Definition of amphibole

A group of dark-colored ferromagnesian silicate

minerals with the general chemical formula:

A2–3 B5 (Si, Al)8O22(OH)2

where A = Mg, Fe+2, Ca, or Na, and B = Mg, Fe+3, Fe+2or Al.

Amphiboles contain continuous double chains of cross-linked

double silicate tetrahedra. The chains are bound together by

cations such as Ca, Mg, and Fe, which satisfy the negative

charges of the polymerized tetrahedra. Most amphiboles are

monoclinic, but some crystallize in the orthorhombic crystal

system. They have good prismatic cleavage intersecting at 56°

and 124° and typically form columnar or fibrous prismatic

crystals. Amphiboles are very common constituents of metamorphic

and igneous rocks and have a chemical composition

similar to pyroxenes. Some of the common amphibole minerals

include hornblende, tremolite, actinolite, anthophyllite,

cummingtonite, riebeckite, and glaucophane.

Amphibole is a fairly common mineral in intermediate to

mafic igneous rocks such as granodiorite, diorite, and gabbro,

forming up to 25 percent of these rocks in some cases.

Since amphibole is a hydrous mineral, it typically forms in

igneous environments where water is available. Amphibole is

best known, however, as a metamorphic mineral indicative of

medium grade pressure-temperature metamorphism of mafic

rocks. When basalt, gabbro, or similar rocks are heated to

930°F–1,300°F (500°C–700°C) at 3–10 kilobars pressure

(equivalent to 6 to 20-mile or 10 to 30-km depth), the primary

mineral assemblage will commonly turn to an assemblage

of amphibole+plagioclase feldspar. Many field geologists will

call such a rock an “amphibolite,” although this term should

be reserved for a description of the metamorphic conditions

(known as facies) at which these rocks formed.

See also MINERALOGY.

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