Harzburgite Identification Guide

What Harzburgite Looks Like

Harzburgite is a coarse-grained ultramafic plutonic rock — a type of peridotite composed mainly of olivine plus orthopyroxene (enstatite), with little or no clinopyroxene. Fresh surfaces are greenish to dark grey-green and granular, with sugary olivine and stubby, bronzy orthopyroxene crystals. Weathered surfaces turn yellow-brown to rusty orange as olivine oxidizes. The rock is dense and heavy, often partly altered to serpentine, giving a waxy, slippery feel.

• Color: dark green to grey-green fresh; yellow-brown to rusty weathered
• Texture: coarse, granular, holocrystalline
• Luster: dull to greasy (especially where serpentinized)
• Habit: massive plutonic bodies; mantle xenoliths; ophiolite sequences

Step-by-Step Field-ID Checklist

1. Heft it. Ultramafic rocks feel heavy for their size (high density).
2. Look for green olivine grains. Granular, glassy olive-green crystals dominate.
3. Spot orthopyroxene. Bronzy, slightly metallic, blocky crystals with a faint sheen distinguish harzburgite from pure dunite.
4. Check for serpentine alteration. Greasy green-black patches and slickensided surfaces are common.
5. Note the setting. Ophiolites, alpine peridotite massifs, or mantle xenoliths point to harzburgite.

Key Diagnostic Tests

• Hardness: olivine ~6.5–7 and pyroxene ~5.5–6 scratch glass; serpentinized parts are softer.
• Density (SG): high, ~3.2–3.3 — distinctly heavy.
• Magnetism: weak; alteration produces magnetite, giving a slight magnetic response.
• Acid: no reaction (silicate, no carbonate).
• Color index: essentially all dark (mafic/ultramafic) minerals; no quartz or feldspar.
• Mineral mix: olivine + orthopyroxene dominant, clinopyroxene scarce — the key to naming it harzburgite vs. lherzolite.

Common Look-Alikes and How to Tell Them Apart

• Dunite: almost pure olivine (>90%); harzburgite contains noticeable orthopyroxene crystals.
• Lherzolite: contains significant clinopyroxene (often with reddish-brown to emerald-green grains, sometimes chrome diopside); harzburgite is clinopyroxene-poor.
• Pyroxenite: dominated by pyroxene with little olivine; harzburgite is olivine-rich.
• Basalt/gabbro: contain plagioclase feldspar (pale laths); ultramafic harzburgite lacks feldspar.
• Serpentinite: the fully altered product — soft, waxy, and lacking fresh olivine/pyroxene grains.

The deciding factors are an olivine + orthopyroxene assemblage with minimal clinopyroxene, no feldspar, and high density.

Where Harzburgite Is Found

Named for the Harz Mountains of Germany, harzburgite is a major component of the residual upper mantle. It is exposed in ophiolite complexes (oceanic mantle thrust onto land), alpine-type peridotite massifs, and as mantle xenoliths carried up by basalts and kimberlites. Classic localities include ophiolites in Oman, Cyprus (Troodos), and the Alps.

Quick Field Summary

A heavy, coarse, green-to-rusty ultramafic rock made of granular olivine and bronzy orthopyroxene, with little clinopyroxene and no feldspar, is harzburgite — distinct from olivine-only dunite, clinopyroxene-bearing lherzolite, and feldspar-bearing gabbro.