Olivine Basalt

Igneous, Extrusive

Basalt with Olivine phenocrysts

Also known as: Basalt with Olivine phenocrysts

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Description

Olivine basalt is a common type of mafic extrusive igneous rock characterized by a fine-grained (aphanitic) to porphyritic texture, with visible phenocrysts of olivine set in a dark, fine-grained groundmass. The groundmass is typically composed of plagioclase feldspar (labradorite to bytownite), pyroxene (augite), and often magnetite, with minor amounts of other minerals. The olivine phenocrysts are typically green to yellowish-green and can range in size from a few millimeters to several centimeters. The rock is generally dark gray to black.

How to Identify

Color: Dark gray to black groundmass with distinctive yellowish-green to green olivine phenocrysts.
Luster: Dull to earthy in the groundmass; vitreous to greasy on fresh olivine phenocrysts.
Texture: Porphyritic, with larger, often euhedral to subhedral olivine crystals (phenocrysts) embedded in a very fine-grained to aphanitic (microcrystalline) groundmass. Vesicular textures (containing gas bubbles) are common.
Crystal Form: Olivine phenocrysts often exhibit characteristic stubby prismatic or equant forms. Groundmass minerals are too small to discern individual crystal forms without magnification.
Cleavage: Olivine has poor to indistinct cleavage. Pyroxene in the groundmass has two distinct cleavages at nearly 90 degrees. Plagioclase has two distinct cleavages at nearly 90 degrees. These are typically not visible in the fine-grained groundmass.
Geological Environment: Mid-ocean ridges, oceanic islands (hotspots), continental rift zones, and continental flood basalt provinces. It is a primary component of oceanic crust.

Key Facts

Hardness: 6-7 on Mohs scale (for olivine phenocrysts; groundmass is similar but variable)
Specific Gravity: 2.9-3.3 g/cm³
Crystal System: Orthorhombic (olivine); Monoclinic (pyroxene, plagioclase in groundmass)
Color: Dark gray to black with green olivine phenocrysts
Luster: Vitreous to greasy (olivine), dull to earthy (groundmass)
Transparency: Opaque (rock), transparent to translucent (olivine phenocrysts)
Fracture: Conchoidal to uneven (olivine), irregular (rock)
Cleavage: Poor to indistinct in olivine; two distinct cleavages at nearly 90 degrees in pyroxene and plagioclase (not typically visible in groundmass)
Composition: Predominantly plagioclase feldspar (labradorite-bytownite), pyroxene (augite), and olivine, with minor magnetite and other accessory minerals. SiO2 content typically 45-52 wt%.

Quick Check

Color: Dark gray to black with green spots
Luster: Dull to earthy (groundmass), vitreous to greasy (olivine)
Streak: White to grayish-white (difficult to obtain due to hardness)

Physical Characteristics

Crystal Habit: Olivine phenocrysts are typically euhedral to subhedral, stubby prismatic or equant. Groundmass is aphanitic.
Cleavage Type: Olivine: Poor {010} and {100}. Pyroxene and Plagioclase: Two distinct cleavages at nearly 90 degrees.
Fracture Type: Conchoidal to uneven for olivine; irregular for the bulk rock.
Tenacity: Brittle
Luster Type: Vitreous to greasy for olivine; dull to earthy for the fine-grained groundmass.

Formation

Olivine basalt forms from the rapid cooling of mafic lava (low silica content, rich in magnesium and iron) at or very near the Earth's surface. The presence of olivine phenocrysts indicates that olivine crystallized early from the magma at depth, before eruption, and was then carried to the surface within the remaining melt. The rapid cooling of the lava then forms the fine-grained basaltic groundmass around these larger olivine crystals.

Usage

Used extensively as crushed stone for road construction, aggregate in concrete, railway ballast, and as a source for basaltic fibers (rock wool). Its durability and resistance to weathering make it a valuable construction material. In some cultures, it has been used for tools and building materials.

Age Distribution

Common throughout Earth's history, particularly abundant in Cenozoic and Mesozoic oceanic crust and continental flood basalts.

Where to Find

Mid-Ocean Ridges

The most extensive source, though often submerged. Samples can be obtained from ophiolite complexes where oceanic crust has been obducted onto continents.

Oceanic Islands

Volcanic islands formed over hotspots, such as Hawaii, Iceland, and the Galapagos, are rich in olivine basalt flows.

Continental Flood Basalt Provinces

Large igneous provinces like the Deccan Traps (India), Siberian Traps (Russia), and Columbia River Basalt Group (USA) contain vast quantities of olivine basalt.

Continental Rift Zones

Areas where continents are pulling apart, such as the East African Rift Valley, often feature olivine basalt volcanism.

Finding Tips

Look for Dark, Fine-Grained Rocks

Basalts are typically dark. The fine-grained nature indicates rapid cooling.

Identify Green Phenocrysts

The key diagnostic feature is the presence of visible, often glassy-looking, yellowish-green to green olivine crystals. Use a hand lens if necessary.

Check for Vesicles

Many basalts are vesicular, containing small, rounded holes formed by gas bubbles. This is a common feature but not exclusive to olivine basalt.

Consider the Geological Context

If you are in a volcanic region, especially one associated with oceanic crust or rift zones, the likelihood of finding olivine basalt increases.

Similar Rocks

Tholeiitic Basalt

Also known as: Oceanic Basalt

Alkali Basalt

Also known as: Alkali Olivine Basalt

Picrite

Also known as: Olivine-rich Basalt

Gabbro

Also known as: Coarse-grained Basalt

Scientific Classification

Mineral Class: Igneous Rock
Group: Basalt Group
Crystal System: Not applicable for the rock as a whole; constituent minerals have their own systems (e.g., Olivine: Orthorhombic)
Chemical Formula: Variable, but generally (Mg,Fe)2SiO4 for olivine; Ca(Mg,Fe)Si2O6 for augite; Na(AlSi3O8)-Ca(Al2Si2O8) solid solution for plagioclase.
Composition: Mafic, low silica (45-52% SiO2), rich in Fe, Mg, Ca. Key minerals: Plagioclase (labradorite-bytownite), Pyroxene (augite), Olivine. Accessory minerals: Magnetite, Ilmenite.

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