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Pyroxene (diopside) crystals from Afghanistan

The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula XY(Si,Al)2O6,[1] where X represents ions of calcium (Ca), sodium (Na), iron (Fe(II)) or magnesium (Mg) and more rarely zinc, manganese or lithium, and Y represents ions of smaller size, such as chromium (Cr), aluminium (Al), magnesium (Mg), cobalt (Co), manganese (Mn), scandium (Sc), titanium (Ti), vanadium (V) or even iron (Fe(II) or Fe(III)). Although aluminium substitutes extensively for silicon in silicates such as feldspars and amphiboles, the substitution occurs only to a limited extent in most pyroxenes. They share a common structure consisting of single chains of silica tetrahedra. Pyroxenes that crystallize in the monoclinic system are known as clinopyroxenes and those that crystallize in the orthorhombic system are known as orthopyroxenes.

The name pyroxene is derived from the Ancient Greek words for 'fire' (πυρ, pur) and 'stranger' (ξένος, xénos). Pyroxenes were so named due to their presence in volcanic lavas, where they are sometimes found as crystals embedded in volcanic glass; it was assumed they were impurities in the glass, hence the name meaning "fire stranger". However, they are simply early-forming minerals that crystallized before the lava erupted.

The upper mantle of Earth is composed mainly of olivine and pyroxene minerals. Pyroxene and feldspar are the major minerals in basalt, andesite, and gabbro rocks.[2][3]

Structure

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Pyroxenes are the most common single-chain silicate minerals (the only other important group of single-chain silicates, the pyroxenoids, are much less common.) Their structure consists of parallel chains of negatively-charged silica tetrahedra bonded together by metal cations. In other words, each silicon ion in a pyroxene crystal is surrounded by four oxygen ions forming a tetrahedron around the relatively small silicon ion. Each silicon ion shares two oxygen ions with neighboring silicon ions in the chain.[4]

The tetrahedra in the chain all face in the same direction, so that two oxygen ions are located on one face of the chain for every oxygen ion on the other face of the chain. The oxygen ions on the narrower face are described as apical oxygen ions. Pairs of chains are bound together on their apical sides by Y cations, with each Y cation surrounded by six oxygen ions. The resulting pairs of single chains have sometimes been likened to I-beams. The I-beams interlock, with additional X cations bonding the outer faces of the I-beams to neighboring I-beams and providing the remaining charge balance. This binding is relatively weak and gives pyroxenes their characteristic cleavage.[4]

  • A single chain of silicon tetrahedra viewed in the [100] direction
    A single chain of silicon tetrahedra viewed in the [100] direction
  • A single chain of silica tetrahedra viewed in the [010] direction
    A single chain of silica tetrahedra viewed in the [010] direction
  • Structure of pyroxene looking along the silica chains. "I-beams" are outlined in green. Silicon ions are oversized to emphasize the silicon chains.
    Structure of pyroxene looking along the silica chains. "I-beams" are outlined in green. Silicon ions are oversized to emphasize the silicon chains.

Chemistry and nomenclature

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The chain silicate structure of the pyroxenes offers much flexibility in the incorporation of various cations and the names of the pyroxene minerals are primarily defined by their chemical composition. Pyroxene minerals are named according to the chemical species occupying the X (or M2) site, the Y (or M1) site, and the tetrahedral T site. Cations in Y (M1) site are closely bound to 6 oxygens in octahedral coordination. Cations in the X (M2) site can be coordinated with 6 to 8 oxygen atoms, depending on the cation size. As of 1989, twenty mineral names are recognised by the International Mineralogical Association's Commission on New Minerals and Mineral Names and 105 previously used names have been discarded.[5]

Pyroxene nomenclature
Pyroxene quadrilateral nomenclature of the calcium, magnesium, iron pyroxenes
Pyroxene triangle nomenclature of the sodium pyroxenes

A typical pyroxene has mostly silicon in the tetrahedral site and predominantly ions with a charge of +2 in both the X and Y sites, giving the approximate formula XYT2O6. The names of the common calcium–iron–magnesium pyroxenes are defined in the 'pyroxene quadrilateral'. The enstatite-ferrosilite series ([Mg,Fe]SiO3) includes the common rock-forming mineral hypersthene, contains up to 5 mol.% calcium and exists in three polymorphs, orthorhombic orthoenstatite and protoenstatite and monoclinic clinoenstatite (and the ferrosilite equivalents). Increasing the calcium content prevents the formation of the orthorhombic phases and pigeonite ([Mg,Fe,Ca][Mg,Fe]Si2O6) only crystallises in the monoclinic system. There is no complete solid solution in calcium content and Mg-Fe-Ca pyroxenes with calcium contents between about 15 and 25 mol.% are not stable with respect to a pair of exolved crystals. This leads to a miscibility gap between pigeonite and augite compositions. There is an arbitrary separation between augite and the diopside-hedenbergite (CaMgSi2O6−CaFeSi2O6) solid solution. The divide is taken at > 45 mol.% Ca. As the calcium ion cannot occupy the Y site, pyroxene with more than 50 mol.% calcium is not possible. A related mineral, wollastonite (CaSiO3), has the formula of the hypothetical calcium end member (Ca2Si2O6), but important structural differences mean that it is instead classified as a pyroxenoid.

