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Amphibole

Amphibole defines an important group of 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. In chemical composition and general characteristics they are very similar to the pyroxenes and, like them, fall into three series according to their system of crystallization. The chief difference between amphiboles and pyroxenes is in cleavage: amphiboles form oblique cleavage planes, while pyroxenes have cleavages of approximately 90 degrees. Amphiboles are also specifically less dense than the corresponding pyroxenes, and in their optical characteristics they are distinguished by their stronger pleochroism and by the wider angle of extinction on the plane of symmetry. Amphiboles are the primary constituent of amphibolites.

Amphiboles are minerals of either original or secondary origin; in the former case occurring as constituents (hornblende) of igneous rocks, such as granite, diorite, andesite and others. Those of secondary origin have either been developed in limestones by contact metamorphism (tremolite) or have resulted by the alteration of augite by dynamo-metamorphism (actinolite). Pseudomorphs of amphibole after pyroxene are known as uralite.

The name amphibole (Greek amfibolos, “ambiguous”) was used by RJ Haüy to include tremolite, actinolite, and hornblende; 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 conform to the general metasilicate formula RSiO3.

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 NaFe2+3Fe3+2Si8O22(OH)2
- Richterite Na2Ca(Mg,Fe)5Si8O22(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; the specific gravity, for example, varies from 2.9 in tremolite to 3.8 in aenigmatite.

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

Actinolite is an important 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 crystalline schists. The name (from aktis, a ray, and 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 occurring in crystalline schists, and are the result of dynamo-metamorphic processes; the latter two are dark green minerals which occur as original constituents of igneous rocks rich in soda, such as nepheline-syenite and phonolite.

Aenigmatite and its variety cossyrite are rare minerals forming constituents of igneous rocks of the nepheline-syenite and phonolite groups.

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Amphibole


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