Garnet in Basalt

Igneous rock with mineral inclusion

Garnet (mineral) in Basalt (igneous rock)

Also known as: Garnet-bearing Basalt, Basalt with Garnet Xenocrysts

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Description

Garnet in basalt refers to the occurrence of garnet mineral crystals embedded within a basaltic rock matrix. Basalt is a fine-grained, dark-colored extrusive igneous rock, rich in ferromagnesian minerals and plagioclase feldspar. The garnets, often appearing as reddish to brownish-red, subhedral to anhedral grains, are typically xenocrysts, meaning they are foreign crystals incorporated into the magma from the surrounding mantle or lower crust during its ascent. Their presence indicates a deep-seated origin for the magma and/or the entrained material.

How to Identify

Color: Basalt matrix is typically dark gray to black. Garnet xenocrysts are commonly reddish-brown, dark red, or purplish-red.
Luster: Basalt matrix is dull to sub-vitreous. Garnet xenocrysts exhibit a vitreous to resinous luster.
Texture: Basalt is fine-grained to aphanitic, sometimes porphyritic with phenocrysts of olivine or pyroxene. Garnets appear as distinct, often rounded or fractured, coarser grains within the fine-grained matrix.
Crystal Form: Garnets are typically anhedral to subhedral xenocrysts, often rounded due to resorption or mechanical abrasion during transport. Euhedral dodecahedral or trapezohedral forms are rare but can occur.
Cleavage: Basalt does not exhibit macroscopic cleavage. Garnet has no true cleavage, but may show parting due to fractures.
Geological Environment: Found in volcanic fields and diatremes where basaltic magmas have rapidly ascended from mantle depths, often associated with continental rifts, ocean islands, and subduction zones.

Key Facts

Hardness: Basalt: 5-6 (Mohs). Garnet: 6.5-7.5 (Mohs).
Specific Gravity: Basalt: 2.8-3.0. Garnet: 3.5-4.3 (varies with composition).
Crystal System: Basalt: Not applicable (rock). Garnet: Isometric.
Color: Basalt: Dark gray to black. Garnet: Red, reddish-brown, purplish-red.
Luster: Basalt: Dull to sub-vitreous. Garnet: Vitreous to resinous.
Transparency: Basalt: Opaque. Garnet: Translucent to opaque.
Fracture: Basalt: Conchoidal to uneven. Garnet: Conchoidal to uneven.
Cleavage: Basalt: None. Garnet: None (parting may occur).
Composition: Basalt: Primarily plagioclase feldspar, pyroxene, olivine, and iron oxides. Garnet: Complex silicate, typically pyrope-rich (Mg3Al2(SiO4)3) when mantle-derived, with varying amounts of almandine (Fe3Al2(SiO4)3) and grossular (Ca3Al2(SiO4)3).

Quick Check

Color: Dark gray to black matrix with reddish-brown to dark red inclusions.
Luster: Dull to sub-vitreous for basalt, vitreous to resinous for garnet.
Streak: White to grayish-white for basalt (if powdered), colorless for garnet.

Physical Characteristics

Crystal Habit: Basalt: Aphanitic to porphyritic. Garnet: Anhedral to subhedral grains, often rounded or fractured xenocrysts.
Cleavage Type: Basalt: None. Garnet: None.
Fracture Type: Basalt: Conchoidal to uneven. Garnet: Conchoidal to uneven.
Tenacity: Basalt: Brittle. Garnet: Brittle.
Luster Type: Basalt: Dull to sub-vitreous. Garnet: Vitreous to resinous.

Formation

Garnet in basalt typically forms when basaltic magma, rising rapidly from the Earth's mantle, entrains pre-existing garnet crystals (xenocrysts) or garnet-bearing mantle xenoliths. These garnets are not in equilibrium with the basaltic melt at shallow crustal pressures and temperatures, indicating their deep-seated origin. The rapid ascent prevents significant reaction or dissolution of the garnets within the basalt.

Usage

Primarily of scientific interest for understanding mantle composition, dynamics, and the petrogenesis of deep-seated magmas. Garnet xenocrysts can provide valuable information about the pressure-temperature conditions of their source region. Occasionally, well-formed garnets can be extracted for lapidary purposes, though this is rare for basalt-hosted garnets due to their typically small size and fractured nature.

Age Distribution

Variable, depending on the age of the basaltic eruption and the source of the garnet xenocrysts. Can range from Cenozoic to Precambrian.

Where to Find

Kimberley region, South Africa

While primarily known for diamonds in kimberlite, some basaltic rocks in the region can contain mantle-derived garnets.

Colorado Plateau, USA

Various volcanic fields, particularly those with alkali basalts, contain mantle xenoliths and xenocrysts, including garnets.

Eastern Australia (e.g., New South Wales, Queensland)

Cenozoic basaltic provinces are known for mantle xenoliths and xenocrysts, including pyrope garnets.

Siberian Platform, Russia

Associated with large igneous provinces and kimberlite fields, some basaltic occurrences may contain deep-seated garnets.

Hawaii, USA

While less common than olivine, some deep-seated xenoliths and xenocrysts, including garnet, can be found in Hawaiian basalts, particularly those from deeper mantle sources.

Finding Tips

Look for Volcanic Fields

Focus on areas with recent or ancient basaltic volcanism, especially those known for mantle xenoliths.

Examine Outcrops Closely

Garnets can be small and easily overlooked. Look for distinct reddish or purplish grains contrasting with the dark basalt matrix.

Check Weathered Surfaces

Weathering can sometimes make the garnets more prominent as they may resist erosion differently than the surrounding basalt.

Consult Geological Maps and Literature

Research specific localities known for mantle xenocrysts or xenoliths in basaltic rocks.

Similar Rocks

Peridotite Xenolith in Basalt

Basalt with Peridotite Xenolith

Also known as: Mantle Xenolith in Basalt

Eclogite Xenolith in Basalt

Basalt with Eclogite Xenolith

Also known as: Garnet-Pyroxene Xenolith in Basalt

Garnet in Kimberlite

Kimberlite with Garnet (mineral)

Also known as: Kimberlitic Garnet

Scientific Classification

Mineral Class: Silicate (for Garnet)
Group: Garnet Group (for Garnet)
Crystal System: Isometric (for Garnet)
Chemical Formula: Basalt: (Na,Ca)(Si,Al)2O4 (plagioclase), (Mg,Fe)SiO3 (pyroxene), (Mg,Fe)2SiO4 (olivine). Garnet: X3Y2(SiO4)3, where X can be Ca, Mg, Fe2+, Mn2+ and Y can be Al, Fe3+, Cr3+.
Composition: Basalt: Mafic igneous rock. Garnet: Nesosilicate, typically pyrope-rich in mantle xenocrysts.

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