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Enstatite

Enstatite is a magnesium-rich silicate mineral with the chemical formula Mg₂Si₂O₆, serving as the pure magnesium end-member of the orthopyroxene group within the broader pyroxene mineral class.^[1]^[2] It crystallizes in the orthorhombic system, typically forming prismatic crystals or granular masses, and is characterized by its refractory nature, derived from the Greek word enstates meaning "adversary" or "opponent," reflecting its resistance to chemical alteration. It was first described in 1855 by German mineralogist Gustav Adolf Kenngott.^[2]^[1]^[3] Physically, enstatite exhibits a vitreous to pearly luster, with colors ranging from white and grayish to yellowish, greenish, olive-green, or brown, and it appears colorless in thin section.^[1]^[2] It has a Mohs hardness of 5–6, a specific gravity of 3.2–3.9, and displays good cleavage on {210} planes with uneven fracture and brittle tenacity.^[1]^[2] Optically, it is biaxial positive with refractive indices α = 1.649–1.667, β = 1.653–1.671, and γ = 1.657–1.680, and a 2V angle of 55°–90°, making it translucent to opaque.^[1] Common impurities include iron, calcium, aluminum, and trace elements like chromium and nickel, which can lead to varieties such as bronzite (iron-bearing) and hypersthene (intermediate compositions in the enstatite-ferrosilite series).^[2] Enstatite primarily occurs in ultramafic igneous rocks such as pyroxenites, peridotites, and dunites, as well as in mafic volcanics and ultramafic inclusions within alkalic basalts and kimberlites; it is less common in metamorphic rocks like granulites and ophiolites, and rarely in felsic igneous settings.^[1]^[2] It is frequently associated with olivine, diopside, phlogopite, spinel, and pyrope.^[1] Notably, enstatite is a key component in certain meteorites, including enstatite chondrites and achondrites, where it forms under highly reducing conditions, providing insights into early solar system processes.^[1] Rare transparent varieties from localities like Sri Lanka are used as gemstones due to their durability and subtle green or brownish hues.^[1]

Overview

Definition and Composition

Enstatite is the magnesium endmember of the orthopyroxene subgroup within the pyroxene group of minerals, characterized by the chemical formula Mg₂Si₂O₆ (or equivalently MgSiO₃).^[4] This composition represents the pure magnesian variant, distinguishing it from iron-bearing members in the series.^[5] The molecular weight of enstatite is 200.78 g/mol, with an ideal oxide composition of 40.15% MgO and 59.85% SiO₂.^[5] As an inosilicate, enstatite features a chain silicate structure consisting of single chains of corner-sharing SiO₄ tetrahedra, where magnesium cations occupy octahedral sites to link the chains laterally.^[6] Enstatite serves as a major constituent in the Earth's upper mantle, particularly in peridotite assemblages under conditions of moderate pressure and temperature where orthopyroxene is stable.^[7]

Etymology and Discovery

The name enstatite derives from the Greek term enstatēs, meaning "adversary" or "opponent," a reference to the mineral's refractory quality and resistance to decomposition by acids or under the blowpipe flame.^[2]^[8] This etymology highlights its chemical inertness, distinguishing it from more reactive silicates. Enstatite was first formally described as a distinct mineral species in 1855 by German-Swiss mineralogist Gustav Adolph Kenngott, based on specimens collected from serpentinized rocks at Mount Zdjar near Sobotín (then Schönberg) in Moravia (now Czech Republic). Prior to this, similar magnesium-rich pyroxenes such as bronzite and hypersthene had been recognized since the early 19th century, but enstatite was initially conflated with these varieties due to overlapping optical and physical traits, leading to inconsistent classifications among mineralogists.^[2]^[9] The distinctions among these pyroxenes were clarified in the 20th century through advancements in X-ray diffraction techniques, which allowed precise determination of crystal structures and confirmed enstatite as the pure MgSiO₃ endmember of the orthopyroxene solid solution series. Pioneering studies, such as those by B. E. Warren and D. I. Modell in 1930 on hypersthene (a ferroan enstatite variety), revealed the orthorhombic lattice parameters and silicate chain configurations unique to enstatite, resolving prior ambiguities in mineral identification.^[10]^[11] These structural analyses, building on earlier powder diffraction work from the 1920s, established enstatite's polymorphs and its role in pyroxene series, providing a foundation for modern petrological interpretations.

Crystal Structure and Polymorphs

Orthorhombic Polymorphs

The orthorhombic polymorphs of enstatite, MgSiO₃, include orthoenstatite and protoenstatite, which differ in their structural arrangements and thermal stability. Orthoenstatite is the low-temperature stable form at ambient conditions, characterized by space group Pbca and a unit cell with parameters approximately a = 18.23 Å, b = 8.84 Å, and c = 5.19 Å.^[2]^[10] In this structure, magnesium cations occupy two distinct octahedral sites, M1 and M2, which are coordinated by oxygen anions in a distorted chain of silica tetrahedra.^[10] Orthoenstatite commonly exhibits prismatic, lamellar, or massive habits, often forming striated crystals parallel to the c-axis.^[5] Protoenstatite represents the high-temperature orthorhombic polymorph, with space group Pbcn and unit cell parameters approximately a = 9.25 Å, b = 8.78 Å, and c = 5.32 Å.^[12]^[13] This form is metastable and unstable at room temperature, reverting to orthoenstatite or other phases upon cooling due to its higher free energy under ambient conditions.^[14] The transition between orthoenstatite and protoenstatite is reversible and occurs at approximately 1000°C under low pressure, with protoenstatite stable above this temperature.^[15]^[16] These polymorphs highlight the structural flexibility of enstatite, where subtle distortions in the silicate chains drive the phase change without altering the overall orthorhombic symmetry.^[17]

Monoclinic Polymorphs

Clinoenstatite represents the low-temperature monoclinic polymorph of enstatite (MgSiO₃), characterized by a space group of P2₁/c and featuring distorted silicate chains due to the monoclinic symmetry that tilts the tetrahedral units relative to the orthorhombic form.^[18] This structure arises from a displacive transformation, resulting in a denser packing compared to the stable orthorhombic polymorph under ambient conditions.^[19] High clinoenstatite, the high-temperature monoclinic variant, adopts the space group C2/c, with silicate chains exhibiting less distortion and a configuration that stabilizes at elevated temperatures above approximately 1000°C.^[20] This form is quenchable under rapid cooling but reverts to lower-symmetry structures upon slow annealing.^[21] Both polymorphs are metastable at room temperature, where the orthorhombic enstatite is thermodynamically favored; clinoenstatite typically forms through inversion from the high-temperature protoenstatite (an orthorhombic phase) during cooling, a process that is kinetically hindered and common in extraterrestrial settings such as meteorites.^[22] In meteoritic enstatite chondrites, clinoenstatite occurs as phenocrysts that preserve this inverted structure due to rapid quenching from igneous processes.^[23] Twinning in clinoenstatite often develops during the inversion from protoenstatite, involving rotation of silicate chains by approximately 180° around the axis or through cell-twinning mechanisms that produce fine lamellae parallel to (100).^[24] The inversion kinetics from clinoenstatite to orthoenstatite exhibit an apparent activation energy of approximately 67 kJ/mol (16 kcal/mol) within 800–1000°C, reflecting a reconstructive mechanism that proceeds via nucleation and growth of orthorhombic domains, though the reverse transformation is faster and displacive.^[25] This kinetic barrier contributes to the persistence of monoclinic forms in natural samples despite their metastability.^[26]

Physical and Optical Properties

Physical Characteristics

Enstatite exhibits a Mohs hardness of 5 to 6, making it moderately hard and suitable for identification in hand specimens.^[1] Its specific gravity ranges from 3.2 to 3.3 g/cm³ for typical magnesium-rich compositions, with calculated values for pure Mg₂Si₂O₆ at approximately 3.19 g/cm³, reflecting minor variations due to polymorphs such as orthoenstatite and protoenstatite.^[1]^[27] The mineral displays good cleavage on {210} planes at angles of about 88°, often described as prismatic, with additional partings on {100} and {010}, resulting in splintery fragments upon breaking.^[1] Fracture is uneven, contributing to its brittle tenacity.^[1] Luster is vitreous to pearly on cleavage surfaces, though the bronzite variety shows a submetallic sheen due to fine lamellar inclusions.^[1]^[27] Enstatite occurs in colors ranging from colorless or white to greenish, yellowish, olive-green, or brown, with a white to gray streak that aids in distinguishing it from similar pyroxenes.^[1] It demonstrates thermal stability up to its incongruent melting point of approximately 1550°C, beyond which it decomposes into forsterite and liquid in the MgO-SiO₂ system.^[27]

Optical and Spectroscopic Features

Enstatite exhibits biaxial positive optical character, with refractive indices typically ranging from nα = 1.650–1.658, nβ = 1.653–1.663, and nγ = 1.659–1.668, though these values increase slightly with iron substitution in solid solution.^[28] The birefringence is low at approximately 0.009, resulting in first-order interference colors under crossed polars.^[29] The optic axial angle (2V) varies between 55° and 90°, with pure Mg-endmember enstatite showing a 2Vγ of about 55°.^[27] In thin section, enstatite displays weak pleochroism, appearing colorless to pale green or pinkish, particularly in iron-bearing varieties.^[29] Extinction is parallel in longitudinal sections and symmetrical in basal sections, aiding its distinction from clinopyroxenes.^[29] Spectroscopically, enstatite's infrared and Raman spectra feature prominent peaks associated with silicate vibrations. In infrared spectra, the Si-O stretching mode appears as a broad band around 1000 cm⁻¹ (near 10 μm), characteristic of the pyroxene chain structure.^[30] Raman spectra show key modes including M-O stretching at ~343 cm⁻¹ (ν3) and Si-O-Si stretching at ~665 cm⁻¹ (ν11) and ~687 cm⁻¹ (ν12), with additional low-frequency peaks varying by orientation and polymorph.^[31] These signatures enable precise identification in mineral mixtures. Ultraviolet-visible (UV-Vis) absorption in enstatite arises from trace transition metal impurities such as iron and vanadium, producing bands in the visible and near-infrared ranges (e.g., Fe²⁺ features at ~430, 500, and 550 nm; V³⁺ bands around 420–490 nm and 600–700 nm in some samples), superimposed on a steep UV cutoff, which imparts subtle coloration.^[32]^[33]

Varieties and Series

Common Varieties

Enstatite sensu stricto represents the nearly pure magnesium silicate end-member with the formula MgSiO₃, typically exhibiting colorless to pale green hues due to its high purity and minimal impurities.^[5] Bronzite, a widespread variety, incorporates 5–15 wt% FeO substituting for MgO, resulting in a distinctive submetallic luster attributed to the schiller effect from oriented lamellae of iron oxides such as ilmenite or rutile inclusions.^[34]^[35] This iron content imparts bronze-brown tones, distinguishing it from purer forms while maintaining the orthopyroxene structure. Chrome-enstatite features up to 1 wt% Cr₂O₃, which introduces a vivid green coloration through chromium substitution, and occurs notably in kimberlite-hosted mantle xenoliths.^[1] Gem-quality enstatite remains rare, with transparent to translucent specimens primarily sourced from localities like Tanzania and Myanmar; these are often cut as cabochons to highlight chatoyancy in bronzite varieties, where aligned fibrous inclusions produce a cat's-eye effect.^[36] In modern applications, synthetic enstatite is manufactured via sol-gel methods or powder sintering for advanced ceramics, yielding dense, high-purity MgSiO₃ phases suitable for thermal and electrical insulators.^[37] These varieties occupy the magnesium-rich portion of the enstatite-ferrosilite solid solution series.

Relation to Ferrosilite Series

Enstatite and ferrosilite form a complete solid solution series in the orthopyroxene group, spanning compositions from pure enstatite (MgSiO₃, denoted as En₁₀₀) at the magnesium-rich end to pure ferrosilite (FeSiO₃, Fs₁₀₀) at the iron-rich end. This series, with the general formula (Mg,Fe)₂Si₂O₆, allows for continuous substitution of Fe²⁺ for Mg²⁺ in the octahedral sites of the crystal structure, resulting in a wide range of intermediate compositions commonly found in igneous and metamorphic rocks.^[38]^[39] Despite the complete solid solution at low temperatures, a miscibility gap exists at higher temperatures (above approximately 1000–1200°C, depending on pressure), where intermediate compositions become unstable and exsolve into magnesium- and iron-enriched lamellae. This exsolution produces fine-scale intergrowths observable under microscopy, which record cooling histories in rocks like peridotites and norites. The gap is asymmetric, broader toward the iron-rich side, and influences the textural evolution of pyroxenes during magmatic crystallization or metamorphic recrystallization.^[40]^[41] Nomenclature within the series is based on the enstatite (En) to ferrosilite (Fs) ratio, with enstatite applied to compositions exceeding 90% En (less than 10% Fs), hypersthene to those with 50–70% Mg (approximately En₅₀Fs₅₀ to En₇₀Fs₃₀), and ferrosilite to iron-dominant members. Monoclinic intermediates, stable at higher temperatures, are termed pigeonite, particularly for compositions around En₅₀–₇₅Fs₂₅–₅₀ with minor calcium. These terms, while varietal, aid in describing natural occurrences without implying distinct mineral species.^[42]^[43] In the phase diagram of the enstatite-ferrosilite system at atmospheric pressure, orthopyroxene occupies the low-temperature stability field below the pigeonite solvus, which closes at the end-members but widens for intermediates around 1100°C. This configuration explains the prevalence of orthorhombic forms in slowly cooled rocks and the inversion to monoclinic pigeonite upon heating. Recent studies on Fe-Mg partitioning between orthopyroxene and coexisting phases in mantle xenoliths have refined thermodynamic models, revealing deviations from ideal behavior due to pressure and minor elements like aluminum, which affect temperature estimates for upper mantle equilibration (typically 900–1200°C at 1–3 GPa). These findings enhance geobarometry applications in peridotite xenoliths from kimberlites and basalts.^[44]^[45]^[46]

