Garnet Amphibolite

Description

Garnet amphibolite is a medium- to coarse-grained metamorphic rock characterized by the presence of both amphibole (typically hornblende) and garnet, along with plagioclase feldspar. The rock exhibits a foliated or lineated texture, often with porphyroblastic garnets. The color is generally dark, ranging from dark green to black, with reddish-brown to purplish-red garnets conspicuously embedded within the matrix. The mineral assemblage reflects its formation under specific pressure and temperature conditions within the Earth's crust.

How to Identify

Color

Dark green to black matrix with prominent reddish-brown to purplish-red garnets.

Luster

Vitreous to sub-vitreous for garnets, vitreous to silky for amphiboles, dull to vitreous for plagioclase.

Texture

Medium to coarse-grained, typically foliated or lineated, often porphyroblastic with large garnet crystals.

Crystal Form

Garnets typically form euhedral to subhedral dodecahedral or trapezohedral crystals. Amphiboles are prismatic to acicular. Plagioclase is anhedral to subhedral.

Cleavage

Amphiboles exhibit two distinct cleavages at approximately 56° and 124°. Garnets have no cleavage. Plagioclase has two cleavages at nearly 90°.

Geological Environment

Regional metamorphic terrains, often associated with convergent plate boundaries, within medium to high-grade metamorphic zones (amphibolite facies).

Key Facts

• Hardness: 6.5-7.5 (garnet), 5-6 (amphibole), 6-6.5 (plagioclase)
• Specific Gravity: 3.1-4.3 (garnet), 2.9-3.4 (amphibole), 2.6-2.8 (plagioclase)
• Crystal System: Isometric (garnet), Monoclinic (amphibole, plagioclase)
• Color: Dark green to black matrix with reddish-brown to purplish-red garnets
• Luster: Vitreous to sub-vitreous (garnet), vitreous to silky (amphibole), dull to vitreous (plagioclase)
• Transparency: Translucent to opaque (garnet), translucent to opaque (amphibole), translucent (plagioclase)
• Fracture: Conchoidal to uneven (garnet), uneven to splintery (amphibole), uneven (plagioclase)
• Cleavage: Garnet: None; Amphibole: Two distinct cleavages at ~56° and ~124°; Plagioclase: Two cleavages at nearly 90°
• Composition: Primarily hornblende (amphibole), garnet (almandine-pyrope series common), and plagioclase feldspar. Minor constituents may include quartz, biotite, epidote, ilmenite, magnetite, and rutile.

Quick Check

• Color: Dark green to black with red garnets
• Luster: Vitreous to sub-vitreous (garnet), vitreous to silky (amphibole)
• Streak: White to grayish (for the rock as a whole, individual minerals vary)

Physical Characteristics

• Crystal Habit: Garnets are typically euhedral to subhedral dodecahedra or trapezohedra. Amphiboles are prismatic to acicular. Plagioclase is anhedral to subhedral granular.
• Cleavage Type: Amphiboles exhibit perfect prismatic cleavage. Garnets have no cleavage. Plagioclase has perfect basal and good pinacoidal cleavage.
• Fracture Type: Garnets show conchoidal to uneven fracture. Amphiboles and plagioclase typically have uneven to splintery fracture.
• Tenacity: Brittle for all major constituent minerals.
• Luster Type: Vitreous to sub-vitreous for garnets, vitreous to silky for amphiboles, dull to vitreous for plagioclase.

Formation

Garnet amphibolite forms under medium to high-grade regional metamorphic conditions, typically within the amphibolite facies. It is derived from the metamorphism of mafic igneous rocks (like basalt or gabbro) or certain types of sedimentary rocks (like marls or impure limestones) that contain sufficient iron, magnesium, and aluminum. The presence of garnet indicates higher pressures and/or temperatures compared to a typical amphibolite without garnet.

Usage

Due to its hardness and durability, garnet amphibolite is sometimes used as a dimension stone, for crushed aggregate in construction, and as a decorative stone. The garnets themselves, if of sufficient quality and size, can be used as abrasive materials or, rarely, as gemstones.

Age Distribution

Precambrian to Cenozoic, depending on the specific metamorphic event.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Silicate (for all major constituents)

Group

Metamorphic Rock

Crystal System

Isometric (garnet), Monoclinic (amphibole, plagioclase)

Chemical Formula

Variable, reflecting the mineral assemblage. E.g., Garnet: (Fe,Mg,Ca,Mn)3Al2(SiO4)3; Hornblende: Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2; Plagioclase: (Na,Ca)(Al,Si)AlSi2O8

Composition

Predominantly hornblende, garnet, and plagioclase feldspar. Accessory minerals may include quartz, biotite, epidote, ilmenite, magnetite, and rutile.

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