Serpentinite Identification Guide

What Serpentinite Looks Like

Serpentinite is a metamorphic rock composed mostly of serpentine-group minerals, formed by alteration of ultramafic rocks (peridotite/dunite). It is typically dark to bright green, olive, grey-green, or black, often mottled, streaked, or veined with paler green or white. Surfaces frequently show a waxy, greasy, or polished/slickensided sheen because the rock is sheared. It can contain fibrous chrysotile veins and relict crystals of olivine or pyroxene. Texture is generally fine-grained and massive, sometimes scaly or fibrous.

Step-by-Step Field ID Checklist

1. Color and pattern — green to blackish-green, mottled with lighter veins.
2. Surface feel — smooth, soapy, often with shiny slickensided (polished) surfaces from shearing.
3. Hardness — soft overall (Mohs ~2.5–4 for the serpentine matrix); a knife scratches it.
4. Look for veins — white/green chrysotile or magnetite stringers; magnetite makes parts weakly magnetic.
5. Setting — found in ophiolite belts, fault zones, and subduction mélange.
6. Streak — pale/white.

Key Diagnostic Tests

• Mohs hardness: matrix ~2.5–4, knife-scratchable; harder relict grains may persist.
• Streak: white to pale green.
• Magnetism: often weakly to moderately magnetic due to magnetite formed during serpentinization.
• Density: ~2.5–2.7 g/cm³.
• Acid: generally inert in dilute HCl (unless it contains carbonate veins, which will fizz).
• Texture: sheared, slickensided, greasy surfaces are characteristic of tectonized serpentinite.

Common Look-Alikes and How to Tell Them Apart

• Greenstone/metabasalt: also green but harder and less greasy; serpentinite is softer (knife-scratchable) and feels soapy.
• Soapstone (steatite): even softer (fingernail scratches it, Mohs 1) and dominated by talc; serpentinite is slightly harder and often more strongly green/mottled.
• Greenschist: foliated with visible chlorite/actinolite sheen; serpentinite is more massive and waxy.
• Eclogite: hard, dense, with red garnet and green omphacite; very different hardness and density.
• Massive serpentine (mineral): serpentinite is the rock made largely of that mineral; the distinction is scale (rock vs. mineral specimen).

Where Serpentinite Is Typically Found

Serpentinite occurs in ophiolite complexes, subduction mélanges, and major fault zones where seawater altered mantle rocks. Classic localities include the California Coast Ranges (state rock of California), the Alps, the Lizard in Cornwall (England), the Apennines (Italy), and ophiolite belts in Oman, Cyprus (Troodos), and New Caledonia. It often hosts asbestos, chromite, and nickel deposits.