Information AboutAmphibole |
| CATEGORIES ABOUT AMPHIBOLE | |
| inosilicates | |
| asbestos | |
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)]] Amphibole defines an important group of generally dark-colored rock-forming Inosilicate Mineral s composed of double chain SiO4 tetrahedra linked at the vertices and generally containing Ion s of Iron and/ or Magnesium in their structures. In chemical composition and general characteristics they are very similar to the Pyroxene s and, like them, fall into three series according to their system of Crystallization . The chief difference between amphiboles and pyroxenes are that (i) they contain hydroxl ions (OH) (ii) the basic structure is a double chain of tetrahedra (as opposed to the single chain structure of the pyroxene). Most apparent in hand specimen is that amphiboles form oblique cleavage planes (at around 120 degrees), 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 Amphibolite s. Amphiboles are minerals of either original or secondary origin; in the former case occurring as constituents ( Hornblende ) of Igneous Rock s, such as Granite , Diorite , