Hornblende Schist

Description

Hornblende schist is a medium- to coarse-grained metamorphic rock characterized by a strong schistosity and a significant proportion of the amphibole mineral hornblende. It typically forms from the metamorphism of mafic igneous rocks (basalts, gabbros) or sometimes from calcareous shales. The rock exhibits a distinct foliation, often appearing as parallel bands or layers of hornblende crystals, sometimes interlayered with lighter-colored minerals like plagioclase feldspar or quartz. The hornblende crystals are typically prismatic and dark green to black, giving the rock its characteristic dark color. Other common accessory minerals include epidote, garnet, biotite, and magnetite.

How to Identify

Color

Typically dark green to black, often with a speckled appearance due to the presence of other minerals.

Luster

Hornblende crystals exhibit a vitreous to sub-vitreous luster. The overall rock may appear dull to somewhat shiny on foliation planes.

Texture

Schistose, meaning it has a well-developed foliation (parallel alignment of platy or prismatic minerals). Medium- to coarse-grained, with individual hornblende crystals often visible to the naked eye.

Crystal Form

Hornblende crystals are typically prismatic, elongated, and often acicular (needle-like) or bladed. They are aligned parallel to the schistosity.

Cleavage

Hornblende itself has two distinct cleavages at approximately 56 and 124. In the rock, the schistosity represents a plane of easy splitting.

Geological Environment

Commonly found in regional metamorphic terrains associated with convergent plate boundaries, mountain belts, and ancient shield areas. It forms at medium metamorphic grades (amphibolite facies) from mafic protoliths.

Key Facts

• Hardness: Variable, generally 5-6 on Mohs scale (due to hornblende and other silicates)
• Specific Gravity: 2.9-3.4 g/cm (reflecting the density of hornblende and other mafic minerals)
• Crystal System: Monoclinic (for hornblende, the dominant mineral)
• Color: Dark green, black, often with lighter bands of other minerals
• Luster: Vitreous to sub-vitreous (hornblende), overall dull to somewhat shiny
• Transparency: Opaque (in hand specimen)
• Fracture: Irregular to splintery, but typically splits along schistosity planes
• Cleavage: Excellent schistosity (rock cleavage) due to mineral alignment; hornblende itself has two distinct cleavages at 56 and 124
• Composition: Primarily hornblende and plagioclase feldspar, with variable amounts of quartz, epidote, garnet, biotite, chlorite, and accessory minerals like magnetite, ilmenite, and sphene.

Quick Check

• Color: Dark green to black
• Luster: Vitreous to sub-vitreous (hornblende), overall dull to somewhat shiny on foliation planes
• Streak: Grayish-green to colorless (for hornblende, but not practical for the rock)

Physical Characteristics

• Crystal Habit: Prismatic, elongated, bladed, or acicular crystals of hornblende, aligned to form schistosity.
• Cleavage Type: Excellent schistosity (rock cleavage) parallel to the alignment of hornblende crystals. Hornblende mineral has two distinct prismatic cleavages.
• Fracture Type: Irregular to splintery across the foliation, but the rock preferentially splits along the schistosity planes.
• Tenacity: Brittle
• Luster Type: Vitreous to sub-vitreous (individual hornblende crystals), overall dull to somewhat shiny on foliation surfaces.

Formation

Hornblende schist forms through regional metamorphism of mafic igneous rocks (like basalt or gabbro) or certain sedimentary rocks (like marls or impure dolomites) under conditions of medium-grade metamorphism (greenschist to amphibolite facies). The presence of hornblende indicates temperatures typically ranging from 500C to 700C and pressures from 4 to 10 kilobars. The schistosity develops due to directed stress during metamorphism, causing the platy or prismatic minerals (like hornblende and micas) to align.

Usage

Hornblende schist is not typically used as a primary industrial mineral due to its variable composition and schistose nature. However, it can be used locally as crushed stone for road construction, aggregate, or fill material. Historically, some varieties have been used as building stone, though its foliation can make it prone to splitting. It is also of significant interest to geologists for understanding metamorphic processes and tectonic history.

Age Distribution

Precambrian to Cenozoic, depending on the specific metamorphic event and protolith age. Common in ancient shield areas and orogenic belts.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Silicate (specifically, an inosilicate for hornblende)

Group

Metamorphic Rock (specifically, a schist)

Crystal System

Monoclinic (for hornblende)

Chemical Formula

Variable, as it's a rock. Hornblende's general formula is (Ca,Na)(Mg,Fe,Al)SiO(OH)

Composition

Dominated by hornblende and plagioclase feldspar, with accessory minerals such as quartz, epidote, garnet, biotite, chlorite, magnetite, ilmenite, and sphene. The exact proportions vary depending on the protolith and metamorphic conditions.

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