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Hypersthene
Hypersthene
Hypersthene
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Hypersthene
General
CategoryInosilicate
Formula(Mg,Fe)SiO3
Strunz classification8/F.02-20
Crystal systemOrthorhombic
Identification
ColorGray, brown, or green
TwinningOn [100]
Cleavage{100} Perfect, {010} Perfect
FractureUneven
Mohs scale hardness5.5–6
LusterVitreous to pearly
StreakGreyish white, greenish
Specific gravity3.4–3.9
Optical propertiesBiaxial (−)
Refractive indexnα = 1.669 – 1.755 nβ = 1.674 – 1.763 nγ = 1.680 – 1.773
Birefringenceδ = 0.011 – 0.018
DispersionWeak

Hypersthene is a common rock-forming inosilicate mineral belonging to the group of orthorhombic pyroxenes.[1] Its chemical formula is (Mg,Fe)SiO3. It is found in igneous and some metamorphic rocks as well as in stony and iron meteorites. Many references have formally abandoned this term, preferring to categorise this mineral as enstatite or ferrosilite. It forms a solid solution series with the minerals enstatite and ferrosilite, being a mid-way member between the two. Pure enstatite contains no iron, while pure ferrosilite contains no magnesium; hypersthene is the name given to the mineral when a significant amount of both elements are present. Enstatite is stable at atmospheric pressure, but ferrosilite is stable only at elevated pressure, decomposing into quartz and fayalite at atmospheric pressure unless stabilized by magnesium or other impurities.[2]

Distinctly developed crystals are rare, the mineral being usually found as foliated masses embedded in the igneous rocks norite and hypersthene-andesite, of which it forms an essential constituent.[1] The coarse-grained labradorite-hypersthene-rock (norite) of Paul's Island off the coast of Labrador has furnished the most typical material; for this reason, the mineral has been known as Labrador hornblende or paulite.[1]

Color is often gray, brown, or green, and the luster is usually vitreous to pearly. The pleochroism is strong, the hardness is 5–6, and the specific gravity is 3.4–3.9. On certain surfaces it displays a brilliant copper-red metallic sheen, or schiller, which has the same origin as the bronzy sheen of bronzite, but is even more pronounced. Like bronzite, it is sometimes cut and polished as a gemstone.[1]

The name "hypersthene" comes from the Greek and means "over strength", and is an allusion to its being harder than the amphibole mineral hornblende (a mineral with which it is often confused.[3]

Hypersthene has been discredited by the International Mineralogical Association and is no longer considered a valid mineral type; specimens labelled as hypersthere are usually ferroan enstatite.[4]

References

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Hypersthene

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Hypersthene is a traditional name for a variety of orthopyroxene, a rock-forming in the group with the approximate chemical formula (Mg,Fe)SiO₃, representing an intermediate composition between the magnesium-rich and iron-rich ferrosilite endmembers. Although the International Mineralogical Association (IMA) discredited the name "hypersthene" in 1988 as part of revised , replacing it with "orthopyroxene" or the specific endmember terms based on the 50% rule for solid solutions, the term persists in geological descriptions, , and historical contexts for Mg-Fe pyroxenes with roughly equal proportions of magnesium and iron. Physical and Optical Properties
Hypersthene typically exhibits a vitreous to pearly or submetallic luster, often with a variety displaying an iridescent schiller effect due to thin lamellae of exsolution. It has a Mohs hardness of 5 to 6, a specific gravity of , and perfect cleavage in two directions nearly at right angles (87° and 93°), producing a conchoidal to uneven . Colors range from grayish-green to brownish-black, and it is usually translucent to opaque, though rarely transparent; it may show weak from light pink to light green in thin sections. Optically, it is biaxial, with refractive indices of nα = 1.669–1.755, nβ = 1.674–1.763, and nγ = 1.680–1.773, and a 2V angle of 50° to 90° that varies with composition—positive near and negative for intermediate ranges—along with parallel extinction relative to cleavage. Occurrence and Formation
Hypersthene forms primarily in and ultramafic igneous rocks through the slow cooling of magnesium- and iron-rich magmas, such as in gabbros, norites, peridotites, and basalts, and is also common in high-grade metamorphic rocks like granulites under conditions of elevated temperature and pressure. Notable localities include the in New York, , and the Bushveld Complex in , with additional occurrences in stony-iron meteorites and lunar basalts. Uses and Significance
While not economically mined on a large scale, hypersthene serves as an important index mineral in petrology for classifying igneous and metamorphic rocks due to its stability in specific geochemical environments. In gemology, attractive specimens with metallic luster or schiller are occasionally cut as cabochons or faceted gems for jewelry and collector items, though it is relatively rare and fragile for such applications. Its presence in extraterrestrial materials also aids studies in planetary geology.

