1911 Encyclopædia Britannica/Amphibole

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AMPHIBOLE, an important group of rock-forming minerals, very similar in chemical composition and general characters to the pyroxenes, and like them falling into three series according to the system of crystallization. They differ from the pyroxenes, however, in having an angle between the prismatic cleavage of 56° instead of 87°; they are specifically lighter than the corresponding pyroxenes; and, in their optical characters, they are distinguished by their stronger pleochroism and by the wider angle of extinction on the plane of symmetry.

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

The name amphibole (from the Gr. ἁμφἰβολος, ambiguous) was used by R. J. 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 R′′SiO3.

Orthorhombic Series.

Anthophyllite . . (Mg, Fe)SiO3.

Monoclinic Series.

Tremolite . . CaMg3(SiO3)4.
Actinolite . . Ca(Mg, Fe)3(SiO3)4.
Cummingtonite . (Fe, Mg)SiO3.
Richterite . . (K2, Na2, Mg, Ca, Mn)SiO3.
Hornblende . . Ca(Mg, Fe)3(SiO3)4 with
NaAl(SiO3)2 and (Mg, Fe) (Al, Fe)2SiO6.
Glaucophane . . NaAl(SiO3)2•(Fe, Mg)SiO3.
Crocidolite . . NaFe(SiO3)2•FeSiO3.
Riebeckite . . 2NaFe(SiO3)2•FeSiO3.
Arfvedsonite . . Na8(Ca, Mg)3(Fe, Mn)14(Al, Fe)2 Si21O45.

Anorthic Series.

Aenigmatite . . Na4Fe′′9Al Fe′′ '(Si, Ti)12O38.

Of these, tremolite, hornblende and crocidolite, as well as the important varieties, asbestos and jade, are treated under their own headings. Brief mention only need be here made of some of the others. Naturally, on account of the wide variations in chemical composition, the different members vary considerably in characters 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 colour. It occurs frequently as a constituent of crystalline schists. The name (from άκτίς, a ray, and λίθος, 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 two former are blue fibrous minerals occurring in crystalline schists, and are the result of dynamo-metamorphic processes; the two latter 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. (L. J. S.)