Pyroxene: Structure and classification

Pyroxenes are the most significant and abundant group of rock-forming ferromagnesian silicates. They are found in almost every variety of igneous rock and also occur in rocks of widely different compositions formed under conditions of regional and contact metamorphism. The name pyroxene is derived from the Greek pyro, meaning “fire,” and xenos, meaning "stranger", and was given by Haüy to the greenish crystals found in many lavas which he considered to have been accidentally included there.

Pyroxenes have a basic structural unit consisting of linked SiO4 tetrahedra that each share 2 of their oxygens in such a way as to build long chains of SiO4. The basic structural group is thus Si2O6 with. Pyroxenes have a general structural formula of:

XYZ2O6

where X = Na+, Ca2+, Mn2+, Fe2+, or Mg2+ filling octahedral sites called M2
Y = Mn2+, Fe2+, Mg2+ , Al3+, Cr3+, or Ti4+ filling smaller octahedral sites called M1
Z = Si4+ or Al3+ in tetrahedral coordination.

The basic building blocks of the pyroxene structure are single chains of SiO4 tetrahedra. The chains are infinitely long and run parallel to the z-direction. The chain repeats after every two SiO4 tetrahedra. Two silicate chains are joined together via a ribbon of edge-sharing octahedral sites (labelled M1) Together, these make a structural unit often referred to as an "I-beam".

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Pyroxene silicate chains.



Viewed down the z-axis, we can see how these I-beams are joined together to form the rest of the structure. The bases of the I-beams are held together by cation sites labelled M2. The M2 sites are larger and more distorted than the M1 sites. The coordination number can vary from 6 to 8, depending on the specific structure and the size of the cation occupying it.

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(A) Schematic projection of the monoclinic pyroxene structure perpendicular to thec axis. (B) Control of cleavage angles by the I beams in the pyroxene structure.



The most important natural pyroxene minerals are formed from solid solutions involving Mg, Fe2+, and Ca. The range of compositions can be represented by the pyroxene quadrilateral. There are three important solid solutions contained within the quadrilateral: the orthopyroxene solid solution (often referred to as hypersthene), the pigeonite solid solution, and the augite solid solution.

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The nomenclature of Pyroxene



there is complete Mg-Fe substitution and small amounts of Ca substitution into the Orthopyroxene solid solution series. Mg-rich varieties of orthopyroxene are called hypersthene, whereas Fe-rich varieties are called Ferrosilite. There is also complete Mg-Fe solid solution between Diopside and Ferrohedenbergite, with some depletion in Ca. The pure Ca endmember Wollastonite (Ca2Si2O6) is not a pyroxene mineral. Although it has a structure based on chains of SiO4 tetrahedra, the chains have a three-tetrahedra repeat, rather than a two-tetrahedra repeat. Wollastanite belongs to a class of structures called the pyroxenoids.

The pyroxenes can be divided into several groups based on chemistry and crystallography:

• Clinopyroxenes are monoclinic pyroxenes and are either calcic or sodic. Clinopyroxenes include augite, diopside, pigeonite, hedenbergite, aegirine, jadeite and omphacite. Augite, pigeonite and diopside can contain exsolution lamellae of orthopyroxene if cooled slowly.

• Orthopyroxenes are orthorhombic pyroxenes. They are low-Ca ferromagnesian pyroxenes. The maximum birefringence of orthopyroxenes increases with Fe content. Orthopyroxenes consist of a range of compositions between enstatite - MgSiO3 and ferrosilite -FeSiO3

All pyroxenes show perfect (110) cleavage. When viewed looking down the c-crystallographic axis, the cleavages intersect at near 90° angles (the angles are actually 92 – 93° and 87 – 88°). This 90 degree cleavage angle is most useful in distinguishing pyroxenes from amphiboles (in amphiboles the cleavages are at 56° and 124°.

Bibliography



• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc
Stephen. A. Nelson