 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 chondrites 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 mineral 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 shergottite): Within shock-melt veins. 5

Additional Information:

3D Model of Single Chain Inosilicate (Pyroxene Si2O6). Modified from Source: wps.prenhall.com

Terrestrial pyroxenes are found in mafic and ultramafic igneous rocks and high-temperature granulite facies metamorphic rocks. The pyroxene, diopside also occurs in amphibolite facies calc-silicates and marbles; whereas, pigeonite is mainly restricted to meteorites and terrestrial volcanic rocks. Na-rich pyroxenes occur in high pressure eclogite-facies metamorphism of mafic igneous rocks. Pyroxene transforms into a garnet structure at high pressure to form majorite, (MgVIII)3(SiVI)2(SiIV)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.

Some or all content above used with permission from J. H. Wittke.