Geological Occurrence

Terrestrial Formations

Enstatite, a magnesium-rich orthopyroxene, is a primary constituent of ultramafic rocks on Earth, particularly peridotites, where it occurs alongside olivine and clinopyroxene in mantle-derived assemblages.^[47] In kimberlites, enstatite appears as xenocrystic grains or in discrete nodules, often with low iron and calcium contents, reflecting its origin in the lithospheric mantle.^[48] Komatiites, ancient ultramafic volcanic rocks, commonly feature enstatite in their metamorphic variants, especially at higher grades where it coexists with anthophyllite and diopside.^[49] Prominent terrestrial localities for enstatite include the Bushveld Complex in South Africa, a vast layered intrusion where it forms part of gabbronorite layers rich in ferroan enstatite associated with chromite deposits.^[50]^[51] In the United States, the Stillwater Complex in Montana hosts enstatite within gabbroic and noritic units of this Neoarchean intrusion, contributing to its ultramafic lower zones.^[52]^[53] At Mount Etna in Italy, enstatite occurs as high-magnesium orthopyroxene inclusions in olivine within tholeiitic lavas, indicating primitive magmatic processes.^[54] Enstatite typically forms through the crystallization of mafic to ultramafic magmas under high-temperature (above 1200°C) and low-pressure conditions (less than 1 GPa), as part of the early stages of fractional crystallization where it stabilizes alongside olivine.^[55] In metamorphic settings, it is prominent in granulite-facies rocks, where it participates in dehydration reactions involving phlogopite and quartz, and in eclogites as fine-grained coronas around quartz inclusions in garnet-clinopyroxene matrices.^[4]^[56] Recent sample collections from volcanic pipes have advanced understanding of enstatite-bearing xenoliths; for instance, 2024 studies of ultramafic xenoliths from Mt. Vulture in southern Italy reveal enstatite in lherzolites equilibrated at upper mantle depths, providing insights into CO₂-rich fluid interactions.^[57] Enstatite's prevalence in these assemblages underscores its role in defining the magnesium-rich composition of the upper mantle.^[47]

Mantle and Igneous Associations

Enstatite, as the magnesium-rich endmember of orthopyroxene, constitutes a significant component of the Earth's upper mantle, typically comprising 20-30% by volume in residual peridotites. These peridotites represent the depleted residues left after the extraction of basaltic melts during partial melting processes at mid-ocean ridges or in the mantle wedge. In fertile lherzolites, orthopyroxene abundance can reach up to 40% in some cratonic suites, reflecting variations in initial mantle composition and degree of melting.^[58]^[59] In mafic igneous magmas, enstatite crystallizes early in the sequence, typically following olivine but preceding clinopyroxene, as dictated by Bowen's discontinuous reaction series. This order occurs in tholeiitic basalts where cooling promotes the formation of magnesium-rich orthopyroxene from primitive melts with MgO contents around 10-15 wt%. The presence of enstatite in such cumulates provides insights into the thermal and compositional evolution of magma chambers.^[55] Geothermobarometry exploits the Fe-Mg partitioning between enstatite (En) and ferrosilite (Fs) components in orthopyroxene coexisting with olivine, with the exchange coefficient K_D = \frac{(Fe/Mg)_{opx}}{(Fe/Mg)_{ol}} approximately 0.09 at 1200°C under mantle conditions. This partitioning is temperature-sensitive and forms the basis of calibrations like those of Brey and Köhler (1990), enabling estimates of equilibration temperatures in peridotite xenoliths up to 1300-1400°C. Such applications are crucial for reconstructing mantle thermal structures without direct pressure dependence in simple systems. Experimental petrology demonstrates enstatite's stability in upper mantle assemblages up to approximately 15 GPa (corresponding to depths of ~450 km), where it persists as a major phase in peridotitic compositions under anhydrous to moderately hydrous conditions. Beyond this pressure, it undergoes phase transitions to denser polymorphs like clin-enstatite or majorite, but remains integral to the mineralogy below the lithosphere-asthenosphere boundary. Recent high-pressure experiments confirm its role in buffering silica activity during melting.^[60] Recent elasticity studies of orthopyroxene from 2022–2023 demonstrate that enstatite enrichment in the mantle wedge, resulting from metasomatism by siliceous melts derived from subducting slabs, can produce low Vp/Vs ratios (1.65–1.72 at low pressure and high temperature). This explains seismic observations of reduced Vp/Vs in subduction zones, such as beneath Alaska, where orthopyroxene formation from reactions with olivine lowers the ratio by up to 0.1–0.15 depending on its volume fraction.^[61]