Etymology and history

Name origin

The name hypersthene derives from the Greek terms hyper (ὑπέρ), meaning "over" or "above," and sthenos (σθένος), meaning "strength" or "power," reflecting its perceived superior hardness relative to , a common with which it was frequently confused. This etymology was established by French mineralogist René Just Haüy in 1804, who introduced the term in his systematic classification of minerals based on crystallographic and physical properties. Haüy's coinage stemmed from observations of the mineral's robust appearance and metallic luster, leading to its initial recognition as a separate species rather than a variant within existing groups like amphiboles. The sheen, often bronze-like due to schiller effects, and its resistance to scratching—registering 5.5 to 6 on the —prompted the emphasis on "over-strength" to distinguish it from softer, similar-looking rocks. Hypersthene was first described from specimens collected in , , where it appears in dark plutonic rocks such as s and gabbros, contributing to early understandings of orthorhombic pyroxenes during the mineralogical advancements of the era. Similar occurrences were later documented in the of New York.

Discovery and nomenclature evolution

Hypersthene was first identified in samples from rocks in , , during the early 19th century, with initial descriptions appearing in René Just Haüy's Traité de Minéralogie in 1801, where it was referred to as "labradorite ." German mineralogist also studied the mineral and named it "paulite" after St. Paul Island in . Haüy formalized the name "hypersthene" in 1804 upon recognizing it as a distinct mineral species, based on its properties and occurrences in these coarse-grained igneous rocks. Similar hypersthene-bearing s were later documented in the of New York, contributing to its early characterization as a common component of basic igneous formations. Throughout the 19th and 20th centuries, hypersthene gained recognition as a key orthopyroxene mineral, integral to petrological studies of and ultramafic rocks, with its schiller effect and metallic luster distinguishing it from related silicates. In 1988, the International Mineralogical Association's Commission on New Minerals and Names (CNMMN) Pyroxene Subcommittee discredited hypersthene as a valid name, deeming it redundant for intermediate compositions in the (Mg₂Si₂O₆)–ferrosilite (Fe₂Si₂O₆) series, and recommending the use of end-member terms instead. This decision was part of a broader revision of , which formally discarded 105 obsolete names to simplify classification based on the 50% rule for series members. Despite its discreditation, hypersthene remains in common use within petrological literature for orthopyroxenes with approximately 50 mol% Fe-Mg substitution, often specified as "ferroan " to denote these intermediate compositions. This persistent application reflects its descriptive utility in rock classification and historical precedence, even as strict IMA guidelines favor more precise end-member .

Chemical composition

Molecular formula

Hypersthene possesses the ideal end-member chemical formula (\ceMg,Fe)SiO3(\ce{Mg,Fe})SiO_3, denoting a 1:1 magnesium-iron composition where magnesium and iron substitute for one another in varying proportions. This formula captures the essential stoichiometry of the mineral as part of the orthopyroxene group within the broader pyroxene family. At the atomic level, the structure features magnesium and iron cations occupying distorted octahedral sites (M1 and M2), coordinated by oxygen anions, while silica tetrahedra (\ceSiO4\ce{SiO4}) form infinite single chains parallel to the c-axis, cross-linked by these cations to create the framework. The core elemental makeup remains centered on Mg, Fe, Si, and O, though minor trace substitutions such as aluminum (Al) for silicon in tetrahedral sites or calcium (Ca) and manganese (Mn) in octahedral sites may occur without significantly altering the ideal formula. The formula weight for the pure \ceMgSiO3\ce{MgSiO3} end-member is approximately 100.39 g/mol, calculated from atomic masses (Mg: 24.305 g/mol, Si: 28.085 g/mol, O: 15.999 g/mol × 3); this increases with iron substitution, reaching about 131.93 g/mol for \ceFeSiO3\ce{FeSiO3}.

