Definition
Hypersthene is a silicate mineral of the orthorhombic pyroxene group, with a chemical composition that lies between the end‑members enstatite (MgSiO₃) and ferrosilite (FeSiO₃). It is commonly found as a constituent of igneous and metamorphic rocks.
Overview
Hypersthene occurs worldwide in a variety of geological settings, including mafic and ultramafic igneous rocks such as gabbro, norite, and diorite, as well as in high‑grade metamorphic rocks like granulite. Historically, the term “hypersthene” was used to describe orthopyroxenes that displayed a noticeable degree of iron substitution, but modern mineralogical classification generally subsumes it under the broader term “orthopyroxene.” Crystalline aggregates may appear as euhedral prisms or as granular to massive textures in hand specimen.
Etymology/Origin
The name derives from the Greek words hyper (“above” or “exceedingly”) and sthenos (“strength”), a reference to the mineral’s relatively high specific gravity compared with other silicates. The term was introduced in the early 19th century by mineralogists seeking to distinguish this iron‑bearing orthopyroxene from the magnesium‑rich enstatite.
Characteristics
- Chemical formula: (Mg,Fe)SiO₃, with typical Fe/(Fe+Mg) ratios ranging from 0.1 to 0.5.
- Crystal system: Orthorhombic; space group Pbnm.
- Physical properties:
- Color: Typically dark brown, greenish‑brown, or black; may exhibit a brownish sheen.
- Luster: Vitreous to sub‑metallic.
- Hardness: 5.5–6 on the Mohs scale.
- Specific gravity: 3.2–3.4, higher than that of pure enstatite.
- Cleavage: Two nearly perpendicular cleavage planes at 90° (characteristic of pyroxenes).
- Fracture: Uneven to conchoidal.
- Optical properties: Biaxial (+) with refractive indices nα ≈ 1.650–1.680, nβ ≈ 1.660–1.690, nγ ≈ 1.660–1.700; 2V angle typically 70°–85°.
Related Topics
- Orthopyroxene group – the broader mineral series that includes hypersthene, enstatite, and ferrosilite.
- Enstatite – the magnesium‑rich end‑member of the orthopyroxene series.
- Ferrosilite – the iron‑rich end‑member of the orthopyroxene series.
- Pyroxene mineral group – a major class of chain silicates significant in igneous and metamorphic petrology.
- Mafic and ultramafic rocks – rock types where hypersthene is a common accessory mineral.
- Metamorphic facies – particularly the granulite facies, where orthopyroxenes such as hypersthene are stable.