How to identify Peridotite

To correctly identify Peridotite (Peridotite), 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 black, often with a yellowish-green hue from olivine. Can be mottled or speckled.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to greasy on fresh surfaces, especially from olivine. Serpentinized varieties can have a waxy or silky luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Coarse-grained, phaneritic (crystals visible to the naked eye). Can be granular or poikilitic.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual mineral grains (olivine, pyroxene) are typically anhedral to subhedral, interlocking. Olivine often forms equant grains.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Olivine has poor cleavage. Pyroxenes exhibit two distinct cleavages at approximately 90 degrees (orthopyroxene) or 87/93 degrees (clinopyroxene).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in ophiolite complexes (obducted oceanic crust and upper mantle), as xenoliths in kimberlites and other volcanic rocks, and as cumulates in large layered mafic intrusions. Represents mantle material.

Key Facts

Hardness

6.5-7 (olivine), 5-6 (pyroxene), 2.5-4 (serpentine if altered)

Specific Gravity

3.2-3.4 g/cm³ (fresh peridotite), decreasing with serpentinization (2.5-2.8 g/cm³ for serpentinite)

Crystal System

Orthorhombic (olivine, orthopyroxene), Monoclinic (clinopyroxene)

Color

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

Luster

Vitreous to greasy (olivine), vitreous (pyroxene), waxy to silky (serpentine).

Transparency

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

Fracture

Conchoidal to uneven (olivine), uneven (pyroxene).

Cleavage

Poor to indistinct (olivine), good in two directions at ~90° (orthopyroxene) or ~87/93° (clinopyroxene).

Composition

Predominantly olivine ((Mg,Fe)2SiO4) and pyroxenes (Mg,Fe)SiO3 and Ca(Mg,Fe)Si2O6. Accessory minerals may include chromite, magnetite, garnet, spinel, amphibole, and phlogopite.

Physical characteristics

• Crystal Habit: Granular, anhedral to subhedral interlocking crystals.
• Cleavage Type: Poor in olivine, distinct prismatic in pyroxenes.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Vitreous to greasy on fresh surfaces.

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