Extraterrestrial Contexts

Meteorites and Achondrites

Enstatite chondrites represent a rare class of meteorites, comprising approximately 1% of known falls and finds, and are subdivided into EH (high total iron) and EL (low total iron) subtypes based on metal and sulfide abundances. These meteorites formed under highly reducing conditions in the inner solar nebula, close to the proto-Sun, where high C/O ratios exceeding 0.8–1.0 facilitated the condensation of anhydrous silicates and sulfides at temperatures around 700–1000 K. Enstatite serves as the dominant mineral phase, with modal abundances ranging from 16% in some unequilibrated samples to over 70% in equilibrated ones, underscoring its role as the primary silicate component in these reduced assemblages.^[62]^[63] Aubrites, also known as enstatite achondrites, consist predominantly of coarse-grained, nearly pure enstatite (75–98 wt%, with compositions En_{98.76–99.54}), interpreted as cumulates from fractional crystallization processes under reducing conditions similar to those of enstatite chondrites. These meteorites exhibit brecciated textures, with most samples classified as monomict breccias containing angular fragments of enstatite and minor plagioclase, while exceptions like Shallowater display unbrecciated, cm-sized enstatite grains. Their bulk chemistry and oxygen isotope ratios closely resemble those of petrologic type 6 enstatite chondrites, suggesting a shared parent body origin in the inner solar system.^[64] Oldhamite (CaS) occurs as inclusions within enstatite grains in both EH and EL chondrites, serving as a key indicator of the low oxygen fugacity prevalent during their formation and metamorphism. In unequilibrated EH3 chondrites, oldhamite displays convex downward rare-earth element patterns with positive Eu and Yb anomalies (up to 4–5× CI-normalized), consistent with condensation from a residual gas under C/O ratios >0.5, where divalent Eu²⁺ and Yb²⁺ partitioning into CaS is favored. Equilibrated samples show flatter patterns due to metamorphic redistribution, while EL varieties exhibit negative Eu anomalies linked to partial melting and FeS vapor loss, further evidencing the ultra-reduced environment (ΔIW -4 to -7) that stabilized sulfide minerals over oxides.^[65] Oxygen isotope analyses reveal ¹⁶O enrichment in enstatite chondrites, with δ¹⁷O and δ¹⁸O values aligning closely with terrestrial standards (Δ¹⁷O ≈ 0‰), supporting their role as building blocks for Earth-like reservoirs in the inner protoplanetary disk. This isotopic similarity, distinct from other chondrite groups, implies minimal mass-dependent fractionation during accretion and links enstatite chondrite material to the proto-Earth's silicate fraction. Clinoenstatite is prevalent in low petrologic type (3–4) samples, occurring alongside orthoenstatite in chondrules and matrices.^[66] Recent studies of Antarctic meteorites, including primitive enstatite chondrites recovered from collections like Allan Hills and Elephant Moraine, have identified presolar grains embedded within enstatite phases, providing evidence of pre-solar nebula processing and heterogeneous dust reservoirs in the inner disk. These grains, including silicon carbide and graphite, exhibit anomalous isotopic compositions (e.g., ¹²C/¹³C > 100) indicative of origins in asymptotic giant branch stars or supernovae, with abundances up to 100 ppm in unequilibrated EH3 and EL3 lithologies. Such findings highlight the preservation of pristine nebular materials despite the reducing conditions.^[67]^[63]

Astronomical and Planetary Occurrences

Enstatite has been identified through spectral analysis in E-type asteroids, which are characterized by their enstatite-rich compositions resembling aubrite meteorites. These asteroids exhibit diagnostic absorption features near 0.9–1.0 μm and 2.0 μm attributable to low-iron pyroxene, consistent with enstatite dominance in their surfaces. Recent James Webb Space Telescope (JWST) observations of such bodies, including potential E/M-type transitional objects, have refined these identifications by providing higher-resolution mid-infrared spectra that reveal crystalline silicate signatures, enhancing models of their formation in reduced, inner solar system environments.^[68]^[69] For asteroid (16) Psyche, an M-type body targeted by NASA's Psyche mission, spectroscopic data indicate a surface composition including low-Fe pyroxene, interpreted as enstatite mixed with metallic iron-nickel alloys at approximately 7% by weight. JWST observations from 2023–2024, using the Mid-Infrared Instrument (MIRI), have detected hydration signals potentially linked to silicates, consistent with a heterogeneous regolith including enstatite-bearing components over a metallic core. This aligns with bulk density constraints suggesting a partially differentiated body.^[70]^[71] In planetary nebulae, enstatite appears as crystalline dust grains, as evidenced by Infrared Space Observatory (ISO) and JWST spectra of NGC 6302, the "Butterfly Nebula." The 2001 ISO analysis revealed enstatite emission features at 10–40 μm, with abundances comparable to forsterite and temperatures around 80 K, likely originating from a toroidal disk around the central star. JWST/MIRI data from 2025 confirm these findings, showing enstatite emission features longward of 15 μm alongside quartz and forsterite in the nebula's UV-irradiated outflows, highlighting its role in post-asymptotic giant branch dust processing.^[72]^[73]^[74] Enstatite is also detected in cometary dust, particularly through the Rosetta mission's analysis of comet 67P/Churyumov-Gerasimenko. In situ measurements by the COSIMA instrument identified low-calcium pyroxene, including enstatite, in refractory grains, comprising up to 10% of the dust mineralogy alongside amorphous silicates and forsterite. These findings, corroborated by laboratory spectra calibrated for Rosetta's filters, indicate enstatite formation via direct condensation in the solar nebula under reducing conditions.^[75]^[76] In exoplanet models, enstatite-rich mantles are predicted for super-Earths forming in reducing atmospheres, where low oxygen fugacity favors pyroxene over olivine dominance. Phase equilibrium studies of reduced precursors like enstatite chondrites show that inner nebula regions yield mantles with >50% pyroxene (primarily enstatite), influencing seismic profiles and volatile retention. This composition is crucial for understanding magma ocean solidification and atmospheric interactions on planets like those in the TRAPPIST-1 system.^[77]^[78] Brown dwarf atmospheres, such as that of 2M2224-0158 (an L7-type dwarf), feature enstatite in silicate clouds, as inferred from near-infrared spectra showing polarization signatures of crystalline pyroxene at effective temperatures around 1500 K. Retrieval analyses indicate enstatite and quartz layers above iron condensates, comprising 20–30% of cloud opacity and contributing to the object's red colors. These clouds form via equilibrium chemistry in metal-poor environments, providing analogs for young exoplanet hazes.^[79]^[80] Recent sample returns have bolstered astronomical interpretations of enstatite occurrences. NASA's OSIRIS-REx mission, analyzing Bennu samples returned in 2023, identified enstatite fragments as anhydrous pyroxene in <10% of the regolith, with 2024–2025 Raman and X-ray diffraction confirming low-Fe variants amid hydrated matrix, linking to E-type spectral analogs. Similarly, JAXA's Hayabusa2 samples from Ryugu (2020 return) revealed minor enstatite in non-carbonaceous inclusions, resembling enstatite chondrites and informing remote sensing of C-type asteroids' hidden refractory components.^[81]^[82]