Magnesium, calcium and iron are by no means the only cations that can occupy the X and Y sites in the pyroxene structure. A second important series of pyroxene minerals is the sodium-rich pyroxenes, corresponding to the 'pyroxene triangle' nomenclature. The inclusion of sodium, which has a charge of 1+, into the pyroxene implies the need for a mechanism to make up the "missing" positive charge. In jadeite and aegirine, this is added by the inclusion of a 3+ cation (aluminium and iron(III), respectively) on the Y site. Sodium pyroxenes with more than 20 mol.% calcium, magnesium or iron(II) components are known as omphacite and aegirine-augite. With 80% or more of these components, the pyroxene is classified using the quadrilateral diagram.

First X-ray diffraction view of Martian soil – CheMin analysis reveals feldspar, pyroxenes, olivine and more (Curiosity rover at "Rocknest")[6]

A wide range of other cations can be accommodated in the different sites of pyroxene structures.

Order of cation occupation in the pyroxenes
T Si Al Fe3+
Y Al Fe3+ Ti4+ Cr V Ti3+ Zr Sc Zn Mg Fe2+ Mn
X Mg Fe2+ Mn Li Ca Na

In assigning ions to sites, the basic rule is to work from left to right in this table, first assigning all silicon to the T site and then filling the site with the remaining aluminium and finally iron(III); extra aluminium or iron can be accommodated in the Y site and bulkier ions on the X site.

Not all the resulting mechanisms to achieve charge neutrality follow the sodium example above, and there are several alternative schemes:

  1. Coupled substitutions of 1+ and 3+ ions on the X and Y sites respectively. For example, Na and Al give the jadeite (NaAlSi2O6) composition.
  2. Coupled substitution of a 1+ ion on the X site and a mixture of equal numbers of 2+ and 4+ ions on the Y site. This leads to e.g., NaFe2+0.5Ti4+0.5Si2O6.
  3. The Tschermak substitution where a 3+ ion occupies the Y site and a T site, leading to e.g., CaAlAlSiO6.

In nature, more than one substitution may be found in the same mineral.

Pyroxene minerals

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A thin section of green pyroxene
Mantle-peridotite xenolith from San Carlos Indian Reservation, Gila Co., Arizona, USA. The xenolith is dominated by green peridot olivine, together with black orthopyroxene and spinel crystals, and rare grass-green diopside grains. The fine-grained gray rock in this image is the host basalt.(unknown scale).
A sample of pyroxenite (meteorite ALH84001 from Mars), a rock consisting mostly of pyroxene minerals

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Pyroxene

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Pyroxene is a group of dark-colored, rock-forming inosilicate minerals that are fundamental components of many igneous and metamorphic rocks, characterized by a single-chain tetrahedral structure and the general XY(Si,Al)2O6, where X and Y sites are occupied by cations such as Ca, Na, Mg, Fe, Al, and others. These minerals typically exhibit prismatic or stubby crystal habits and are distinguished by their cleavage in two directions nearly at right angles, with a Mohs ranging from 5 to 7 and specific between 3 and 4. Pyroxenes are divided into two main subgroups based on crystal symmetry: orthopyroxenes (orthorhombic, e.g., Mg2Si2O6 and ferrosilite Fe2Si2O6) and clinopyroxenes (monoclinic, e.g., CaMgSi2O6, , and NaAlSi2O6), with compositions often plotted on a diagram reflecting series between these end-members. Their colors vary from black and green to rarer hues like lilac in , and they form under high-temperature and/or high-pressure conditions, making them key indicators of magmatic and metamorphic environments. Pyroxenes are abundant in mafic and ultramafic rocks such as , , , and the Earth's and , where they can constitute up to 50% of the mineral assemblage, while occurring as accessories in intermediate rocks like and , and rarely in granites or sedimentary deposits due to their susceptibility to . In metamorphic settings, they appear in eclogites, amphibolites, and granulites, often serving as geothermometers through the analysis of orthopyroxene-clinopyroxene solvus or exsolution textures that record cooling histories. Notable varieties include , prized for its durability in gemstones and carvings, and , a source of used in ceramics, , and formerly as an . Overall, pyroxenes play a critical role in understanding , appearing in meteorites and lunar samples, and their compositional variability provides insights into mantle processes and evolution.

Overview and Properties

Definition and General Characteristics

Pyroxenes are a group of important rock-forming inosilicate minerals commonly found in igneous and metamorphic rocks. They are characterized by a single-chain structure and have the general XY(Si,Al)2O6XY(\mathrm{Si,Al})_2\mathrm{O}_6, where X typically represents Ca, Na, Fe²⁺, or Mg, and Y represents Cr, Al, Fe³⁺, or Mg. This composition allows for extensive series, making pyroxenes key indicators of evolution and metamorphic conditions. Physically, pyroxenes exhibit a hardness of 5 to 7 on the , a specific ranging from 3.0 to 3.6, and a vitreous luster. They commonly occur as prismatic crystals or in granular masses, often displaying colors from dark green to black, though lighter varieties exist. A distinctive feature is their two-directional cleavage, intersecting at angles of approximately 87° and 93°, which produces nearly rectangular fragments. The name "pyroxene" originates from the Greek words pyr (fire) and xenos (stranger), coined in 1796 by René Just Haüy to describe crystals found in volcanic lavas, initially thought to be foreign inclusions not formed by igneous processes. Representative end-member compositions include (MgSiO3\mathrm{MgSiO_3}), an orthopyroxene rich in magnesium, and (CaMgSi2O6\mathrm{CaMgSi_2O_6}), a calcic clinopyroxene.

