Fact Sheet

• Mineral type: Amphibole supergroup; double-chain inosilicate minerals, commonly containing OH, F, Cl, or O at one structural site

• General formula: Amphiboles are commonly represented by AB₂C₅T₈O₂₂W₂. In many common amphiboles, T is mostly Si and W is mostly OH, giving simplified formulas such as AB₂C₅Si₈O₂₂(OH)₂. The A, B, C, T, and W sites allow extensive substitutions by elements such as Na, Ca, K, Mg, Fe, Al, F, Cl, and O.

• Crystal system: Mostly monoclinic; some amphiboles are orthorhombic

• Mohs hardness: Usually 5–6

• Cleavage: Two good cleavages meeting at about 56° and 124°

• Typical colors: Green, black, brown, white, colorless, blue, or lavender, depending on composition

• Common members: Hornblende, tremolite, actinolite, glaucophane, riebeckite, anthophyllite, cummingtonite-grunerite

• Main importance: Rock-forming minerals, indicators of igneous and metamorphic conditions, nephrite jade, and some historically important asbestos varieties

What Is Amphibole?

Amphibole is not one mineral name in the way quartz or calcite is. It is a supergroup of related double-chain silicate minerals, many of which are major rock-forming minerals in igneous and metamorphic rocks. Many common amphiboles contain hydroxyl, written as OH, although the same structural site can also contain F, Cl, or O. Together with their double-chain silicate structure, this helps separate amphiboles from pyroxenes, which are single-chain inosilicates and commonly show cleavage angles closer to 90°.

In the field, amphibole is often recognized by its crystal shape and cleavage. Amphiboles often form long prismatic or needle-like crystals, and in hand specimen they commonly show two cleavage directions that meet at about 56° and 124°. This is one of the classic field clues geologists use when identifying amphibole-bearing rocks.

How Amphibole Forms

Amphiboles form in many geological settings, but they are especially important where water plays a role. In igneous rocks, they can crystallize from water-bearing magmas and are common in rocks such as diorite, granodiorite, some gabbros, and some volcanic rocks.

Different amphiboles can point to different pressure-temperature conditions and rock histories. Hornblende is common in many igneous and metamorphic rocks. Actinolite is typical of low- to medium-grade metamorphic rocks. Tremolite often forms in metamorphosed carbonate rocks such as dolomitic marbles. Glaucophane, with its distinctive blue color, is a key mineral of blueschist rocks, which form under high-pressure, low-temperature conditions in subduction zones.

Types of Amphibole

Hornblende is a familiar field name for dark green to black, complex calcium-rich amphiboles. In hand specimen and thin section, geologists often use “hornblende” for common dark amphiboles in rocks such as diorite, granodiorite, amphibolite, and gneiss, even though modern amphibole nomenclature is more precise.

Tremolite is usually white, pale gray, or light green. It commonly forms in metamorphosed dolomitic rocks and belongs to the tremolite-actinolite series.

Actinolite is the greener, iron-bearing member of the tremolite-actinolite series. It is common in many metamorphic rocks and can occur as bladed, radiating, or fibrous crystals.

Glaucophane is blue to bluish gray and is strongly associated with blueschist-facies metamorphism. Its presence can tell geologists that a rock experienced high pressure at relatively low temperature.

Riebeckite is a dark blue to black sodium amphibole. Its fibrous variety, crocidolite, is better known as blue asbestos.

Anthophyllite and cummingtonite-grunerite are magnesium-iron amphiboles. Their asbestiform varieties include anthophyllite asbestos and grunerite asbestos, better known commercially as amosite.

Uses and Importance

Amphiboles are most important as geological clues. Because they form under particular pressure, temperature, and fluid conditions, they help geologists interpret the history of igneous and metamorphic rocks. Amphibole chemistry can also preserve information about magma evolution, metamorphic grade, and fluid-rock interaction.

Some amphibole-rich materials have ornamental value. Nephrite jade, for example, is made mainly of very fine intergrown tremolite-actinolite amphibole. It has been used for carvings, tools, jewelry, and decorative objects.

Some amphiboles are also important because of their asbestos varieties. Several asbestiform amphiboles were once used industrially because their fibers are heat-resistant and durable. Asbestiform amphiboles, including crocidolite, amosite, tremolite asbestos, actinolite asbestos, and anthophyllite asbestos, were historically used because their fibers are heat-resistant and durable. These uses are now heavily restricted or banned in many countries because inhaled asbestos fibers can cause serious disease, including asbestosis, lung cancer, and mesothelioma.

Health and Safety Note

Not every amphibole specimen is asbestos. The hazard depends strongly on whether the mineral occurs in an asbestiform, respirable fibrous habit. Still, suspected fibrous amphibole material should be treated cautiously. Do not saw, grind, drill, crush, or polish unknown fibrous amphibole specimens without expert identification and appropriate safety controls.

Where Is Amphibole Found?

Amphiboles are found worldwide. Because amphibole is a mineral group rather than a single mineral, it is better to describe its occurrence by rock type instead of listing a few countries. Amphiboles occur in many igneous and metamorphic rocks, including amphibolite, schist, gneiss, diorite, granodiorite, and some altered mafic rocks. Specific amphibole species have their own classic localities, but the group as a whole is globally widespread.