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Contents
Amphibole
Amphibole is 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 Greek amfibolos, ambiguous)
was used by R.J. Hauy 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.
Orthorhombic Series
- Anthophyllite (Mg,Fe)SiO3
Monoclinic Series
- Tremolite CaMg3(SiO3)4
- Actinolire 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)2Si21O45
Anorthic Series
- Aenigmatite Na4FeII9Al FeIII(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 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 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.
Article from 1911 EB
See also: list of minerals
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