Amphibole

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For the logical fallacy, see Amphibology.

Amphibole (pronounced amfi-bowl) defines an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain SiO₄ tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Amphiboles crystallize into two crystal systems, monoclinic and orthorhombic. In chemical composition and general characteristics they are similar to the pyroxenes. The chief differences between amphiboles and pyroxenes are that (i) they contain essential hydroxyl (OH) or halogene (F, Cl) and (ii) the basic structure is a double chain of tetrahedra (as opposed to the single chain structure of pyroxene). Most apparent, in hand specimens, is that amphiboles form oblique cleavage planes (at around 120 degrees), whereas pyroxenes have cleavage angles of approximately 90 degrees. Amphiboles are also specifically less dense than the corresponding pyroxenes. In optical characteristics, many amphiboles are distinguished by their stronger pleochroism and by the smaller angle of extinction (Z angle c) on the plane of symmetry. Amphiboles are the primary constituent of amphibolites.

Amphiboles are minerals of either igneous or metamorphic origin; in the former case occurring as constituents (hornblende) of igneous rocks, such as granite, diorite, andesite and others. Those of metamorphic origin include examples such as those developed in limestones by contact metamorphism (tremolite) and those formed by the alteration of other ferromagnesian minerals (hornblende). Pseudomorphs of amphibole after pyroxene are known as uralite.

The name amphibole (Greek αμφιβολος - amphibolos meaning 'ambiguous') was used by RJ Haüy to include tremolite, actinolite, tourmaline and hornblende. The group was so named by Haüy in allusion to the protean variety, in composition and appearance, assumed by its minerals. This term has since been applied to the whole group. Numerous sub-species and varieties are distinguished, the more important of which are tabulated below in three series. The formulae of each will be seen to be built on the general double-chain silicate formula RSi₄O₁₁.

[edit] Amphibole groups

Orthorhombic Series

Anthophyllite (Mg,Fe)₇Si₈O₂₂(OH)₂

Monoclinic Series

Tremolite Ca₂Mg₅Si₈O₂₂(OH)₂
Actinolite Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂
Cummingtonite Fe₂Mg₅Si₈O₂₂(OH)₂
Grunerite Fe₇Si₈O₂₂(OH)₂
Hornblende Ca₂(Mg,Fe,Al)₅(Al,Si)₈O₂₂(OH)₂
Glaucophane Na₂(Mg,Fe)₃Al₂Si₈O₂₂(OH)₂
Riebeckite Na₂Fe²⁺₃Fe³⁺₂Si₈O₂₂(OH)₂
Arfvedsonite Na₃Fe²⁺₄Fe³⁺Si₈O₂₂(OH)₂
Crocidolite Na₂Fe²⁺₃Fe³⁺₂Si₈O₂₂(OH)₂
Richterite Na₂Ca(Mg,Fe)₅Si₈O₂₂(OH)₂
Pargasite NaCa₂Mg₃Fe²⁺Si₆Al₃O₂₂(OH)₂

Of these, tremolite, hornblende, and crocidolite, as well as the important varieties, asbestos and jade, are treated under their own headings. Brief mention need only be made of some of the others. Naturally, on account of the wide variations in chemical composition, the different members vary considerably in properties and general appearance.

Anthophyllite occurs as brownish, fibrous or lamellar masses with hornblende in mica-schist at Kongsberg in Norway and some other localities. An aluminous related species is known as gedrite and a deep green Russian variety containing little iron as kupfferite.

Hornblende is an important constituent of many igneous rocks. It is also an important constituent of amphibolites formed by metamorphism of basalt.

Actinolite is an important and common member of the monoclinic series, forming radiating groups of acicular crystals of a bright green or greyish-green color. It occurs frequently as a constituent of greenschists. The name (from Greek ακτις/aktis, a 'ray' and λιθος/lithos, a 'stone') is a translation of the old German word Strahlstein (radiated stone).

Glaucophane, crocidolite, riebeckite and arfvedsonite form a somewhat special group of alkali-amphiboles. The first two are blue fibrous minerals, with glaucophane occurring in blueschists and crocidolite (blue asbestos) in ironstone formations, both resulting from dynamo-metamorphic processes. The latter two are dark green minerals, which occur as original constituents of igneous rocks rich in sodium, such as nepheline-syenite and phonolite.

Pargasite is a rare magnesium-rich amphibole with essential sodium, usually found in ultramafic rocks. For instance, it occurs in uncommon mantle xenoliths, carried up by kimberlite. It is hard, dense, black and usually idiomorphic, with a red-brown pleochroism in petrographic thin section.