Geological Significance

Pyroxenes are among the most abundant rock-forming s on , comprising approximately 11% of the crust by volume and serving as key components in and ultramafic igneous rocks, where they can constitute up to 50% of the mineral assemblage. In the , pyroxenes are a dominant phase after in s, typically making up 20-40% of the mineral content and playing a critical role in the composition of mantle-derived rocks. In , pyroxenes are invaluable for reconstructing geological conditions, acting as indicators of , , and magma composition through exchange reactions between coexisting orthopyroxene and clinopyroxene. Single-pyroxene thermobarometry, based on subsolidus phase relations in systems like CaO-MgO-SiO2, enables precise estimates of equilibration conditions in mantle xenoliths and ophiolites, with applications in geothermometry yielding temperatures from 800-1200°C and geobarometry pressures up to 30 kbar. These methods, refined through experimental and thermodynamic evaluations, provide insights into mantle dynamics and metamorphic histories without requiring multiple pairs. Economically, pyroxenes contribute to industrial applications primarily through their host rocks, serving as sources of magnesium, calcium, and iron in fluxing agents for production and as aggregates in construction materials like and black for , tile, and facing. deposits, rich in these elements, are utilized in blast furnaces to enhance and reduce impurities, supporting the iron and industry, though pure pyroxene minerals themselves have limited direct economic value.

Crystal Structure

Silicate Framework

Pyroxenes are classified as inosilicates, characterized by a fundamental framework consisting of single chains of silica tetrahedra. Each SiO₄ tetrahedron in the chain shares two of its oxygen atoms with adjacent tetrahedra at their corners, creating infinite linear chains with the repeating unit [Si₂O₆]⁴⁻ that extend parallel to the crystallographic c-axis. These chains form the backbone of the pyroxene structure, providing rigidity along the chain direction due to the strong covalent Si-O bonds within the tetrahedra and between them. The linkage of tetrahedra results in a zigzag arrangement along the chain, with a repeat of approximately 5.3 for every two tetrahedra, defining the periodicity of the . Adjacent chains in the lattice are oriented such that tetrahedra point in alternating directions (up and down), enhancing the overall stability of the framework. Pairs of these oppositely oriented chains associate to form rigid, -like motifs, where the parallel chains act as the flanges of the , connected along their length by bridging bonds; this configuration contributes to the mechanical strength of the network perpendicular to the chain direction. The weak ionic bonds between these I-beam units, rather than the strong bonds within the chains or motifs, give rise to the characteristic prismatic cleavage of pyroxenes, manifesting as two nearly perpendicular planes at angles of about 87° and 93°. This cleavage reflects the anisotropic nature of the framework, where disruptions preferentially occur along the inter-chain interfaces. Variations in chain kinking can influence polymorphism, but the core single-chain topology remains consistent across pyroxene types.

Cation Coordination and Polymorphism

In the pyroxene structure, cations occupy two primary non-tetrahedral sites: the M1 site and the M2 site, which integrate with the single-chain silicate framework to define the mineral's topology. The M1 site, also denoted as the Y site, is a smaller, nearly regular octahedral polyhedron coordinated by six oxygen anions, preferentially hosting divalent cations like Mg²⁺ and Fe²⁺ or trivalent Al³⁺ with ionic radii typically between 0.53 and 0.83 Å. Mean M1–O bond lengths range from approximately 2.00 to 2.15 Å, reflecting the site's geometric regularity and sensitivity to occupant size. In contrast, the M2 site, or X site, is larger and more distorted, accommodating larger cations such as Ca²⁺ and Na⁺ in irregular 6- to 8-fold coordination, with coordination number varying by cation radius—8-fold for the largest (e.g., Ca²⁺) and reducing to 6-fold for smaller ones like Mg²⁺ or Fe²⁺. This irregularity arises from the M2 polyhedron's proximity to the silicate chain kinks, leading to mean M2–O bond lengths of 2.47 to 2.57 Å for Na⁺-bearing varieties. Polymorphism in pyroxenes manifests as orthorhombic orthopyroxenes ( Pbca) versus monoclinic clinopyroxenes ( C2/c or P2₁/c), driven by differences in chain conformation. Orthopyroxenes, exemplified by , feature nearly straight chains with O3–O3–O3 angles approaching 180° and minimal rotation (approximately 0°), resulting in higher . Clinopyroxenes, such as , exhibit kinked chains with O3–O3–O3 angles of 136° to 170° and chain rotation angles of about 9°, which lowers the symmetry to monoclinic and accommodates larger M2 cations more effectively. These structural differences stem from the interplay between M1–M2 polyhedral dimensions and chain flexibility, as described in seminal models that emphasize topological parity rules for chain linkage. Stability fields for these polymorphs are governed by and composition, with orthopyroxenes favored in high- environments above approximately 1000°C under low-pressure conditions, particularly in low-calcium systems. Clinopyroxenes predominate at lower or in calcium-rich compositions, where the larger M2 site stabilizes the kinked structure; for instance, transitions from P2₁/c to C2/c forms occur around 725–960°C in Fe-Mg pyroxenes, decreasing with increasing iron content. Such phase boundaries reflect energetic preferences, with clinopyroxenes often metastable at ambient conditions in low-Ca systems due to kinetic barriers in chain reconfiguration. Variations in M1–M2 interpolyhedral distances and associated distortions further modulate pyroxene properties, including and mechanical . Shorter M1–M2 separations in orthopyroxenes enhance polyhedral packing efficiency, yielding lower densities compared to the more expanded clinopyroxene structures, while distortions in the M2 site—quantified by higher octahedral elongation—increase with cation size mismatch and influence lattice strain. These bond length adjustments, typically on the order of 0.1–0.2 between polymorphs, arise from differential and directly impact phase stability under varying pressure-temperature conditions. The general for pyroxenes, XY(Si,Al)₂O₆, underscores this site-specific cation distribution, with X at M2 and Y at M1.

