Pyroxene Mineral
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.
[Image will be uploaded soon]
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?
The word pyroxene has been derived from the Greek pyro which means fire, and xenos which means stranger.
Pyroxene is one of the main minerals in basalt, andesite and gabbro.
FAQs on Pyroxene
1. What is pyroxene and where is it commonly found?
Pyroxene is a major group of rock-forming inosilicate minerals that are essential components in many igneous and metamorphic rocks. They are found globally in various geological settings, including:
Igneous Rocks: They are abundant in mafic and ultramafic rocks like basalt, gabbro, and peridotite, which are primary components of the Earth's oceanic crust and upper mantle.
Metamorphic Rocks: They are found in high-grade metamorphic rocks such as granulites and eclogites, which form under high temperature and pressure.
Extraterrestrial Rocks: Pyroxenes are also common minerals found in many meteorites and lunar rocks.
2. What is the general chemical formula for the pyroxene group of minerals?
The generalised chemical formula for the pyroxene group is XY(Si,Al)₂O₆. In this formula:
The X site is typically occupied by larger cations such as magnesium (Mg), iron (Fe²⁺), calcium (Ca), sodium (Na), or lithium (Li).
The Y site is occupied by smaller cations, most commonly magnesium (Mg), iron (Fe²⁺), aluminium (Al), manganese (Mn), and titanium (Ti).
The wide variety of elements that can substitute into these sites gives rise to the numerous different minerals within the pyroxene group.
3. How can you identify pyroxene based on its physical properties?
Pyroxene can be identified in hand specimens by observing a combination of its key physical properties:
Cleavage: This is the most diagnostic feature. Pyroxenes show two good cleavage planes that intersect at nearly 90 degrees (specifically 87° and 93°), giving the mineral a characteristically square or rectangular cross-section.
Hardness: They have a Mohs hardness ranging from 5 to 6.5, making them hard enough to scratch glass.
Colour: Typically, pyroxenes are dark green, brown, or black, with the colour deepening as the iron content increases.
Lustre: They generally exhibit a vitreous (glass-like) to dull lustre on their crystal and cleavage surfaces.
4. What are some common examples of pyroxene minerals?
The pyroxene group includes several important minerals, classified based on their chemistry and crystal structure. Common examples are:
Augite: The most common pyroxene, found in many igneous rocks like basalt and gabbro. Its formula is (Ca,Na)(Mg,Fe,Al)(Si,Al)₂O₆.
Enstatite-Ferrosilite Series: An important orthopyroxene series with magnesium and iron ((Mg,Fe)SiO₃), common in the Earth's mantle.
Diopside-Hedenbergite Series: A clinopyroxene series containing calcium, magnesium, and iron (Ca(Mg,Fe)Si₂O₆), often found in metamorphic rocks.
Jadeite: A sodium- and aluminium-rich pyroxene (NaAlSi₂O₆), which is one of the two minerals classified as true jade.
5. What are the key structural and chemical differences between pyroxenes and amphiboles?
Pyroxenes and amphiboles are often confused but can be distinguished by three fundamental differences:
Silicate Structure: Pyroxenes are single-chain silicates, where SiO₄ tetrahedra link to form one chain. Amphiboles are double-chain silicates, creating a more complex structure.
Chemical Composition: Amphiboles are hydrous minerals, containing essential hydroxyl (OH)⁻ ions in their structure. Pyroxenes are anhydrous, meaning they lack this water/hydroxyl component.
Cleavage Angle: The structural difference leads to different cleavage angles. Pyroxenes have two cleavage planes at nearly 90 degrees, while amphiboles have two planes at approximately 60° and 120°.
6. How does the single-chain silicate structure of pyroxene influence its properties?
The defining single-chain structure, where SiO₄ tetrahedra are linked end-to-end, directly controls the main physical properties of pyroxene minerals. This influence is seen in:
Cleavage: The bonds between the parallel silicate chains are weaker than the bonds within them. This creates two planes of weakness, resulting in the characteristic two-directional cleavage. The specific arrangement of these chains leads to the near-90-degree angle.
Crystal Habit: The structure often leads to pyroxene crystals being short, stout, or "stubby" prisms, in contrast to the more elongated or fibrous habits of double-chain silicates like amphiboles.
Thermal Stability: Being anhydrous, pyroxenes are stable at higher temperatures than their hydrous counterparts (amphiboles), which is why they crystallise earlier from magma and are common in high-temperature rocks.
7. Why are pyroxenes considered significant rock-forming minerals in geology?
Pyroxenes are significant in geology because they are primary constituents of the Earth's crust and upper mantle. Their importance lies in:
Petrologic Indicators: The specific type and chemical composition of a pyroxene mineral can reveal the temperature, pressure, and chemical environment under which its host rock formed. This makes them crucial tools for interpreting Earth's history.
Mantle Composition: They are a major component of peridotite, the dominant rock in the Earth's upper mantle. Studying pyroxenes is therefore essential for understanding mantle dynamics, plate tectonics, and the generation of magma.
Rock Classification: The presence and type of pyroxene are fundamental criteria used in the classification of most mafic and ultramafic igneous rocks.
8. How are pyroxenes classified into types like orthopyroxenes and clinopyroxenes?
Pyroxenes are classified into two main sub-groups based on their crystallographic symmetry, which in turn is dictated by the size of the cations in their chemical structure:
Orthopyroxenes (Opx): These minerals crystallise in the orthorhombic system. Their structure primarily accommodates smaller cations like Mg²⁺ and Fe²⁺. The enstatite-ferrosilite series is the main example.
Clinopyroxenes (Cpx): These minerals crystallise in the monoclinic system. Their more flexible structure can accommodate both small cations (like Mg²⁺, Fe²⁺) and larger cations (like Ca²⁺, Na⁺). This makes them more chemically diverse and includes common minerals like augite and diopside.
