How to identify Peridotite

To correctly identify Peridotite (Peridotite (olivine-rich ultramafic rock)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Typically dark green to yellowish-green, black, or brownish-black. The green color is due to the high olivine content. Weathered surfaces can be reddish-brown due to oxidation of iron.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to greasy on fresh olivine surfaces; pyroxenes can be vitreous. Serpentinized peridotite may have a waxy or silky luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Coarse-grained, phaneritic (visible crystals). Can be equigranular or porphyritic (larger olivine crystals in a finer matrix).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Olivine crystals are typically anhedral to subhedral, often granular. Pyroxenes can be subhedral to euhedral.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Olivine has poor cleavage (or none), often exhibiting conchoidal fracture. Pyroxenes have distinct cleavage (two directions at nearly 90 degrees for orthopyroxene, and 87/93 degrees for clinopyroxene).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in ophiolite complexes (obducted oceanic crust and mantle), as xenoliths in kimberlites and basalts, and as cumulates in layered mafic intrusions. Also occurs in the roots of ancient mountain belts and in mantle diapirs.

Key Facts

Hardness

6.5-7 (olivine), 5-6 (pyroxene)

Specific Gravity

3.2-3.4 g/cm³ (higher than most common rocks)

Crystal System

Orthorhombic (olivine, orthopyroxene), Monoclinic (clinopyroxene)

Color

Dark green, yellowish-green, black, brownish-black

Luster

Vitreous to greasy

Transparency

Opaque to translucent (olivine can be transparent in thin sections)

Fracture

Conchoidal to uneven (olivine), uneven (pyroxene)

Cleavage

Poor to absent (olivine), distinct (pyroxenes)

Composition

Predominantly olivine (>40%), with varying amounts of orthopyroxene, clinopyroxene, and accessory minerals like chromite, garnet, and spinel.

Physical characteristics

• Crystal Habit: Granular, anhedral to subhedral crystals of olivine and pyroxene.
• Cleavage Type: Olivine: Poor to absent. Pyroxenes: Two distinct cleavages at nearly 90 degrees (orthopyroxene) or 87/93 degrees (clinopyroxene).
• Fracture Type: Conchoidal to uneven (olivine), uneven (pyroxene).
• Tenacity: Brittle.
• Luster Type: Vitreous to greasy.

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