Chemical Composition

Elemental Constituents and Formulas

Pyroxenes are a group of silicate minerals dominated by silicon and oxygen as the primary constituents, forming the tetrahedral silicate framework essential to their structure. The major cations include calcium (Ca), magnesium (Mg), and iron (Fe), which occupy octahedral coordination sites, while sodium (Na) and aluminum (Al) play significant roles in modifying the composition, particularly in certain end-members. Minor elements such as titanium (Ti), chromium (Cr), and manganese (Mn) are present in trace amounts, influencing optical and physical properties but not defining the core chemistry. The general formula for pyroxenes is XY(Si,Al)2O6XY(\mathrm{Si,Al})_2\mathrm{O}_6, where X typically accommodates larger divalent or monovalent cations like Ca2+^{2+}, Na+^+, Mg2+^{2+}, or Fe2+^{2+}, and Y hosts smaller trivalent or divalent cations such as Al3+^{3+}, Fe3+^{3+}, Mg2+^{2+}, or Fe2+^{2+}. This notation reflects the single-chain arrangement, with two tetrahedral sites per formula unit. Ideal end-member compositions illustrate the range of primary variations: orthopyroxene end-members include (MgSiO3\mathrm{MgSiO_3}) and ferrosilite (FeSiO3\mathrm{FeSiO_3}), while clinopyroxene end-members encompass (CaMgSi2O6\mathrm{CaMgSi_2O_6}), hedenbergite (CaFeSi2O6\mathrm{CaFeSi_2O_6}), and (NaAlSi2O6\mathrm{NaAlSi_2O_6}). These end-members represent pure stoichiometric forms, though pyroxenes often deviate slightly to minor ionic substitutions. Aluminum incorporates into both tetrahedral sites (substituting for Si) and octahedral sites (M1 and ), with the extent and primary site varying by pyroxene type; for example, tetrahedral substitution is modest in most, but octahedral Al dominates in sodic clinopyroxenes like . This replacement occurs to maintain structural stability and charge balance, distinguishing pyroxenes from other silicates like feldspars where tetrahedral Al is more prevalent. Compositional analysis of pyroxenes relies heavily on electron microprobe techniques, which enable high-spatial-resolution quantification of major elements like Si, O, Ca, Mg, Fe, Na, and Al, as well as minor components, by measuring emissions from targeted spots on polished samples. This method is standard in petrological studies for establishing precise elemental ratios and verifying adherence to the general formula.

