Garnet Amphibolite

Description

Garnetiferous amphibolite is a medium- to coarse-grained metamorphic rock primarily composed of amphibole (typically hornblende) and plagioclase feldspar, with a significant proportion of almandine garnet. The rock exhibits a characteristic dark color, often black, dark green, or greenish-black, due to the abundance of hornblende. The almandine garnets appear as distinct, often euhedral to subhedral, reddish-brown to dark red porphyroblasts, contrasting with the darker matrix. The texture is typically nematoblastic (aligned amphibole crystals) or granoblastic, and a foliation may be present, defined by the alignment of amphibole crystals. The presence of garnet indicates specific metamorphic conditions and bulk rock chemistry.

How to Identify

Color

Dark green, black, or greenish-black matrix with distinct reddish-brown to dark red garnet porphyroblasts.

Luster

Amphibole crystals exhibit a vitreous to silky luster; plagioclase is vitreous; garnet is vitreous to resinous.

Texture

Medium- to coarse-grained, often nematoblastic (aligned amphiboles) or granoblastic. Porphyroblastic texture due to large garnet crystals.

Crystal Form

Hornblende typically forms prismatic crystals; plagioclase forms tabular crystals; almandine garnet forms dodecahedral or trapezohedral crystals, often euhedral to subhedral.

Cleavage

Hornblende exhibits two distinct cleavages at approximately 56 and 124. Plagioclase has two cleavages at nearly 90. Garnet typically shows no cleavage but may exhibit parting.

Geological Environment

Regional metamorphic terrains, especially in ancient shield areas, mountain belts (orogenic zones), and subduction zones where mafic rocks have undergone medium- to high-grade metamorphism.

Key Facts

• Hardness: Hornblende: 5-6; Plagioclase: 6-6.5; Almandine Garnet: 6.5-7.5 (Mohs scale)
• Specific Gravity: Hornblende: 3.0-3.4; Plagioclase: 2.6-2.8; Almandine Garnet: 3.9-4.3
• Crystal System: Hornblende: Monoclinic; Plagioclase: Triclinic; Almandine Garnet: Isometric
• Color: Dark green, black, or greenish-black matrix with reddish-brown to dark red garnets
• Luster: Vitreous to silky
• Transparency: Opaque to translucent (rock); individual minerals can be translucent to opaque
• Fracture: Uneven to conchoidal (garnet); splintery (amphibole)
• Cleavage: Hornblende: two distinct cleavages at 56 and 124; Plagioclase: two cleavages at nearly 90; Garnet: none
• Composition: Predominantly hornblende (calcium amphibole) and plagioclase feldspar, with significant almandine garnet. Minor constituents may include quartz, biotite, epidote, ilmenite, magnetite, and sphene.

Quick Check

• Color: Dark green to black with red garnets
• Luster: Vitreous to silky (amphibole), vitreous (plagioclase), vitreous to resinous (garnet)
• Streak: White to grayish-white (for individual minerals, rock streak is not diagnostic)

Physical Characteristics

• Crystal Habit: Prismatic (hornblende), tabular (plagioclase), dodecahedral/trapezohedral (garnet)
• Cleavage Type: Perfect to good (hornblende, plagioclase); none (garnet)
• Fracture Type: Uneven to subconchoidal
• Tenacity: Brittle
• Luster Type: Vitreous to silky

Formation

Garnetiferous amphibolite forms through regional metamorphism of mafic igneous rocks (like basalt or gabbro) or certain sedimentary rocks (like marls or impure limestones) under amphibolite facies conditions. This involves temperatures typically ranging from 500C to 750C and pressures from 5 to 10 kilobars. The presence of almandine garnet indicates specific bulk rock compositions (sufficient Al and Fe) and often higher pressure-temperature conditions within the amphibolite facies, sometimes approaching granulite facies.

Usage

Historically, amphibolites have been used as building stones, road aggregate, and in some cases, for dimension stone. The presence of garnet can make it attractive for ornamental purposes if the garnets are well-formed and abundant. Almandine garnets themselves are used as abrasives and, if gem-quality, as gemstones.

Age Distribution

Precambrian to Cenozoic, common in ancient shield areas and orogenic belts.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Silicate

Group

Metamorphic Rock (Amphibolite Facies)

Crystal System

Polycrystalline (rock); constituent minerals vary (Monoclinic, Triclinic, Isometric)

Chemical Formula

Variable, reflecting the mineral assemblage (e.g., Hornblende: Ca2(Mg,Fe)5Si8O22(OH)2; Almandine: Fe3Al2(SiO4)3; Plagioclase: (Na,Ca)(Al,Si)AlSi2O8)

Composition

Mafic to intermediate bulk composition, metamorphosed under amphibolite facies conditions.

Explore Garnet Amphibolite