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(a) Crystal structures of protoenstatite and twinned clinoenstatite....
The possible mechanisms of transformation from PEn to CEn are shown in (b–c). The image is from Smyth (1974). (b) Rotation angle of ≈ 180° around the [001] axis ...
[24]
Kinetic and Microstructural Studies of Magnesium-Rich ... - jstor
The apparent activation energy for the transformation is N 16 kcal/mole ... mation of clinoenstatite to orthoenstatite at much lower temperatures. For ...
[25]
First principles study of thermodynamics and phase transition in low ...
Feb 4, 2010 · Using first principles quasi-harmonic theory, we have investigated the vibrational and thermodynamic properties of P21/c and C2/c MgSiO3 ...3. Crystal Structure And... · 4. Thermodynamic Properties · 5. Lp-En To Hp-En Phase...<|control11|><|separator|>
[26]
Enstatite–Ferrosilite (Mg,Fe 2 +)[SiO 3 ] - GeoScienceWorld
Jan 1, 2013 · Minerals in the enstatite–ferrosilite solid solution are the only common orthopyroxenes. They show weak to moderate birefringence and straight extinction.Missing: handbook | Show results with:handbook
[27]
Enstatite gemstone information - Gemdat.org
Optical Properties of Enstatite. Refractive Index, 1.650 to 1.680. Walter ... Birefringence, 0.009 to 0.012. Walter Schumann, Gemstones of the world (2001).
[28]
Orthopyroxene Page - - Clark Science Center
Cleavage, Good cleavage on (210) Parting on (100) and (010), Cleavage in two directions, intersecting at nearly right angles. Hand Sample Color, Brown to green ...
[29]
Quantitative Infrared Spectra of Hydrosilicates and Related Minerals
Band types measured include O—H stretching modes near 3 μm, Si—O stretching motions near 10 μm, Si—O—Si bends near 14 μm, O—Si—O bends near 20 μm, and ...
[30]
Raman spectroscopy of shocked enstatite‐rich meteorites
May 6, 2018 · The principal modes in the enstatite spectra are ν3 (343 cm−1, M-O stretch), ν11 (665 cm−1, Si-O-Si stretch), ν12 (687 cm−1, Si-O-Si stretch), ...
[31]
The Polarized Spectra of Iron in Silicates. I. Enstatite
Jul 23, 2021 · Polarization spectra covering infrared, visible and ultraviolet ranges from 2200-27000 cm -1 have been obtained of enstatite at room and liquid helium ...
[32]
Inclusions and Spectroscopic Features of Yellowish Green Enstatite
The iron absorptions are superimposed by two broad bands around 420–490 nm and 600–700 nm due to the presence of V3+ plus trace Cr3+ in the stone lattice (see ...
[33]
il/iietican%Musdum
The FeO content ranges from 7 to 12 per cent; figures of less than 7 per cent have been recorded, but when checked against the composition of the minerals ...
[34]
[PDF] are given in Tables 1 and 2 and previously published analyses for ...
Bronzite, ultrabasic gneiss, Loch Ossigary, Rodil, South ... lites of rutile or ilmenite, giving rise to the schiller effect visible in some specimens.<|separator|>
[35]
[PDF] Eskolaite associated with diamond from the Udachnaya kimberlite ...
The major-element composition, determined by EMP, contains elemental substitu- tions that effectively lower the Cr2O3 end-member abundance down to 86.8 wt%, ...
[36]
Enstatite Value, Price, and Jewelry Information
Dec 19, 2021 · Most gem enstatites have indices in the range 1.663-1.673. The brown and green gems from Tanzania are enstatites, as are the brownish-green stones from Sri ...
[37]
Enstatite ceramics: a multicomponent system via sol-gel
The powder consists of an amorphous (siliceous) phase + forsterite + periclase. Pure enstatite monoliths made by dispersing, casting and gelling these powders ...
[38]
ALEX STREKEISEN-Pyroxene-
Complete substitution exists between enstatite (Mg2Si2O6) and ferrosilite (Fe2Si2O6), and complete solid solution of iron for magnesium exists between diopside ...
[39]
6.4.7: Pyroxenes - Geosciences LibreTexts
Dec 16, 2022 · Orthopyroxenes, predominantly solid solutions of the end members ferrosilite and enstatite, have the general formula (Mg,Fe)2Si2O6. The photo on ...
[40]
Experimental and infrared characterization of the miscibility gap ...
Apr 1, 2014 · It is also suggested that the miscibility gaps presented here provide a basis for interpreting the degree of equilibrium reached in assemblages ...
[41]
Mixing properties of the enstatite-ferrosilite solid solution
Aug 7, 2025 · A suite of 18 natural orthopyroxene samples with compositions across the enstatite-ferrosilite join was examined. Annealing experiments were ...
[42]
[PDF] Nomenclature of pyroxenes - Mineralogical Society of America