Substitutions and Charge Balance

Pyroxenes exhibit significant chemical variability through ionic substitutions that maintain the overall charge balance and structural integrity of their single-chain framework. These substitutions primarily occur at the M1, , and tetrahedral (T) sites, where cations of similar size but different valence states replace one another, often in coupled pairs to preserve electroneutrality. For instance, the basic pyroxene formula M2M1T2O6 requires that the total positive charge from cations in the octahedral (M) and tetrahedral (T) sites balances the -12 charge from the oxygen anions. Coupled substitutions are essential mechanisms for this balance, allowing pyroxenes to incorporate diverse elements while minimizing lattice strain. The Tschermak substitution, for example, involves the replacement of Mg²⁺ in the M1 site and Si⁴⁺ in the T site by two Al³⁺ ions (MgSi ↔ AlAl), which maintains charge neutrality by introducing two +3 charges to replace one +2 and one +4. This is common in both ortho- and clinopyroxenes, enabling aluminum enrichment without destabilizing the structure. Similarly, the jadeite substitution couples Na⁺ in the M2 site with Al³⁺ in the M1 site to replace Ca²⁺ (M2) and Mg²⁺ (M1), as in (NaAlSi₂O₆) versus (CaMgSi₂O₆), where the +1 and +3 charges balance the two +2 charges. Another key example is the aegirine-augite substitution (NaFe³⁺ ↔ CaMg), which introduces Na⁺ and Fe³⁺ to substitute for Ca²⁺ and Mg²⁺, preserving charge through the +1/+3 pair replacing two +2 ions and facilitating sodium and ferric iron incorporation in alkaline environments. These coupled mechanisms, detailed in pyroxene standards, underscore the mineral's adaptability to varying geochemical conditions. Solid solution series in pyroxenes further illustrate the role of substitutions in compositional diversity, with charge balance achieved through isomorphic replacements of similar-sized ions. In orthopyroxenes, complete exists between the magnesium-rich (Mg₂Si₂O₆) and iron-rich ferrosilite (Fe₂Si₂O₆) end-members, driven by the Mg²⁺ ↔ Fe²⁺ substitution in both M1 and M2 sites, as their ionic radii (0.72 Å and 0.78 Å, respectively) allow extensive mixing without significant distortion. In Ca-rich clinopyroxenes, extensive occurs between the (CaMgSi₂O₆) and hedenbergite (CaFeSi₂O₆) end-members via Mg²⁺ ↔ Fe²⁺ in the M1 site; however, a at low temperatures restricts full miscibility and can lead to exsolution features. These series highlight how valence-equivalent substitutions enable broad chemical ranges while tetrahedral Si⁴⁺ remains dominant, with minor Al³⁺ incorporation balanced by octahedral adjustments. Variations in iron oxidation states, particularly the Fe²⁺/Fe³⁺ ratio, influence pyroxene properties through substitutions that affect charge balance and site occupancy. Fe²⁺ typically substitutes for Mg²⁺ in octahedral sites, but oxidation to Fe³⁺ requires coupling with other ions, such as Na⁺ (as in ) or Al³⁺, to offset the +1 charge increase; this can shift Fe³⁺ into M1 or M2 sites, altering lattice parameters. These ratios impact color, with Fe²⁺-Fe³⁺ intervalence charge transfer bands near 0.77 μm producing green to brown hues in terrestrial pyroxenes, and influence stability by favoring Fe³⁺-rich compositions in oxidized, alkaline magmas where higher valence states enhance compatibility with silica-poor melts. Analytically, pyroxene —oscillatory, sector, or normal—records these substitutions and reflects evolving conditions, providing insights into history. Compositional gradients, such as increasing Al or Fe³⁺ toward rims, indicate coupled substitutions responding to changes in , , or melt composition during growth, with rapid undercooling promoting sector via differential cation partitioning across crystal faces. Such , observable via electron microprobe, thus serves as a proxy for magmatic processes like recharge or differentiation.

Nomenclature and Classification

Historical Development

The term "pyroxene" was coined in by French mineralogist René Just Haüy to describe the monoclinic pyroxene mineral found in volcanic rocks, derived from the Greek words pyr (fire) and xenos (stranger), reflecting its unexpected presence in lava without melting during analysis. Initially applied specifically to what is now known as , the name soon encompassed a broader group of structurally similar silicates. In the early 19th century, German mineralogist Gustav Rose distinguished orthorhombic pyroxenes from the monoclinic varieties, recognizing their distinct crystal symmetry in analyses from 1835 onward, which laid the groundwork for separating species like and . This differentiation addressed early confusions in classifying pyroxenes based solely on optical or macroscopic properties, marking a shift toward crystallographic criteria. During the 1920s, Norwegian geochemist Victor Moritz Goldschmidt advanced pyroxene understanding through his foundational work on crystal chemistry, establishing rules for ionic substitutions and site preferences in structures, including the M1 and M2 cation sites in pyroxenes that influence behaviors. His principles, outlined in seminal publications like Die Gesetze der Krystallochemie (1926), explained how elements like Mg, Fe, Ca, and Al distribute within the pyroxene lattice, providing a theoretical basis for later refinements. By the mid-20th century, challenges arose from extensive solid solutions among pyroxene compositions, leading to overlapping names and inconsistent classifications for intermediate members, such as those blending , hedenbergite, and . These issues prompted interventions by the International Mineralogical Association (IMA), culminating in Morimoto et al.'s 1988 report formalizing 20 distinct species based on end-member compositions and structural types. Further expansions in the 1980s addressed Al-rich pyroxenes, incorporating additional species like omphacite and esseneite to account for complex substitutions while resolving historical ambiguities.

