Hypersthene — bronze calm with subtle luster
Hypersthene — a classic, smoky brown to dark slate member of the orthopyroxene family — chemically a magnesium and iron silicate mixture. On polished surfaces it often shows a gentle bronze or silver luster (schiller), like moonlight on oil. The name is old-fashioned; modern mineralogy merges "bronzite" and "hypersthene" into the orthopyroxene series, but the character remains the same: dark, quiet, and unexpectedly reflective. Imagine it as the mineral equivalent of well-tailored black shirts — reserved until light hits at the right angle.
Identity and name 🔎
Old name, modern family
Hypersthene historically meant iron-rich orthopyroxene; bronzite — magnesium-rich. Today mineralogists more often use the neutral umbrella term orthopyroxene, encompassing enstatite (Mg-end member), ferrosilite (Fe-end member), and intermediate compositions. In the gemstone/mineral world, "hypersthene" is still used as a common term for dark, bronze-colored material.
Etymology
From Greek hyper ("very") + sthenos ("strength") — a 19th-century hint at strong cleavage planes and metallic luster. Strong, but not scratch-resistant (see hardness below).
How and where it forms 🌍
Igneous environments
Hypersthene crystallizes from basaltic to andesitic magmas and is abundant in norites (orthopyroxene-rich gabbros). It can occur as phenocrysts in volcanic rocks and is a major component of coarse intrusive bodies along with plagioclase and clinopyroxene.
High-grade metamorphism
In the granulite facies (high temperature, moderate pressure) orthopyroxene appears in charnockites (granite rocks containing hypersthene) and mafic granulites, often recording dry, hot lower crust conditions.
Notable areas
Classic localities: parts of Canada (noritic belts), Adirondacks (USA), Scandinavia, Greenland, India (charnockite areas), and South Africa. Where cooling of mafic intrusions was slow — or the crust was "baked" by heat — orthopyroxene is likely.
Appearance and schiller 👀
Colors and mood
- Coal to black shale — common in polished cabochons.
- Smoky brown / sepia — the classic warmth of "bronzite."
- Olive gray — especially in Mg-richer material.
Transparency is usually opaque to semi-translucent at thin edges. Fresh surfaces are glassy; cleavage planes may appear silky to metallic luster.
Where does the sheen come from?
A bronze/silver schiller is created by aligned microscopic inclusions and exsolution lamellae inside the crystal (often oxides or very thin compositional layers). Light scatters and reflects off these planes, producing a soft, directional glow. Turn the stone — and a "curtain" of light slides across it: subtle, calm, and very pleasant.
Home observation: Shine a small flashlight across the polished surface and slowly rotate the stone; watch how the sheen follows the light like a slow meteor.
Personality sketch: quiet self-confidence. Hypersthene doesn't shout — its gaze is a knowing nod when the light finds it.
Physical and optical properties 🧪
| Property | Typical limit / note |
|---|---|
| Chemical composition | (Mg,Fe)SiO3 Orthopyroxene; composition between enstatite and ferrosilite |
| Crystal system | Orthorhombic; prismatic crystal habit |
| Hardness | ~5.5–6 (beware of abrasives and impacts) |
| Relative density | ~3.3–3.5 (Fe-richer varieties feel somewhat heavier) |
| Cleavage | Two well-expressed cleavages ~90° (typical for pyroxenes) |
| Luster | Glassy to silky; on schiller planes — metallic-like |
| Optical properties | Biaxial (+). Refractive index ~1.69–1.77 (increases with Fe content); birefringence is small |
| Pleochroism | Visible in thin sections: greenish → brownish tones, depending on orientation |
| Streak color | White to gray |
Under loupe / microscope 🔬
Polished cabochons
At 10× magnification you can see very fine parallel lamellae or dot-like submicron inclusions aligned along cleavage. These are "mirrors" that create schiller.
Thin sections (crossed polarizers)
- Low to medium interference colors (1st order).
- Parallel extinction relative to cleavage traces.
- Simple twinning in orthopyroxene is rare (compared to being more common in clinopyroxenes).
Geological note
Exsolution textures (orthopyroxene with clinopyroxene lamellae or oxide lamellae) preserve cooling histories — tiny "time stamps" of deep magmas.
Woods where hypersthene is found 🧱
Norite ("hypersthene gabbro")
Intrusive mafic rock with plagioclase + orthopyroxene (hypersthene) as main minerals. Common in layered intrusions and impact crater structures.
Charnockites and granulites
High-temperature crustal rocks; orthopyroxene with feldspars and quartz indicates dry, hot metamorphism.
Basalt and andesite
As phenocrysts in volcanic lavas — small prismatic crystals that weather to dark, earthy shades.
Similar minerals and how to distinguish 🕵️
Obsidian (glassy varieties)
Volcanic glass with shiny inclusions; no cleavage, conchoidal fracture dominates. Hypersthene shows right-angle cleavage in good light.
Labradorite / spectrolite
Iridescence ("labradorescence") of feldspar flashes colors (blue/green/golden). Hypersthene's luster is a single-tone bronze/silver sheen, not a rainbow "plate" effect.
Hematite / metallic oxides
Truly metallic luster and significantly higher SG; streak is reddish brown (hematite). Hypersthene's metallic impression is superficial — streak remains light.
Amphibole (hornblende)
Similar dark appearance, but cleavage angles ~60°/120°. If angles look "V" shaped — amphibole likely; if "box" shaped — pyroxene.
Bronzite and hypersthene
Both are orthopyroxenes. Bronzite is usually Mg-rich (often warmer brown), hypersthene is Fe-rich (usually darker). In practice, the names are more descriptive than strict.
Quick checklist
- Two ~90° cleavages (distinguishing feature of pyroxenes).
- Subtle single-color schiller, not multicolored iridescence.
- Invisible, but thin edges can be translucent; fresh fractures are glassy.
Care and stability 🧼
Everyday use
- Medium hardness (~5.5–6). Handle like a favorite lens — no pocket sand.
- Because of cleavage, avoid sudden impacts on edges and corners.
- Gently wipe before photographing; luster shows best on clean, dry surfaces.
Cleaning
- Mild soap + lukewarm water + soft cloth/brush; rinse well and dry.
- Avoid ultrasonic/steam cleaners — microcracks and cleavage don’t “like” them.
- No aggressive acids/bases or abrasive powders.
Storage
- Keep separate from harder silicates and quartz to protect the polished surface.
- Cushion specimens with inert foam or acid-free paper; support by the base, not the cleavage planes.
Questions ❓
Is “hypersthene” still an official mineral name?
In strict modern classification, most samples are called orthopyroxene with compositions between enstatite and ferrosilite. The name “hypersthene” informally remains for dark, bronze material (and it works well for a general audience).
Why do some pieces look almost metallic?
Because aligned microscopic inclusions and lamellae reflect light inside the crystal, creating a surface metallic effect called schiller.
Can hypersthene be translucent?
On very thin edges — yes, smoky greenish brown. Most collector pieces and cabochons appear opaque.
Does it ever show asterism (a star)?
Usually not. If you see a bright star in a dark stone, it's more likely black star diopside than hypersthene.
How else do pyroxenes and amphiboles differ?
Look at the cleavage angles. Pyroxenes ≈90°; amphiboles ≈60°/120°. That simple geometric clue solves many identity puzzles.