Hornblende

Description

Hornblende is a complex inosilicate mineral series within the amphibole group. It is one of the most common rock-forming minerals in both igneous and metamorphic rocks. Characterized by its dark color, prismatic habit, and distinctive 56° and 124° cleavage angles, hornblende is a key mineral for understanding the petrogenesis of many crustal rocks. Its chemical composition is highly variable, reflecting the diverse geological environments in which it forms.

How to Identify

Color

Typically dark green, dark brown, or black.

Luster

Vitreous to sub-vitreous, sometimes silky in fibrous varieties.

Texture

Prismatic, often elongated crystals. Can be massive or granular.

Crystal Form

Monoclinic system, forming long, slender to stout prismatic crystals with a roughly hexagonal or rhombic cross-section. Often terminated by dome faces.

Cleavage

Two distinct cleavage planes intersecting at approximately 56° and 124° (characteristic of amphiboles). This is a crucial diagnostic feature.

Geological Environment

Common in intermediate to felsic igneous rocks (e.g., diorite, granodiorite, andesite) and a wide range of metamorphic rocks (e.g., amphibolite, hornblende schist, gneiss).

Key Facts

• Hardness: 5-6 on the Mohs scale
• Specific Gravity: 3.0-3.4 (variable depending on composition)
• Crystal System: Monoclinic
• Color: Dark green, dark brown, black
• Luster: Vitreous to sub-vitreous
• Transparency: Translucent to opaque
• Fracture: Uneven to conchoidal
• Cleavage: Perfect in two directions, intersecting at 56° and 124°
• Composition: Complex calcium, magnesium, iron, aluminum silicate with hydroxyl (OH) groups. The general formula is (Ca,Na)2-3(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2.

Quick Check

• Color: Dark green to black
• Luster: Vitreous to sub-vitreous
• Streak: White to grayish-green

Physical Characteristics

• Crystal Habit: Prismatic, columnar, acicular, fibrous, massive, granular.
• Cleavage Type: Perfect {110} cleavage, producing two planes at 56° and 124°.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to sub-vitreous, sometimes silky in fibrous varieties.

Formation

Hornblende forms under a wide range of metamorphic and igneous conditions. In igneous rocks, it crystallizes from cooling magmas, particularly in intermediate to felsic compositions (e.g., diorite, granodiorite, andesite). In metamorphic rocks, it forms during regional and contact metamorphism of mafic and ultramafic rocks, as well as pelitic sediments, under amphibolite facies conditions. It can also form during hydrothermal alteration.

Usage

Hornblende itself has limited direct industrial uses due to its common occurrence as a rock-forming mineral rather than a primary ore. However, rocks rich in hornblende (e.g., amphibolite) are used as crushed stone for construction aggregate, road base, and landscaping. Its presence can also be an indicator of specific metamorphic or igneous conditions in geological studies.

Age Distribution

Found in rocks of all geological ages, from Precambrian to Cenozoic.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Inosilicate (chain silicate)

Group

Amphibole Group

Crystal System

Monoclinic

Chemical Formula

Ca2(Mg,Fe)4Al(Si7Al)O22(OH,F)2 (generalized, as it's a series)

Composition

A complex hydrous calcium, magnesium, iron, and aluminum silicate. The exact composition varies significantly, with substitutions of Fe2+, Fe3+, Mg, Al, Na, and K, leading to different end-members within the hornblende series (e.g., magnesiohornblende, ferrohornblende, tschermakite).

Explore Hornblende