Lherzolite Identification Guide

What Lherzolite Looks Like

Lherzolite is a coarse-grained ultramafic plutonic rock, a type of peridotite made mostly of green olivine plus both pyroxenes (orthopyroxene and clinopyroxene). It is typically a granular, sugary-textured rock in shades of olive-green to yellowish-green, often mottled with darker pyroxene grains and sometimes flecks of chromite or red-brown spinel/garnet. Fresh surfaces look glassy-green and granular; weathered surfaces turn brownish or rusty as olivine alters. It is dense and heavy in the hand. Lherzolite is a classic mantle rock and the common host rock from which much of Earth's upper mantle is sampled.

Step-by-Step Field ID Checklist

1. Note the green, granular look. A dense rock built of interlocking green olivine grains with darker pyroxenes is the starting point.
2. Confirm coarse grain. Crystals are visible to the naked eye (plutonic, not glassy).
3. Look for two pyroxenes. Both bronzy orthopyroxene and darker green clinopyroxene grains distinguish lherzolite from olivine-only or single-pyroxene peridotites.
4. Check weight. It feels notably heavy.
5. Look for accessory spinel or garnet (small red-brown or black grains), indicating mantle origin.

Key Diagnostic Tests

• Hardness: Olivine and pyroxene are Mohs ~6.5 to 7; the rock scratches glass.
• Streak: White to gray on the silicate minerals.
• Cleavage/Fracture: Pyroxenes show cleavage at near 90 degrees; olivine fractures conchoidally; rock breaks granularly.
• Density: High, ~3.2 to 3.4 g/cm3, heavier than typical crustal rocks.
• Acid: No reaction to HCl (silicate). Serpentinized portions feel slippery and may be softer.
• Magnetism: May be weakly magnetic if it contains chromite/magnetite from alteration.

Common Look-Alikes and How to Tell Them Apart

• Harzburgite: A peridotite with olivine plus orthopyroxene but little or no clinopyroxene; lherzolite contains both pyroxenes in significant amounts. Look for the green clinopyroxene to call it lherzolite.
• Dunite: Almost entirely olivine (over 90 percent), more uniformly green with few dark pyroxene grains; lherzolite is more mottled with pyroxenes.
• Wehrlite: Olivine plus clinopyroxene but little orthopyroxene, the mirror of harzburgite.
• Serpentinite: Alteration product of peridotite, softer (Mohs ~3 to 4), slippery/waxy, and greener-gray; lherzolite is hard and granular.
• Gabbro: Darker, plagioclase-bearing, and lacks the dominant green olivine of peridotite.

Where Lherzolite Is Typically Found

Lherzolite originates in the upper mantle and is found as massifs (the type locality is Lherz, in the French Pyrenees), in ophiolite sequences, and as mantle xenoliths carried up in basalts and kimberlites. Look for it in orogenic peridotite bodies, alpine-type massifs, and within alkali basalt xenolith fields worldwide.