Modern IMA Standards

The modern standards for pyroxene and were established by the International Mineralogical Association (IMA) in 1988 through the report of its Subcommittee on Pyroxenes, which formalized 20 distinct grouped into six chemical subdivisions based on dominant cation occupancy in the M2 structural sites and overall crystal-chemical similarities. These subdivisions include Mg-Fe pyroxenes (e.g., , ferrosilite), Ca-Mg-Fe pyroxenes (e.g., , hedenbergite), Ca-Na pyroxenes (e.g., omphacite, -augite), Na pyroxenes (e.g., , ), Li-Al pyroxenes (e.g., ), and minor-element pyroxenes (e.g., johannsenite, kosmochlor). This framework emphasizes the general pyroxene formula M2M1T2O6, where site allocations determine identity, ensuring systematic identification amid extensive solid-solution series. Classification relies on graphical tools tailored to compositional ranges, such as the pyroxene for Ca-Mg-Fe-Mn-bearing varieties, which plots (Ca2Si2O6), (Mg2Si2O6), ferrosilite (Fe2Si2O6), and intermediate components to delineate orthopyroxene, pigeonite, , and diopside-hedenbergite trends. For sodic pyroxenes, ternary diagrams are employed, such as the (NaAlSi2O6)- (NaFe3+Si2O6)- projection, to classify Na-rich end-members and solid solutions. These diagrams facilitate precise plotting of electron microprobe analyses, accounting for charge balance via coupled substitutions like Na+Al3+ ↔ Ca2+Mg2+ in M2 and M1 sites, respectively. Naming conventions prioritize the dominant cations in the M1 and M2 octahedral sites, with end-member species names assigned when a component exceeds 50% in the relevant site occupancy; for example, designates Ca-dominant M2 and Mg-dominant M1. Minor elements are indicated by prefixes or superscripts, such as "Al" for aluminum content or numerical subscripts for variable ratios (e.g., esseneite as Ca(Fe3+,Al)AlSiO6 with Fe3+-Al3+ disorder). Polysynthetic names like "aegirine-augite" denote intermediate compositions along specific joins, while discarded historical names (105 in total) are redirected to these standards to avoid ambiguity. Since 1988, the IMA has approved additional pyroxene species to accommodate rare compositions, particularly those involving high-charge cations or extraterrestrial occurrences, expanding the group beyond the original 20 while adhering to the core nomenclature. Notable additions include davisite (CaScAlSiO6, approved 2008), the scandium analogue of esseneite from the ; grossmanite (CaTi3+AlSiO6, approved 2008), a titanium-rich variant also from Allende; and ryabchikovite (CuMgSi2O6, approved 2021), a copper-bearing pyroxene from volcanic exhalations at Tolbachik, . These approvals address gaps in rare-earth and transition-metal substitutions, enhancing the framework's applicability to meteoritic and fumarolic pyroxenes without altering the primary classification scheme.

Specific Pyroxene Minerals

Orthopyroxenes

Orthopyroxenes are a of pyroxene minerals characterized by their orthorhombic and compositions primarily along the enstatite-ferrosilite join. Their general formula is (Mg,Fe)SiO₃, reflecting a series where magnesium and iron substitute for one another in the octahedral sites. This series forms the basis for the primary end-members and intermediate varieties, with limited incorporation of other cations like calcium or aluminum in natural specimens. The magnesium-rich end-member is (MgSiO₃), a colorless to pale green mineral with a vitreous luster and Mohs hardness of 5–6. At the iron-rich end is ferrosilite (FeSiO₃), which is rarer in pure form but contributes to the series' variability. Intermediate compositions are represented by , typically with 50–70 mol% ferrosilite, appearing as brownish to greenish-gray crystals often exhibiting bronzite-like metallic sheen due to fine exsolution lamellae. , a variety of Fe-bearing enstatite (around 10–30 mol% ferrosilite), is distinguished by its bronze-colored schiller effect from oriented exsolution of lamellae parallel to the (100) plane. Physical properties of orthopyroxenes vary systematically with iron content: pure has a specific gravity of about 3.2, increasing to 3.9 for ferrosilite-rich varieties, alongside a shift in color from colorless or white to brown or black. These minerals are valued in ceramics for their high (over 1500°C for ) and thermal stability, serving as components in and materials derived from industrial slags. In optical petrography, orthopyroxenes display diagnostic parallel extinction under crossed polars in longitudinal thin sections, low yielding first-order interference colors (gray to yellow), and weak from pale green to . Their orthorhombic symmetry results in approximately 90° cleavage angles, contrasting with the inclined cleavage in monoclinic pyroxenes. Compositions plot along the enstatite-ferrosilite join on the pyroxene , with pigeonite representing a transitional low-calcium variety that inverts to orthopyroxene upon cooling, forming intergrowths. Polymorphism in orthopyroxenes includes low-temperature (Pbca) and high-temperature (Pbcn) forms of .

Clinopyroxenes

Clinopyroxenes are the monoclinic members of the pyroxene group, characterized by their ability to incorporate calcium and sodium alongside magnesium, iron, and aluminum in their crystal structures. Key representatives include diopside (CaMgSi₂O₆), a calcium-magnesium endmember often appearing white to pale green; hedenbergite (CaFeSi₂O₆), its iron-rich counterpart that darkens to black; augite, a complex solid solution with the general formula Ca(Mg,Fe,Al)(Si,Al)₂O₆; jadeite (NaAlSi₂O₆), a sodium-aluminum variant; and aegirine (NaFeSi₂O₆), a sodium-iron member. These minerals typically exhibit colors ranging from green to black, with many showing pleochroism—visible color changes under polarized light—particularly the sodic types like aegirine. In optical microscopy, clinopyroxenes display inclined extinction angles of approximately 40–50 degrees, a hallmark distinguishing them from orthorhombic pyroxenes. Special types within clinopyroxenes include omphacite, a high-pressure Na-Ca-Al with (Ca,Na)(Mg,Fe²⁺,Al)Si₂O₆, commonly found in eclogite facies rocks due to its stability under elevated pressures. , while structurally related as a pyroxenoid with CaSiO₃, differs from true pyroxenes by having a triclinic structure with slightly distorted single silicate chains rather than the ideal pyroxene configuration. Among clinopyroxenes, serves as a prized gem variety known as , distinguished from jade—an mineral—by its pyroxene composition, higher hardness (6.5–7 on the ), and greater translucency and color vibrancy, making it rarer and more valuable.

