Hypersthene — Bronze Calm with a Subtle Shimmer
Hypersthene is the classic, smoky‑brown to deep slate member of the orthopyroxene family—chemically a mix of magnesium and iron silicate. On polished faces it often shows a soft bronze or silvery sheen (schiller) that feels like moonlight on oil. The name is old‑school; modern mineralogy collapses “bronzite” and “hypersthene” into the orthopyroxene series, but the character remains: dark, quiet, and unexpectedly reflective. Think of it as the mineral version of a well‑tailored black shirt—understated until the light hits just right.
Identity & Naming 🔎
Old name, modern family
Hypersthene historically referred to iron‑rich orthopyroxene; bronzite to magnesium‑rich. Today, mineralogists prefer the neutral umbrella orthopyroxene, spanning enstatite (Mg‑end), ferrosilite (Fe‑end), and intermediates. In the gem/mineral world, “hypersthene” persists as a familiar label for dark, bronzy material.
Etymology
From Greek hyper (“very”) + sthenos (“strong”)—a 19th‑century nod to robust cleavage surfaces and a metallic‑looking sheen. Strong but not scratch‑proof (see hardness below).
How & Where It Forms 🌍
Igneous settings
Hypersthene crystallizes from basaltic to andesitic magmas and is abundant in norite (orthopyroxene‑rich gabbro). It can appear as phenocrysts in volcanic rocks and as a major constituent of coarse intrusive bodies alongside plagioclase and clinopyroxene.
High‑grade metamorphism
At granulite facies (high temperature, moderate pressure), orthopyroxene pops up in charnockites (hypersthene‑bearing granitic rocks) and in mafic granulites, often recording dry, hot conditions in the lower crust.
Notable regions
Classic occurrences include parts of Canada (norite belts), the Adirondacks (USA), Scandinavia, Greenland, India (charnockite terrains), and Southern Africa. Anywhere mafic intrusions cooled slowly—or crust baked hot—orthopyroxene has a good chance.
Appearance & Schiller 👀
Colors & mood
- Charcoal to blackish slate — common in polished cabochons.
- Smoky brown / sepia — classic “bronzite” warmth.
- Olive‑gray — especially in more Mg‑rich material.
Translucency is typically opaque to semi‑translucent on thin edges. Fresh surfaces are vitreous; cleavage planes can look silky to metallic.
Why the sheen?
The bronzy/silver schiller comes from aligned microscopic inclusions and exsolution lamellae within the crystal (often oxides or ultra‑thin compositional layers). Light scatters and reflects off these planes, producing a soft, directional gleam. Tilt the stone and a “curtain” of light sweeps across—subtle, relaxing, and very satisfying.
Home observation: Aim a small flashlight across a polished surface and rock the stone slowly; watch the sheen track the light like a slow meteor.
Personality sketch: quiet confidence. Hypersthene doesn’t shout—its shimmer is a knowing nod when the light finds it.
Physical & Optical Properties 🧪
| Property | Typical Range / Note |
|---|---|
| Chemistry | (Mg,Fe)SiO3 orthopyroxene; composition between enstatite and ferrosilite |
| Crystal system | Orthorhombic; prismatic habit |
| Hardness | ~5.5–6 (careful with abrasives and knocks) |
| Specific gravity | ~3.3–3.5 (Fe‑rich varieties feel slightly heftier) |
| Cleavage | Two good cleavages at ~90° (typical pyroxene “stubby” cleavage) |
| Luster | Vitreous to silky; metallic‑like on schiller planes |
| Optics | Biaxial (+). RI commonly ~1.69–1.77 (rises with Fe); birefringence modest |
| Pleochroism | Visible in thin slices: greenish → brownish tones depending on orientation |
| Streak | White to gray |
Under the Loupe / Microscope 🔬
Polished cabochons
At 10×, you may see ultra‑fine parallel lamellae or speckled sub‑micron inclusions that line up along cleavage. They’re the mirrors that make schiller.
Thin sections (crossed polars)
- Low to moderate interference colors (1st order).
- Parallel extinction relative to cleavage traces.
- Simple twinning rare in orthopyroxene (contrast with clinopyroxenes).
Geology bonus
Exsolution textures (orthopyroxene with clinopyroxene lamellae, or oxide lamellae) preserve cooling histories—tiny time stamps from deep‑seated magmas.
Rocks that Host Hypersthene 🧱
Norite (“hypersthene gabbro”)
Intrusive mafic rock with plagioclase + orthopyroxene (hypersthene) as key players. Famous in layered intrusions and impact structures.
Charnockite & granulites
High‑temperature crustal rocks; orthopyroxene with feldspars and quartz signals dry, hot metamorphism.
Basalt & andesite
As phenocrysts in volcanic lavas—small prismatic crystals that weather dark and earthy.
Look‑Alikes & How to Tell 🕵️
Obsidian (sheen varieties)
Volcanic glass with glittery inclusions; no cleavage, conchoidal fractures dominate. Hypersthene shows right‑angle cleavage under good light.
Labradorite / Spectrolite
Iridescence (“labradorescence”) in feldspar flashes in colors (blue/green/gold). Hypersthene’s sheen is a one‑tone bronze/silver glide, not a rainbow plate.
Hematite / metallic oxides
Truly metallic luster and much higher SG; streak is red‑brown (hematite). Hypersthene’s metallic impression is shallow—the streak stays pale.
Hornblende (amphibole)
Similar dark look, but cleavage angles are ~60°/120°. If the angles look “V‑shaped,” think amphibole; if boxy, think pyroxene.
Bronzite vs. Hypersthene
Both are orthopyroxene. Bronzite skews Mg‑rich (often warmer brown); hypersthene skews Fe‑rich (often darker). In practice the names are descriptive rather than strict.
Quick checklist
- Two ~90° cleavages (pyroxene hallmark).
- Subtle single‑color schiller, not multicolor iridescence.
- Opaque to translucent edges; vitreous on fresh breaks.
Care & Stability 🧼
Everyday handling
- Hardness is mid‑range (~5.5–6). Treat like a favorite camera lens—no pocket sand.
- Cleavage means avoid sharp knocks on edges and corners.
- Wipe gently before photos; the sheen shows best on clean, dry surfaces.
Cleaning
- Mild soap + lukewarm water + soft cloth/brush; rinse and dry well.
- Avoid ultrasonic/steam cleaners—micro‑fractures plus cleavage aren’t fans.
- No harsh acids/alkalis or abrasive powders.
Storage
- Store separately from harder silicates and quartz to protect the polish.
- Pad specimens with inert foam or acid‑free tissue; support along the base rather than cleaved faces.
Questions ❓
Is “hypersthene” still an official mineral name?
In strict modern classification, most specimens are called orthopyroxene with a composition specified between enstatite and ferrosilite. The name “hypersthene” survives informally for dark, bronzy material (and that’s fine for general readers).
Why do some pieces look almost metallic?
Because aligned microscopic inclusions and lamellae reflect light from within the crystal, creating a shallow metallic mimic known as schiller.
Can hypersthene be translucent?
On very thin edges, yes—smoky green‑brown. Most cabinet pieces and cabs read opaque.
Does it ever show asterism (a star)?
Not typically. If you see a sharp star in a dark stone, think black star diopside, not hypersthene.
What’s the difference between pyroxene and amphibole again?
Look at the cleavage angle. Pyroxene ≈90°; amphibole ≈60°/120°. That simple geometry solves many ID puzzles.