Pyroxenite Identification Guide

What Pyroxenite Looks Like

Pyroxenite is a coarse-grained, dark, ultramafic intrusive igneous rock composed mostly (over ~90%) of pyroxene minerals (augite, diopside, enstatite, hypersthene), with minor olivine, hornblende, biotite, or chromite/magnetite. It is dark green, brown-green, to nearly black, with a dull to slightly glittery look from cleavage faces catching light. Grains are typically visible (phaneritic), blocky, and stubby. It is heavy and feels dense in the hand.

Step-by-Step Field ID Checklist

1. Note the color: dark green to black overall — distinctly mafic/ultramafic.
2. Confirm coarse grains: you should see individual interlocking mineral crystals (phaneritic).
3. Identify the dominant mineral: blocky pyroxene with two cleavages meeting at nearly 90 degrees (versus amphibole's ~60/120 degrees).
4. Check for olivine: little or no green sugary olivine — if olivine dominates, it's peridotite/dunite instead.
5. Heft it: high density (SG ~3.1–3.4) for a hand sample.
6. Test hardness: pyroxene is 5–6; scratches glass.

Key Diagnostic Tests

• Mineralogy: dominated by pyroxene; the rock is named by its pyroxene content, not quartz/feldspar (it has essentially none).
• Cleavage angle: pyroxene cleavages near 87/93 degrees (~right angles) — the key separator from amphibole-rich rocks.
• Hardness: pyroxene 5–6 (scratches glass).
• Density: high, ~3.1–3.4; heavier than typical granite or basalt.
• Magnetism: may be weakly magnetic if magnetite/chromite is present.
• No acid reaction; no quartz or feldspar to speak of.

Common Look-Alikes and How to Tell Them Apart

• Peridotite/dunite: also dark, dense, ultramafic, but dominated by olivine (sugary, olive-green grains) rather than blocky pyroxene. If olivine is the main mineral, it's peridotite (or dunite if nearly all olivine).
• Hornblendite/amphibolite: dominated by amphibole; distinguish by cleavage angle (~60/120 degrees) and often a more bladed/needle habit, versus pyroxene's stubby ~90-degree cleavage.
• Gabbro/norite: contains substantial plagioclase feldspar (light gray laths) alongside pyroxene; pyroxenite has little to no feldspar. Look for the light plagioclase to call it gabbro.
• Basalt: fine-grained (aphanitic) volcanic equivalent of mafic magma; pyroxenite is coarse-grained and intrusive.

Where Pyroxenite Is Found

Pyroxenite occurs in layered mafic-ultramafic intrusions (e.g., the Bushveld Complex, South Africa; Stillwater Complex, Montana), in ophiolite sequences, as cumulate layers in magma chambers, and as mantle xenoliths in basalts and kimberlites. It is economically important because pyroxenite layers host chromite, platinum-group elements, and nickel sulfides.