Occurrence and Formation

In Igneouses Rocks

Pyroxenes are essential components of and ultramafic igneous rocks, forming through crystallization from mantle-derived magmas rich in magnesium and iron. In rocks such as and , serves as the primary clinopyroxene, typically coexisting with and to define the rock's dark, fine- to coarse-grained texture. In ultramafic rocks like and , orthopyroxenes such as are major components, typically comprising 10–50% of the mineral assemblage alongside dominant , reflecting the high-temperature, low-silica conditions of their formation. , another clinopyroxene, may occur in these settings where calcium content is elevated. In magmatic processes, pyroxenes crystallize early within the discontinuous branch of , following as the cools from temperatures around 1200–1300°C. This sequence arises because pyroxenes are stable at intermediate temperatures (approximately 1100–1200°C), where they incorporate silica more readily than . If early-formed pyroxenes remain in contact with the evolving melt, they can undergo reactions, such as incongruent breakdown to form at higher temperatures or transformation into at lower temperatures and higher water contents, thereby influencing the overall mineralogy and composition of the residual . Zoned pyroxene crystals are common in slowly cooling magmas, recording progressive changes in melt chemistry due to fractional and temperature decline. These zones often feature orthopyroxene cores, which form under initial silica-saturated conditions, rimmed by clinopyroxene overgrowths as the melt becomes relatively calcium-enriched during cooling. Such highlights the dynamic nature of magmatic differentiation, with core-to-rim variations in magnesium, iron, and aluminum contents spanning several weight percent. Volcanic igneous rocks exhibit greater diversity in pyroxene types due to rapid eruption and cooling. Pigeonite, a low-calcium clinopyroxene, is notably present in andesites, where it crystallizes as phenocrysts or groundmass grains under subalkaline conditions at temperatures of 1000–1100°C. This mineral's occurrence in andesitic lavas, such as those from the Tongariro volcanic center, underscores its role in intermediate magmas, often alongside and , and provides insights into pre-eruptive volatile contents and oxidation states.

In Metamorphic and Other Rocks

Pyroxenes play a significant role in metamorphic environments, particularly through calc-silicate reactions in carbonate-bearing rocks. , a calcium-rich clinopyroxene, commonly forms in and marbles via metasomatic processes involving the interaction of siliceous fluids or rocks with or dolomite sequences. These reactions produce alongside other calc-silicates like and , often in contact metamorphic aureoles around intrusions or in regional of impure carbonates. In marbles, appears as green crystals disseminated in a matrix, reflecting the addition of silica and magnesia to the . Omphacite, a sodic clinopyroxene, is a hallmark in high-pressure metamorphic rocks such as eclogites, where it coexists with and indicates subduction-zone conditions exceeding 1.5 GPa and 400–700°C. Its jadeite component (up to 50 mol%) stabilizes under these ultra-high-pressure regimes, forming solid solutions with and hedenbergite, and serves as a key petrological indicator of deep crustal or mantle involvement in tectonic cycles. Eclogites with omphacite typically derive from basaltic protoliths transformed during continental collision or oceanic . A representative reaction in greenschist-facies (approximately 300–500°C and 0.2–0.5 GPa) involves the progressive devolatilization of hydrous phases, such as + 3 + 2 → 5 + 3 CO₂ + H₂O, which releases CO₂ and water while stabilizing pyroxenes in calc-silicate assemblages; a related lower-grade variant simplifies to + + → + CO₂ + H₂O. This transition marks the shift from hydrous minerals like and in lower-grade marbles to pyroxene-dominated rocks at higher temperatures within the facies. Authigenic pyroxenes occur rarely in sedimentary settings, forming diagenetically in sandstones through from pore fluids or low-grade alteration, often as overgrowths on detrital grains without implying full . These secondary pyroxenes, typically or , result from silica- and calcium-rich fluids during burial and can influence , though they are uncommon compared to clays or cements. of primary pyroxenes in source areas contributes detrital grains to sandstones, but authigenic formation remains minor and localized to specific geochemical environments. In mantle-derived materials, orthopyroxene (such as or ) is abundant in xenoliths entrained in volcanic , like kimberlites or basalts, providing direct samples of the . These nodules, often spinel or lherzolites, contain 10–50% orthopyroxene in equilibrium with and clinopyroxene, reflecting depletion or refertilization processes at depths of 30–150 km. Such xenoliths from volcanic conduits offer insights into mantle composition and dynamics without igneous crystallization context.

