Hypersthene

Description

Hypersthene is an orthopyroxene mineral, an intermediate member of the enstatite (MgSiO3) - ferrosilite (FeSiO3) solid solution series. Its chemical formula is (Mg,Fe)SiO3, with iron (Fe) content typically ranging from 30% to 50% of the total magnesium and iron. It is characterized by its dark color, often with a bronze-like metallic luster or a distinctive schiller effect (play of color) due to minute exsolution lamellae of other pyroxenes or iron oxides. It is an important rock-forming mineral in many mafic and ultramafic igneous and high-grade metamorphic rocks.

How to Identify

Color

Dark brown, greenish-black, grayish-black. Often exhibits a bronze-like sheen or schiller.

Luster

Vitreous to pearly on cleavage surfaces, submetallic to metallic on fractured surfaces, especially with schiller.

Texture

Typically massive, granular, or in lamellar aggregates. Can be prismatic.

Crystal Form

Orthorhombic system, typically forming short, stout prismatic crystals, often with a rectangular cross-section. Can also be massive or lamellar.

Cleavage

Two distinct cleavages at nearly 90 degrees (orthorhombic pyroxene cleavage), specifically {210} and {100} or {010} at 87 and 93 degrees.

Geological Environment

Mafic and ultramafic igneous rocks (e.g., norites, gabbros, pyroxenites), high-grade metamorphic rocks (e.g., granulites), and some chondritic meteorites.

Key Facts

• Hardness: 5 - 6 on Mohs scale
• Specific Gravity: 3.4 - 3.9 (increases with Fe content)
• Crystal System: Orthorhombic
• Color: Dark brown, greenish-black, grayish-black; often with bronze-like schiller
• Luster: Vitreous to pearly on cleavage surfaces; submetallic to metallic on fractured surfaces (especially with schiller)
• Transparency: Translucent to opaque
• Fracture: Uneven to conchoidal
• Cleavage: Perfect on {210}, good on {100} or {010} (nearly at 90 degrees)
• Composition: (Mg,Fe)SiO3, where Fe/(Mg+Fe) is between 0.3 and 0.5

Quick Check

• Color: Dark brown, greenish-black, grayish-black, often with bronze sheen
• Luster: Vitreous to pearly, submetallic to metallic (schiller)
• Streak: Grayish-white to pale brown

Physical Characteristics

• Crystal Habit: Prismatic, massive, lamellar, granular
• Cleavage Type: Perfect prismatic (two directions at ~87° and ~93°)
• Fracture Type: Uneven to conchoidal
• Tenacity: Brittle
• Luster Type: Vitreous, pearly, submetallic, metallic

Formation

Hypersthene forms primarily in high-temperature, high-pressure metamorphic and igneous environments. It is a common constituent of mafic and ultramafic igneous rocks such as norites, gabbros, and pyroxenites, and also occurs in high-grade metamorphic rocks like granulites. It crystallizes from magmas rich in magnesium and iron, or forms through the metamorphism of iron- and magnesium-rich sediments or igneous rocks.

Usage

Hypersthene is primarily of scientific interest for understanding petrogenesis and metamorphic processes. It is occasionally used as a minor gemstone, particularly varieties exhibiting schiller or chatoyancy. Due to its hardness and resistance to weathering, it can be a component of some industrial abrasives, though not a primary source. Its iron content can contribute to its magnetic properties, which are sometimes exploited in geological surveys.

Age Distribution

Found in rocks of various ages, from Precambrian to Cenozoic, depending on the geological setting.

Where to Find

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Similar Rocks

Scientific Classification

Mineral Class

Silicates

Group

Pyroxene Group (Orthopyroxene Subgroup)

Crystal System

Orthorhombic

Chemical Formula

(Mg,Fe)SiO3

Composition

Magnesium iron silicate, with iron content between 30% and 50% of the total Mg+Fe

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