Subdivision names of the in- termediate solid-solution ranges, such as bronzite, hypersthene, and eulite of the enstatite-ferrosilite series and salite and ...
[43]
Orthopyroxene | Silicate, Igneous, Olivine - Britannica
orthopyroxene ; enstatite, 0 to 50% Fe ; ferrosilite, 50 to 100% Fe ...
[44]
Phase transitions between high- and low-temperature ...
Jun 1, 2016 · We concluded that the stability field of low-temperature orthopyroxene was below 1000 °C and that of high-temperature orthopyroxene was above ...
[45]
Phase Transition of Enstatite‐Ferrosilite Solid Solutions at High ...
May 12, 2020 · These phase transitions require temperatures higher than 900 K, which are easily satisfied in the normal mantle or relatively warm subduction ...
[46]
Determination of Fe3+ and Fe2+ Partition Coefficients between ...
B. J.. (. 2022. ). Oxidation state of iron and Fe-Mg partitioning between olivine and basaltic martian melts . American Mineralogist. 107. ,. 1442. –. 1452.
[47]
Peridotites - ALEX STREKEISEN
mostly composed of olivine, orthopyroxene (commonly enstatite), and clinopyroxene (diopside), and have relatively high proportions of basaltic ingredients ( ...
[48]
Kimberlites from the Wawa area, Ontario - GeoScienceWorld
Mar 9, 2017 · Two newly discovered kimberlites from the Wawa area, Ontario, are Group-1 kimberlites in their petrographic, mineralogical, geochemical, and ...
[49]
Komatiites - ALEX STREKEISEN
Komatiites are fascinating volcanic rocks. They are among the most ancient lavas of the Earth following the 3.8 Ga pillow basalts at Isua.
[50]
gabbronorite HS - Smith College
Gabbronorite from the Bushveld Igneous Complex, South Africa. Dark brown enstatite and dark green clinopyroxene surrounded by grayish white plagioclase feldspar ...
[51]
[PDF] Stratiform Chromite Deposit Model - USGS Publications Warehouse
Bushveld chromium, because the Bushveld Complex in South. Africa is the only ... chromite deposits include ferroan enstatite (bronzite) and.
[52]
[PDF] Stillwater Complex, Montana Structure, Mineralogy, and Petrology of ...
This paper discusses the Stillwater Complex in Montana, focusing on the structure, mineralogy, and petrology of the basal zone, with emphasis on sulfides.
[53]
BH250-93 · BH250 Mineralogy Teaching Collection
Gabbro (Stillwater Complex). Description. Major Minerals: Plagioclase, Diopside (clinopyroxene), Enstatite (orthopyroxene) Mineralogical Notes: The texture ...
[54]
[PDF] Primitive magmatism of Mt. Etna: insights from mineralogy and melt ...
Orthopyroxene Mga 86 mol.% has been found as inclusions in the most magnesian olivine in tholeiitic lavas only. Crystallization temperatures determined from ...<|separator|>
[55]
Pyroxenes in the Crystallization of Basaltic Magma
The early and middle stages of crystallization of saturated basaltic magma are normally characterized by three reaction series; the two continuous series of ...
[56]
Granulite-facies Overprinting of Ultrahigh-pressure Metamorphic ...
An enstatite-bearing eclogitic rock consists mainly of large grains of garnet and clinopyroxene, and enstatite is present as fine-grained coronas around quartz ...
[57]
(PDF) CO 2 ‐Rich Xenoliths at Mt. Vulture Volcano (Southern Italy)
Aug 29, 2024 · This study provides new mineral chemistry data together with micro‐thermometric measurements on fluid inclusions hosted in ultramafic xenoliths ...Missing: 2020s | Show results with:2020s
[58]
Origin of high-Mg# orthopyroxene-rich cratonic mantle
Aug 15, 2024 · High-Mg# orthopyroxene-rich cratonic peridotites can form by high-degree anhydrous melting of a non-pyrolitic silica-rich mantle at relatively high pressures.
[59]
[PDF] Experimental petrology of peridotites, including effects of water and ...
Dec 13, 2014 · As residual enstatite contains 2.8 % CaO and 7 % Al2O3 and there may be ~1 % trapped melt, the residual lithosphere after MORB extraction ...<|control11|><|separator|>
[60]
Melting of enstatite (MgSiO 3 ) from 10 to 16.5 GPa and the forsterite ...
Sep 10, 1990 · A split sphere multianvil apparatus (USSA 2000) has been used to determine the melting curve of enstatite (MgSiO3) from 10 to 16.5 GPa and ...
[61]
Seismic signals induced by the Metasomatism of mantle wedge by ...
Jan 5, 2023 · In this study, we determined the elasticity of Fe-bearing opx (Mg 0.875 Fe 0.125 )SiO 3 at high pressure and temperature (PT) conditions.
[62]
Enstatite chondrites: condensation and metamorphism under ...
May 20, 2022 · Enstatite chondrites contain a unique mineral inventory of sulfides of typical lithophile elements, Si-bearing metal, silicide and phosphide, ...