Solid solution series

Hypersthene forms part of the continuous series between (Mg2Si2O6\mathrm{Mg_2Si_2O_6}) and ferrosilite (Fe2Si2O6\mathrm{Fe_2Si_2O_6}), the two end-members of the orthopyroxene group, where magnesium and iron substitute freely at the octahedral sites in the crystal lattice. This series is characterized by complete across the compositional range, allowing for a wide variety of intermediate members with varying Mg/Fe ratios. The compositional notation for members of this series is generally expressed as (MgxFe1−x)2Si2O6(\mathrm{Mg}_x \mathrm{Fe}_{1-x})_2 \mathrm{Si}_2 \mathrm{O}_6, where xx denotes the atomic fraction of magnesium substituting for iron. Hypersthene specifically designates the intermediate compositions within this series, typically those containing approximately 30-50 mol% of the ferrosilite component (FeSiO3\mathrm{FeSiO_3}), corresponding to x≈0.5−0.7x \approx 0.5-0.7. Historically, the term hypersthene has been applied to Fe-rich varieties of enstatite, though it is now considered an informal or varietal name for these iron-bearing intermediates rather than a distinct mineral species. Precise determination of the Mg/Fe ratios in hypersthene samples is achieved through electron microprobe analysis, which provides quantitative elemental compositions at the micron scale and is essential for classifying specimens within the series. This method allows geologists to map subtle variations in composition that influence the mineral's stability and occurrence in natural settings.

Crystal structure

Orthorhombic pyroxene framework

Hypersthene, as a member of the orthopyroxene group, exhibits an inosilicate chain structure characterized by single chains of corner-sharing SiO₄ tetrahedra. These tetrahedra link to form infinite, one-dimensional chains with a repeating Si₂O₆ unit, where each silicon atom is coordinated by four oxygen atoms, resulting in Si-O bond lengths of approximately 1.62 Å. The chains are laterally connected by strips of edge-sharing octahedra occupied by Mg²⁺ and Fe²⁺ cations at M1 and M2 sites, with Mg-O bonds averaging about 2.1 Å and Fe-O bonds about 2.2 Å, forming a robust framework that accommodates the mineral's composition. This arrangement crystallizes in the orthorhombic system with Pbca, which imposes higher symmetry compared to the monoclinic clinopyroxenes. In the Pbca structure, the silicate chains run parallel to the c-axis and feature two distinct types (A and B chains) with slight differences in kinking, contributing to the overall stability of the lattice. The octahedral strips alternate with the tetrahedral chains, creating a layered framework parallel to the (100) plane, while the interchain linkages generate open channels along the chain direction that influence the mineral's packing efficiency.

Unit cell and polymorphism

Hypersthene possesses an orthorhombic with lattice parameters varying with composition; for example, near the Mg-rich endmember a ≈ 18.22 , b ≈ 8.82 , c ≈ 5.18 (volume ≈ 832 ³), increasing systematically with Fe content to a ≈ 18.43 , b ≈ 9.08 , c ≈ 5.24 (volume ≈ 877 ³) for ferrosilite, with intermediate hypersthene compositions around 850 ³ and containing Z = 8 formula units. These parameters reflect the framework of edge-sharing SiO₄ tetrahedra chains linked by M1 and M2 octahedral sites occupied by Mg and Fe cations, consistent with the overall orthorhombic structure. Variations in these dimensions occur with increasing Fe content, leading to systematic expansion of the a and b axes while the c axis shows minor changes. Orthopyroxenes, including hypersthene, display polymorphism, with the low-temperature orthorhombic form (orthoenstatite for Mg-rich, extending to intermediate hypersthene compositions) stable under ambient conditions predominating at surface temperatures and pressures. For Mg-rich compositions, it inverts to the high-temperature orthorhombic polymorph protoenstatite above approximately 1000°C; upon slow cooling, it reverts at the same temperature, while rapid below ~700°C preserves metastable monoclinic clinoenstatite. In intermediate hypersthene compositions, transition temperatures are lower (~800–900°C), and the proto form has restricted stability, with clino variants forming more readily under rapid cooling, though less common overall. X-ray diffraction is a primary method for characterizing hypersthene, with powder patterns exhibiting diagnostic peaks that confirm its orthorhombic and distinguish it from clinopyroxene analogs. Key reflections include strong lines at d-spacings of 3.24 corresponding to the (110) plane and 2.98 for the (021) plane, alongside other notable peaks such as 2.88 (040) and 2.51 (600). These d-spacings arise from the periodic arrangement in the unit cell and shift slightly with compositional variations, aiding in precise identification via . Thermal stability studies indicate that hypersthene remains intact up to about 1000°C under , beyond which polymorphic transitions to protoenstatite occur, accompanied by changes in parameters such as increased chain kinking. This transition temperature can vary modestly with Fe/Mg ratio, with more Fe-rich compositions showing slightly lower stability thresholds, but hypersthene's ambient form is robust against typical geological cooling paths.

