How to identify Peridot in Basalt

To correctly identify Peridot in Basalt (Olivine (variety Peridot) in Basalt), 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

   Basalt matrix is typically dark gray to black. Peridot crystals are olive green, yellowish-green, or brownish-green.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Basalt matrix is dull to earthy, sometimes vitreous in fresh fractures. Peridot crystals exhibit a vitreous (glassy) luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Basalt is typically fine-grained (aphanitic) to porphyritic (with larger crystals in a fine-grained matrix). It can also be vesicular (containing gas bubbles). Peridot crystals are typically anhedral to subhedral, sometimes euhedral, embedded within the basalt.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Peridot crystals are often anhedral to subhedral grains, sometimes showing characteristic stubby prismatic forms if well-developed. They are typically disseminated throughout the basalt or concentrated in nodules.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Peridot (olivine) has poor to indistinct cleavage in two directions at 90 degrees, often appearing as conchoidal fracture. Basalt does not exhibit cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Volcanic settings, particularly in areas of active or recent basaltic volcanism, such as oceanic islands, continental rifts, and flood basalt provinces. Found in lava flows, volcanic bombs, and tuffs.

Key Facts

Hardness

Basalt: 6-7 (Mohs); Peridot (Olivine): 6.5-7 (Mohs)

Specific Gravity

Basalt: 2.7-3.3; Peridot (Olivine): 3.27-3.37

Crystal System

Basalt: Not applicable (rock); Peridot (Olivine): Orthorhombic

Color

Basalt: Dark gray to black; Peridot: Olive green, yellowish-green, brownish-green

Luster

Basalt: Dull to earthy; Peridot: Vitreous

Transparency

Basalt: Opaque; Peridot: Transparent to translucent

Fracture

Basalt: Irregular to conchoidal; Peridot: Conchoidal

Cleavage

Basalt: None; Peridot: Poor to indistinct in two directions at 90 degrees

Composition

Basalt: Primarily plagioclase feldspar, pyroxene, and often olivine; Peridot: (Mg,Fe)2SiO4 (Magnesium Iron Silicate)

Physical characteristics

• Crystal Habit: Peridot: Granular, disseminated grains, stubby prismatic crystals, often anhedral to subhedral. Basalt: Aphanitic (fine-grained) to porphyritic.
• Cleavage Type: Peridot: Poor to indistinct {010} and {100}. Basalt: None.
• Fracture Type: Peridot: Conchoidal. Basalt: Irregular to sub-conchoidal.
• Tenacity: Peridot: Brittle. Basalt: Brittle.
• Luster Type: Peridot: Vitreous. Basalt: Dull, earthy, sometimes sub-vitreous.

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