[63]
Mineralogical and spectroscopic investigation of enstatite chondrites ...
Jul 20, 2010 · Rietveld refinement of high-resolution powder X-ray diffraction (XRD) data was used to determine quantitative major mineral abundances. Sample- ...
[64]
[PDF] Aubrites: Their origin and relationship to enstatite chondrites
Ten meteorites have been classified as aubrites or enstatite achondrites. Nine of these are falls and one is a find. The classification is based on their unique ...
[65]
The message of oldhamites from enstatite chondrites
Feb 21, 2022 · We have determined rare-earth element (REE) abundances in oldhamites (CaS) from 13 unequilibrated and equilibrated enstatite chondrites (5 EH and 8 EL) and in ...
[66]
The oxygen isotope composition of earth's oldest rocks and ...
Mar 29, 2013 · Of all the meteorite groups, only enstatite chondrites are virtually identical in oxygen isotopic composition to bulk Earth [Javoy et al., 2010] ...
[67]
Presolar Grains in Primitive Enstatite Chondrites | Request PDF
The aim of the paper was to determine mean atomic weight and to predict heat capacity, thermal conductivity, and thermal diffusivity of Sariçiçek howardite.
[68]
E-type asteroid spectroscopy and compositional modeling - ADS
The designation ``E'' was originally linked with the mineral enstatite, common in aubrite meteorites. More recently, the sulfides troilite and oldhamite have ...Missing: identification JWST
[69]
Spectral Analyses of Asteroids (Chapter 19)
E-Type Asteroids: E-type asteroids were initially linked to enstatite achondrites, but ground-based observations suggest that the type includes several ...
[70]
Constraining the Regolith Composition of Asteroid (16) Psyche via ...
May 12, 2021 · We find that the best matching mixture to Psyche consists of 82.5% metal, 7% low-Fe pyroxene, and 10.5% carbonaceous chondrite by weight.
[71]
Estimate of water and hydroxyl abundance on asteroid (16) Psyche ...
Jul 16, 2024 · We present evidence of hydration due mainly to OH on asteroid (16) Psyche, the largest M-class asteroid, using data from the James Webb Space ...Missing: enstatite 2023-2025
[72]
The complete ISO spectrum of NGC 6302 [*]
Forsterite and enstatite do have roughly the same abundance and temperature. The origin and location of the dust in a toroidal disk around the central star are ...
[73]
JWST/MIRI view of the planetary nebula NGC 6302 - Oxford Academic
Aug 27, 2025 · The labels of crystalline dust are: Fo (Forsterite), En (Enstatite), and Qu (Quartz). Some bright atomic lines saturated the detectors, and ...
[74]
COSIMA-Rosetta calibration for in situ characterization of 67P ...
Remote infrared spectra of silicate emission features in comet Hale–Bopp have led to identification of the minerals forsterite and enstatite in both amorphous ...
[75]
Characterization of OSIRIS NAC filters for the interpretation of ...
OSIRIS will contribute to the definition of the comet terrains, lithology, and mineralogy in synergy with the other instruments onboard the Rosetta spacecraft.
[76]
[PDF] Mantle Mineralogy of Reduced Sub‐Earths Exoplanets and Exo ...
We investigated reduced materials such as enstatite chondrites, CH, and CB chondrites, and aubrites, as precursor bulk compositions in phase equilibrium.Missing: atmospheres | Show results with:atmospheres<|separator|>
[77]
Mantle Mineralogy of Reduced Sub‐Earths Exoplanets and Exo ...
Jun 4, 2024 · We find that silicate mantles of exoplanets located in inner regions of nebulae are dominated by pyroxene group minerals, rather than olivine, ...
[78]
enstatite and quartz clouds in the atmosphere of 2M2224-0158
Studies of brown dwarfs such as 2M2224 ... (2006) made a clear prediction for the presence of quartz in brown dwarf atmospheres as a dominant component of the ...THE TARGET: 2M2224-0158 · RESULTS AND ANALYSIS · DISCUSSION
[79]
enstatite and quartz clouds in the atmosphere of 2M2224-0158 - arXiv
May 10, 2021 · A composition toward the extremes of the local distribution is consistent with this target being an outlier in the ultracool dwarf population.Missing: silicate brown
[80]
[PDF] ANHYDROUS SILICATES IN ASTEROID BENNU SAMPLES
Anhydrous silicates in Bennu samples, such as olivine and pyroxene, are visible in images and are less than 10% of the sample.
[81]
The thermal history of Ryugu based on Raman characterization of ...
Ryugu, a C-type asteroid, has been visited by the JAXA's Hayabusa2 spacecraft. During more than a year, remote (e.g., Watanabe et al., 2019) and in situ (e.g., ...