Physical properties

Optical and luster characteristics

Hypersthene exhibits a range of colors influenced by its iron and magnesium content, typically appearing grayish-green to brownish-black, with varieties in , , or tones; in thin section, it is colorless. The mineral displays weak , with X = colorless to pale yellow, Y = pale yellow to , Z = pale green to along different crystallographic axes. Its luster is vitreous to pearly, particularly along cleavage surfaces, but hypersthene is notable for a distinctive schiller effect that produces an iridescent copper-red or sheen. This arises from light reflection off lamellar inclusions or exsolution lamellae within the . Optically, it is biaxial (+ for Mg-rich, - for more Fe-rich compositions), with refractive indices of nα = 1.669–1.755, nβ = 1.674–1.763, and nγ = 1.680–1.773, a of approximately 0.011–0.018, and a 2V of 50° to 90°; is parallel relative to {110} cleavage. It is generally subtranslucent to opaque in hand specimen but appears transparent in petrographic thin sections, where these aid in its identification under polarized light.

Hardness, density, and cleavage

Hypersthene exhibits a of 5 to 6, which allows it to scratch (approximately 5.5 on the ) but not (7 on the ), indicating moderate durability suitable for certain industrial and ornamental uses. The specific gravity of hypersthene ranges from 3.2 to 3.9, with values increasing as the iron (Fe) content rises relative to magnesium (Mg) in its composition; Mg-rich end-members have lower around 3.2–3.3, while Fe-rich ferrosilite approaches 3.9–4.0. Hypersthene displays perfect prismatic cleavage along {110} planes (intersecting at approximately 87°–93°), with partings on {100} and {010}; when cleavage is absent, it exhibits an uneven to subconchoidal . Twinning in hypersthene is common and typically occurs as simple or lamellar forms on {100}, which can complicate cutting by introducing planes of weakness that affect polishing and faceting stability.

Occurrence

Igneous rock associations

Hypersthene is a key mineral in to intermediate s, where it forms through the of magmas in intrusive and extrusive settings. It commonly occurs in , a type of distinguished by its predominance of orthopyroxene over clinopyroxene, as well as in gabbroic bodies and their extrusive equivalents like and . In these environments, hypersthene crystallizes early in the cooling sequence from basaltic to andesitic magmas at temperatures typically ranging from 900 to 1200°C, often coexisting in equilibrium with and pigeonite before inverting from pigeonite upon slower cooling. In volcanic rocks such as and , hypersthene appears as zoned microphenocrysts, reflecting variations in composition and temperature during ascent and eruption; hypersthene indicates silica-saturated conditions, while in certain arc-related melts, processes can alter melt inclusion compositions in . Its presence in these rocks highlights within evolving melts, where it helps define the boundary between saturated and transitional compositions. Hypersthene's paragenesis in norites typically involves association with calcic plagioclase as the dominant phases, alongside accessory olivine and hornblende, forming cumulate layers in layered intrusions. Notable occurrences include the Bushveld Complex in South Africa, where it forms part of hypersthene gabbro and norite layers in the Main Zone; the Stillwater Complex in Montana, USA, featuring hypersthene in noritic cumulates of the Lower Banded series; and the Nain Plutonic Suite in Labrador, Canada, within anorthosite-norite associations.

