Amphibolite

Metamorphic Rock

Amphibolite

Also known as: Hornblende Schist (when strongly foliated)

Got a photo? Identify rocks instantly with the Rockby app

Description

Amphibolite is a medium- to coarse-grained metamorphic rock primarily composed of amphibole (typically hornblende) and plagioclase feldspar, with minor amounts of other minerals such as garnet, quartz, epidote, biotite, and magnetite. It is characterized by its dark color, often black to dark green, and a typically granoblastic or nematoblastic texture. While it can be massive, it often exhibits foliation (schistosity) due to the alignment of amphibole crystals, especially when subjected to directed stress during metamorphism. The presence of amphibole is the defining characteristic, distinguishing it from other mafic metamorphic rocks like granulite.

How to Identify

Color: Typically dark green to black, sometimes with white speckles from plagioclase.
Luster: Vitreous to dull, depending on the mineral components and their grain size.
Texture: Medium- to coarse-grained, granoblastic (equigranular, interlocking grains) or nematoblastic (elongate, aligned amphibole crystals). Can be massive or foliated (schistose).
Crystal Form: Amphibole crystals are typically prismatic, often elongated and aligned, contributing to foliation. Plagioclase is anhedral to subhedral.
Cleavage: Amphibole (hornblende) exhibits two distinct cleavages at approximately 56 and 124. Plagioclase has two cleavages at nearly 90.
Geological Environment: Commonly found in regional metamorphic terrains associated with convergent plate boundaries, ancient shield areas, and mountain belts. It can also occur in contact metamorphic aureoles around large igneous intrusions.

Key Facts

Hardness: 6-7 (Mohs scale, for hornblende)
Specific Gravity: 2.9-3.2 g/cm
Crystal System: Monoclinic (for hornblende and plagioclase)
Color: Dark green, black, sometimes with white or gray patches
Luster: Vitreous to dull
Transparency: Opaque to translucent (for individual mineral grains)
Fracture: Uneven to subconchoidal
Cleavage: Good to perfect in two directions for hornblende (56 and 124); good in two directions for plagioclase (nearly 90)
Composition: Predominantly hornblende and plagioclase feldspar, with variable amounts of quartz, epidote, garnet, biotite, magnetite, ilmenite, and sphene.

Quick Check

Color: Dark green to black
Luster: Vitreous to dull
Streak: White to grayish-white

Physical Characteristics

Crystal Habit: Prismatic (amphibole), anhedral to subhedral (plagioclase)
Cleavage Type: Perfect to good in two directions (amphibole, plagioclase)
Fracture Type: Uneven to subconchoidal
Tenacity: Brittle
Luster Type: Vitreous to dull

Formation

Amphibolite forms under medium-to-high grade metamorphic conditions (amphibolite facies), typically at temperatures between 500C and 700C and pressures ranging from 4 to 12 kilobars. It is commonly derived from the metamorphism of mafic igneous rocks (basalt, gabbro, diabase) or argillaceous sedimentary rocks (marls, graywackes). The protolith's original composition and the metamorphic conditions dictate the specific mineral assemblage. Key reactions involve the breakdown of plagioclase and pyroxene to form amphibole and other minerals.

Usage

Amphibolite is used as crushed stone for road construction, railway ballast, and aggregate in concrete. Its durability and resistance to weathering make it suitable for these applications. Historically, some varieties have been used as building stone and for decorative purposes, particularly those with attractive textures or colors. In some regions, amphibolite-derived soils can be fertile for agriculture.

Age Distribution

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

Where to Find

Fennoscandian Shield

Extensive occurrences in Norway, Sweden, and Finland, often associated with Precambrian greenstone belts and high-grade metamorphic complexes.

Appalachian Mountains, USA

Found in various metamorphic belts throughout the Appalachian orogen, from New England to the southern states.

Canadian Shield

Widespread in Canada, particularly in Ontario, Quebec, and Labrador, within ancient metamorphic terranes.

Himalayan Orogen

Present in the metamorphic core complexes and thrust sheets of the Himalayas, indicating significant crustal thickening and metamorphism.

European Alps

Occurs in the metamorphic units of the Alps, reflecting the complex tectonic history of the region.

Finding Tips

Look for Dark, Dense Rocks

Amphibolite is typically dark-colored (dark green to black) and relatively dense due to its mafic mineral content. Look for these characteristics in outcrop.

Identify Amphibole Crystals

Examine the rock for elongated, prismatic, dark green to black crystals of amphibole (hornblende). These often show a characteristic 'splintery' appearance on broken surfaces and may be aligned.

Check for Foliation

While some amphibolites are massive, many exhibit foliation (schistosity) where the amphibole crystals are aligned, giving the rock a preferred orientation. This can be seen on weathered surfaces or freshly broken faces.

Consider the Geological Context

Amphibolite is a metamorphic rock, so it will be found in areas that have undergone significant regional or contact metamorphism, such as mountain ranges, ancient shield areas, or near large igneous intrusions.

Distinguish from Basalt/Gabbro

While derived from mafic igneous rocks, amphibolite will show metamorphic textures (foliation, granoblastic texture) and mineral assemblages (prominent amphibole, less pyroxene) that distinguish it from its igneous protoliths.

Similar Rocks

Basalt

Also known as: Volcanic Rock

Gabbro

Also known as: Plutonic Rock

Granulite

Also known as: High-grade Metamorphic Rock

Greenschist

Also known as: Low-grade Metamorphic Rock

Eclogite

Also known as: High-pressure Metamorphic Rock

Scientific Classification

Mineral Class: Metamorphic Rock
Group: Amphibolite Facies Rocks
Crystal System: Polycrystalline aggregate; constituent minerals are typically monoclinic (amphibole, plagioclase)
Chemical Formula: Variable, reflecting the mineral assemblage (e.g., (Ca,Na)(Mg,Fe,Al)SiO(OH) for hornblende, NaAlSiO-CaAlSiO for plagioclase)
Composition: Mainly calcium-rich amphibole (hornblende) and plagioclase feldspar. Accessory minerals may include quartz, epidote, garnet, biotite, magnetite, ilmenite, and sphene. The exact composition depends on the protolith and metamorphic conditions.

Explore Amphibolite

Identification Localities & Collecting Look-alikes

Identify rocks anywhere

Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.

Download on theApp Store Get it onGoogle Play Continue onWeb

Open in app