Olivine Basalt

Igneous, Extrusive (Volcanic)

Basalt (mafic volcanic rock) with Olivine (Mg,Fe)2SiO4

Also known as: Picritic Basalt (if olivine content is very high), Oceanite (a type of picritic basalt)

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Description

Basalt with olivine phenocrysts is a dark-colored, fine-grained to aphanitic (microcrystalline) igneous rock characterized by visible, often green to yellowish-green, crystals of olivine embedded within a darker, typically black to dark gray, basaltic groundmass. The groundmass itself is composed primarily of plagioclase feldspar (labradorite), pyroxene (augite), and often magnetite, with minor amounts of other minerals. The olivine phenocrysts indicate a two-stage cooling history: initial slow cooling at depth allowing olivine to grow, followed by rapid cooling at the surface.

How to Identify

Color: Groundmass is typically dark gray to black; olivine phenocrysts are green, yellowish-green, or brownish-green.
Luster: Groundmass is dull to sub-vitreous; olivine phenocrysts are vitreous (glassy).
Texture: Porphyritic, with larger, well-formed olivine crystals (phenocrysts) set in a fine-grained to aphanitic groundmass. The groundmass can be vesicular (containing gas bubbles) or amygdaloidal (vesicles filled with secondary minerals).
Crystal Form: Olivine phenocrysts often exhibit euhedral to subhedral forms, typically equant or short prismatic. The groundmass crystals are too small to discern without a microscope.
Cleavage: Basaltic groundmass minerals (plagioclase, pyroxene) have cleavage but are too small to observe macroscopically. Olivine has poor to indistinct cleavage, often appearing as conchoidal fracture.
Geological Environment: Mid-ocean ridges, oceanic islands (hotspots like Hawaii), continental flood basalts, rift zones, and volcanic arcs. It is a common product of mantle melting.

Key Facts

Hardness: Basalt (bulk rock) 5-6 on Mohs scale; Olivine 6.5-7 on Mohs scale.
Specific Gravity: Basalt 2.8-3.0 g/cm³; Olivine 3.27-3.37 g/cm³ (for forsterite-fayalite series).
Crystal System: Basalt (groundmass minerals): Plagioclase (triclinic), Pyroxene (monoclinic); Olivine (orthorhombic).
Color: Dark gray to black (groundmass), green to yellowish-green (olivine phenocrysts).
Luster: Dull to sub-vitreous (groundmass), vitreous (olivine).
Transparency: Opaque (groundmass), transparent to translucent (olivine).
Fracture: Conchoidal to uneven (basalt), conchoidal (olivine).
Cleavage: Absent to poor (basalt, due to fine grain size), poor to indistinct (olivine).
Composition: Primarily plagioclase feldspar (labradorite), pyroxene (augite), olivine, and often magnetite. SiO2 content typically 45-52 wt%.

Quick Check

Color: Dark gray to black groundmass with green to yellowish-green phenocrysts.
Luster: Dull to sub-vitreous groundmass, vitreous phenocrysts.
Streak: White to grayish-white (for the bulk rock, though individual olivine streak is white).

Physical Characteristics

Crystal Habit: Porphyritic texture with euhedral to subhedral olivine phenocrysts in an aphanitic to fine-grained groundmass.
Cleavage Type: Olivine: Poor to indistinct on {010} and {100}. Groundmass minerals: Plagioclase has two good cleavages, Pyroxene has two good cleavages at nearly 90 degrees, but these are rarely visible macroscopically in basalt.
Fracture Type: Conchoidal to uneven for the bulk rock; conchoidal for individual olivine crystals.
Tenacity: Brittle.
Luster Type: Vitreous for olivine phenocrysts; dull to sub-vitreous for the basaltic groundmass.

Formation

Formed from the rapid cooling of mafic (magnesium and iron-rich) lava at or very near the Earth's surface. The olivine phenocrysts crystallize at depth within the magma chamber or during ascent, where cooling is slower, allowing larger crystals to grow. The remaining melt then erupts and cools quickly, forming the fine-grained basaltic groundmass.

Usage

Used as aggregate in construction (roads, concrete), railway ballast, dimension stone, and in some cases, as a source of olivine for industrial applications (e.g., refractory materials, sandblasting). The presence of olivine phenocrysts generally does not significantly alter its bulk engineering properties compared to other basalts, though very high olivine content can make it slightly more susceptible to weathering.

Age Distribution

Ranges from Precambrian to present day, common in Cenozoic and Mesozoic volcanic provinces.

Where to Find

Hawaii, USA

Extensive occurrences on all Hawaiian islands, particularly Kilauea and Mauna Loa, where olivine-rich basalts are common in lava flows.

Iceland

Widespread across the island, a result of its location on the Mid-Atlantic Ridge and a mantle plume.

Columbia River Basalt Group, USA

Large igneous province in the Pacific Northwest, though olivine phenocrysts are less ubiquitous than in oceanic basalts, they are present in some flows.

Deccan Traps, India

One of the largest flood basalt provinces globally, containing various basaltic compositions, including olivine-bearing types.

Mid-ocean ridges worldwide

Commonly found in samples dredged from mid-ocean ridge systems, representing newly formed oceanic crust.

Finding Tips

Look for dark, fine-grained rocks

Basalt is typically dark gray to black. The presence of visible green crystals is the key indicator for olivine phenocrysts.

Examine fresh surfaces

Weathering can alter the color of olivine to reddish-brown (iddingsite) or obscure its characteristic green hue. Look for freshly broken surfaces.

Check for vesicularity

Many basalts are vesicular (contain gas bubbles), which can be a helpful textural clue, though not diagnostic of olivine presence.

Consider the geological context

Basalts are associated with volcanic activity. Look in areas with lava flows, volcanic cones, or dikes/sills cutting through other rocks.

Similar Rocks

Basalt

Also known as: Volcanic rock

Gabbro

Also known as: Plutonic rock

Peridotite

Also known as: Ultramafic rock

Andesite

Also known as: Intermediate volcanic rock

Scientific Classification

Mineral Class: Silicate (for olivine and other major minerals)
Group: Igneous Rock (Extrusive)
Crystal System: Orthorhombic (olivine), Monoclinic (pyroxene), Triclinic (plagioclase)
Chemical Formula: Basalt is a rock, not a single mineral, so no single formula. Olivine: (Mg,Fe)2SiO4
Composition: Mafic, with high content of ferromagnesian minerals (olivine, pyroxene) and calcic plagioclase. Low silica content.

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