Peridot in Basalt

Description

Peridot in basalt refers to the occurrence of green olivine crystals, specifically the gem variety peridot, embedded within a fine-grained, dark-colored basaltic matrix. The olivine crystals can range from small, disseminated grains to larger, well-formed phenocrysts or even fragments of mantle xenoliths. The contrast between the vibrant green peridot and the dark, often vesicular basalt matrix makes for a visually striking specimen. This association provides direct evidence of the Earth's mantle composition and the processes of magma generation and transport.

How to Identify

Color

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

Luster

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

Texture

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.

Crystal Form

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.

Cleavage

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

Geological Environment

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)

Quick Check

• Color: Dark gray to black matrix with olive green inclusions.
• Luster: Dull to earthy for basalt, vitreous for peridot.
• Streak: White (for peridot, though rarely tested in situ); basalt streak is typically grayish-white to colorless.

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.

Formation

Peridot (gem-quality olivine) forms at high temperatures and pressures deep within the Earth's mantle. Basalt is an extrusive igneous rock formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or moon. When basaltic magma ascends rapidly from the mantle, it can entrain crystals of olivine (peridot) that formed at depth. These olivine crystals, often as phenocrysts (larger, early-formed crystals) or as fragments of mantle xenoliths (pieces of the mantle brought up by the magma), are then incorporated into the basalt as it erupts and solidifies.

Usage

Primarily as a geological specimen for study, demonstrating mantle-derived material and volcanic processes. The peridot crystals themselves, if of sufficient size and quality, can be extracted and cut as gemstones. Basalt itself is widely used as construction aggregate, road base, and in various industrial applications.

Age Distribution

Ranges from Cenozoic to Mesozoic, depending on the specific volcanic activity. Many occurrences are relatively young, associated with active or recently active volcanism.

Where to Find

Finding Tips

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Scientific Classification

Mineral Class

Silicates

Group

Nesosilicates (Olivine Group)

Crystal System

Orthorhombic

Chemical Formula

(Mg,Fe)2SiO4

Composition

Magnesium Iron Silicate. Peridot is the gem-quality variety of olivine, specifically rich in magnesium (forsterite end-member) with some iron (fayalite end-member) contributing to its green color. The basalt matrix is composed primarily of plagioclase feldspar, pyroxene, and often accessory minerals like magnetite and ilmenite.

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