Metamorphic and extraterrestrial settings

Hypersthene forms in granulite-facies metamorphism primarily through subsolidus reactions between olivine and plagioclase in mafic protoliths, such as metagabbros, under elevated pressure and temperature conditions of 600–800°C and 4–8 kbar. These reactions produce orthopyroxene-rich assemblages, often involving the breakdown of primary mafic minerals to form coronas or symplectites around relict olivine grains. It is a characteristic mineral in charnockites, which are orthopyroxene-bearing to intermediate granulites, and in mafic granulites, where it coexists with , clinopyroxene, and in anhydrous assemblages indicative of low during peak . During exhumation and retrogression to amphibolite-facies conditions, hypersthene commonly alters to hydrous phases like via hydration reactions, preserving reaction rims that record the transition from dry to wet metamorphic environments. A prominent example is the charnockites of Enderby Land in , where hypersthene occurs in enderbitic rocks formed during Archean high-grade . Another notable locality is the in New York, USA, where hypersthene is found in granulites and anorthosites. In extraterrestrial settings, hypersthene, identified as low-calcium orthopyroxene, is abundant in achondritic meteorites of the howardite-eucrite-diogenite (HED) clan, particularly in diogenites, which are cumulate rocks dominated by coarse-grained hypersthene (En₆₀–₉₀), and in howardites, which are breccias incorporating diogenitic fragments. These meteorites, likely derived from the asteroid , feature hypersthene with magnesium-rich compositions reflecting fractional crystallization in a differentiated parent body. Hypersthene also appears as orthopyroxene in lunar basalts, such as KREEP-rich varieties from Apollo samples, where it crystallizes early in the sequence alongside pigeonite and . In Martian meteorites, it is present in orthopyroxenites like ALH 84001 and as minor phases in shergottites, recording cumulate and basaltic processes on Mars. It occurs in stony-iron meteorites such as mesosiderites, within the silicate portion alongside metal. The Allende contains manganiferous hypersthene in certain chondrule rims, highlighting its role in early solar system condensates.

Distinctions and varieties

Relation to enstatite-ferrosilite series

In 1988, the International Mineralogical Association's Commission on New Minerals and Mineral Names (CNMMN) ruled hypersthene invalid as a distinct species, reclassifying its compositions within the enstatite-ferrosilite series as (>50 mol.% MgSiO₃ component), ferrosilite (>50 mol.% FeSiO₃ component), or with adjectival modifiers like ferroan enstatite for Mg-dominant intermediates or magnesian ferrosilite for Fe-dominant ones. This decision eliminated subdivision names like hypersthene, which had previously denoted intermediate members of the orthorhombic solid solution between (Mg₂Si₂O₆) and ferrosilite (Fe₂Si₂O₆). Despite this formal discrediting, the term hypersthene persists in petrological contexts for convenience, particularly when describing intermediate orthopyroxenes in hand specimens and thin sections where precise chemical analysis is unavailable. Historically, hypersthene referred to compositions with 30–50% Fe (as ferrosilite component), distinguishing it from (typically >70% Mg) and contrasting with pigeonite, the monoclinic intermediate form stable at higher temperatures. In modern mineral databases, hypersthene is listed as a discredited name, with specimens redirected to the enstatite-ferrosilite series; however, it remains indexed in older geological literature predating the 1988 ruling.

Similar minerals and identification

Hypersthene, an orthopyroxene , can be confused with other dark mafic silicates due to its color and prismatic habit, but it is distinguished primarily through crystal , optical behavior, and physical traits. Compared to augite, a common clinopyroxene, hypersthene exhibits orthorhombic symmetry versus augite's monoclinic structure, leading to parallel extinction under crossed polars in thin section, while augite shows inclined extinction up to 45°; additionally, hypersthene displays lower (first- to second-order interference colors) and weak (pale pink to green), contrasting with augite's higher birefringence and lack of pleochroism. Hornblende, an , differs from hypersthene in cleavage angles—approximately 56°/124° in versus nearly 90° in hypersthene—and lacks the schiller effect (metallic reflections from exsolved lamellae) often seen in hypersthene; hardness is similar (both 5–6 on ), but typically shows stronger (green-brown-yellow) and inclined extinction. Bronzite, another orthopyroxene variety richer in magnesium, closely resembles hypersthene but features a distinctive velvet-like, sub-metallic bronzy sheen, whereas hypersthene's luster is more vitreous with iridescent schiller ranging from silver to copper-red. For definitive identification, immersion measures hypersthene's refractive indices (nα = 1.669–1.755, nβ = 1.674–1.763, nγ = 1.680–1.773), which are generally lower than 's; X-ray diffraction (XRD) confirms the orthorhombic lattice, distinguishing it from monoclinic or amphiboles; electron microprobe analysis (EMPA) determines the Mg/Fe ratio (typically 70:30 to 50:50 for hypersthene versus higher Mg in ).

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