Extraterrestrial and Research Applications

Pyroxenes in Space

Pyroxenes are prevalent in extraterrestrial materials, particularly in meteorites that provide insights into early solar system processes. Enstatite chondrites, formed under highly reduced conditions, are dominated by as the primary orthopyroxene phase, often comprising the bulk of their fraction alongside minor metal and sulfides. In contrast, achondritic meteorites such as eucrites, which are basaltic in composition and linked to differentiated parent bodies like asteroid , feature as a major clinopyroxene component, typically intergrown with pigeonite and in subophitic textures. These occurrences highlight pyroxenes' role in tracing nebular and subsequent . On planetary bodies, pyroxenes constitute key minerals in igneous terrains. Martian basalts, exemplified by shergottite s, commonly contain pigeonite as the dominant pyroxene, reflecting from mafic magmas under low-pressure conditions; the ALH 84001 , an orthopyroxenite from Mars' ancient crust, consists primarily of uniform low-calcium orthopyroxene (En70Fs27Wo3) with and maskelynite. Lunar highlands include lithologies as part of the mafic cumulates in the lower crust, often exposed in impact craters and contributing to the region's anorthositic-dominanted but pyroxene-enriched composition. Venusian volcanics, inferred from and modeling, exhibit pyroxenes in basaltic flows, where they resist rapid atmospheric oxidation and remain spectrally detectable for extended periods. Recent missions have expanded understanding of pyroxenes' distribution and alteration. The Perseverance rover's analysis in Jezero Crater revealed clinopyroxenes (high-Ca varieties) in olivine-rich igneous rocks on the crater floor, alongside evidence of aqueous alteration that formed carbonates and other secondary minerals, indicating prolonged water-rock interactions in Mars' Noachian era. Samples returned by Hayabusa2 from asteroid Ryugu include high-Ca pyroxenes within chondrule-like objects, mixed with low-Ca pyroxenes and olivine, suggesting these minerals originated from high-temperature nebular processes and reaggregation in the asteroid's rubble-pile structure. Similarly, samples from asteroid Bennu returned by NASA's OSIRIS-REx mission in 2023 contain Mg-rich pyroxenes, often associated with hydrated matrices and phyllosilicates, providing further evidence of aqueous alteration in primitive carbonaceous asteroids. In astrobiology, pyroxene textures, such as etch pits and replacement patterns from bioweathering, have been proposed as potential biosignatures in martian meteorites, analogous to microbial alteration observed in terrestrial pyroxenes.

Petrological and Industrial Uses

Pyroxenes serve as essential tools in for estimating the and conditions during through thermobarometry. The Wells method, developed in 1977, utilizes the Mg/Fe ratios in coexisting orthopyroxene and clinopyroxene pairs to calculate temperatures, applying simple mixing models to their solid solutions in both simple and complex systems. This approach has been widely adopted for analysis, providing reliable estimates typically between 800–1200°C based on from electron microprobe analysis. Recent advancements in the 2020s have incorporated techniques to refine pyroxene thermobarometry, enhancing accuracy by training models on large experimental datasets of clinopyroxene compositions to predict and without assuming equilibrium with other phases. For instance, algorithms applied to clinopyroxene-melt pairs have reduced uncertainties in volcanic plumbing system reconstructions. Similarly, deep learning-based tools like use clinopyroxene-only data to estimate intensive parameters in arc magmas, achieving precisions of ±50°C and ±2 kbar. In , pyroxenes facilitate indirect dating of igneous events by co-occurring with uranium-bearing accessory minerals such as in plutonic rocks, allowing U-Pb ages to constrain the timing of pyroxene crystallization. For example, in pyroxene-bearing plutons like Pyroxene Mountain, U-Pb dating yields precise ages (e.g., ~220 Ma) that correlate with the emplacement of pyroxene-rich magmas, aiding in tectonic reconstructions. This association is particularly valuable in to intermediate intrusions where pyroxenes dominate the modal , enabling integrated petrochronologic studies that link mineral growth to specific magmatic episodes. Industrially, , a calcium-rich clinopyroxene, is incorporated into ceramics and refractories due to its low , high creep resistance, and stability up to 1400°C, making it suitable for high-temperature applications like furnace linings. , a pyroxenoid closely related to pyroxenes, functions as a reinforcing filler in plastics (e.g., polyesters and nylons) and paints, improving mechanical strength, weather resistance, and opacity while serving as an substitute. In the gem trade, —a sodium-aluminum clinopyroxene—is prized for its toughness, vibrant green hues, and cultural significance, commanding high values in jewelry markets, particularly from sources. Recent research has explored pyroxenes in carbon capture technologies, leveraging their reactivity in mineral carbonation processes. Studies from 2023 highlight pyroxene-rich basalts and slags for enhanced CO2 sequestration, where pyroxenes like augite facilitate rapid mineralization by reacting with CO2 to form stable carbonates, achieving up to 80% conversion efficiency under moderate conditions. Environmentally, pyroxene-bearing rock flours from basalts are applied in soil remediation through enhanced weathering, promoting the immobilization of heavy metals like lead and arsenic while improving soil fertility and sequestering atmospheric CO2.

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