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Pyroxene Mineral

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Pyroxenes are a group of important rock-forming silicate minerals which consist of elements like calcium, magnesium, sodium, or iron (ll) which predominate. The word pyroxene has been derived from the Greek pyro which means fire, and xenos which means stranger. Pyroxenes were called so because they were present in volcanic lavas where they were found in the form of crystals embedded in volcanic glass. Assuming they were impurities in the glass, this name (fire strangers) was given. Although, they are merely just minerals that form when the process begins and crystallize before the lava erupts. The upper mantle of the Earth is mostly made up of olivine and pyroxene. Pyroxene and feldspar are also one of the main minerals in basalt, andesite and gabbro.

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Pyroxene Structure and Its Properties

Pyroxenes are the most abundant and one of the most important and significant rock-forming ferromagnesian silicates. We can find them in almost every variety of igneous rocks, and they also occur in rocks of variable compositions which have been formed under regional and contact metamorphosis. Pyroxenes are generally found to vary from dark green to black. They can also be present in various shades ranging from dark green to apple-green or from lilac to colorless depending on their chemical composition. Specific gravity values of pyroxenes usually lie between 3 to 4. Pyroxenes are similar to amphiboles in color, lustre, and hardness but due to the absence of hydroxyls, pyroxenes have a moderately higher density than amphiboles. These do not yield water when they are heated in a closed tube like amphiboles. Pyroxenes have two distinctive planes of cleavage with about 87°and 93°intersecting angles. 

Augites are vitreous on cleavage and crystal faces but dull on other surfaces. They are generally translucent to opaque and rarely transparent. They have a specific gravity ranging from 3.2 to 3.6. They are anisotropic in nature.

Pyroxene Chemical Formula

The names of the pyroxene minerals are fundamentally based on their chemical composition. The general formula which represents the chemical composition of the pyroxene group is given by XYZ2O6 in which X can be Na+, Ca2+, Mn2+, Fe2+, Mg2+, Li+, Y can be Mn2+, Fe2+, Mg2+, Fe3+, Al3+, Cr3+, Ti4+, and Z can be Si4+, Al3+. The range of the possible chemical substitutions in pyroxene is severely restricted by the sizes of the available sites in the structure and the charge present in its substituting cations. The sites of the 'X' cation are normally larger than the 'Y' cation sites. An extensive substitution takes place between the ideal end member compositions. For most pyroxenes, there is a limited substitution of aluminium for silicon in the Z (tetrahedral) site. When the charge differs in a substituting ion, electrical neutrality is kept by coupled substitutions. Like, for example, the pair that consists of Na+ and Al3+ substitutes for 2Mg2+.  

Chemical Composition

The most common pyroxenes usually constitute themselves as part of the chemical system CaSiO3 (known as wollastonite, which is a pyroxenoid), MgSiO3 (known as enstatite), and FeSiO3 (known as ferrosilite). Ferrous iron and magnesium freely take part in substitution because their ionic sizes are similar and their charges are identical. There is a complete substitution that exists between enstatite (Mg2Si2O6) and ferrosilite (Fe2Si2O6), and a complete solid solution of iron for magnesium exists between diopside (CaMgSi2O6) and hedenbergite (CaFeSi2O6). Augite, subcalcic augite, and pigeonite lie inside the interior of the pyroxene quadrilateral. Augite is similar to the members of the diopside-hedenbergite series in composition with limited substitution of Na for Ca, Al for Mg and Fe, and Ak for Si in the Z site. Augites with substantial aluminium or sodium are strictly not considered to be a part of the quadrilateral plane.

The coupled substitution that takes place of Na+ and Al3+ for 2 Mg2+ in enstatite yields pyroxene jadeite. The coupled substitution of Na+ and Fe3+ for 2 Mg2+ gives us pyroxene aegirine (acmite). The substitution that takes place of Li+ and Al3+ for 2 Mg2+ produces spodumene. Also, the substitution of Al3+ for Mg2+ and Al3+ for Si4+ results in the formation of an ideal substance called tschermakite component MgAlSiAlO6.

Some less common pyroxenes with their compositions outside the pyroxene quadrangle are johannsenite (CaMnSi2O6) and kosmochlor (NaCrSi2O6). Johannsenite requires manganese substitution for iron in hedenbergite. In the position of iron or aluminium, Kosmochlor has chromium. 

Did You Know?

  1. The word pyroxene has been derived from the Greek pyro which means fire, and xenos which means stranger.

  2. Pyroxene is one of the main minerals in basalt, andesite and gabbro.

FAQ (Frequently Asked Questions)

1. Describe How Pyroxenes Differ From Amphiboles.

Ans: The pyroxenes differ from the amphiboles in their composition in two respects. Although amphiboles are referred to as hydrous silicates, pyroxenes do not contain any essential water in any form in their structure. The second major difference in their chemical properties is the presence of the Asite in amphiboles which contains the large alkali elements, generally sodium and sometimes potassium. There is no presence of an equivalent site in pyroxenes to accommodate potassium. Due to the hydroxyl groups being present in amphiboles, there is a decrease in their thermal stability relative to the more refractory pyroxenes. Amphiboles decompose to form anhydrous minerals when kept at extreme temperatures.   

2. Discuss the Crystal Structure of the Pyroxenes.

Ans: The pyroxene group of minerals contains minerals that form both in the orthorhombic and monoclinic crystal systems. Orthorhombic pyroxenes are known as orthopyroxenes whereas monoclinic pyroxenes are known as clinopyroxenes. The key feature of all pyroxene structures is the silicon-oxygen tetrahedrons’ linkage by the sharing of two of the four corners to form continuous chains. The chains which extend in an indefinite manner parallel to the crystallographic axis have the composition of (SiO3)n. The SiO3 chains are chained to a given layer of octahedrally coordinated cation bands which in turn extend parallelly to the crystallographic axis. This octahedral layer consists of two unique cation sites.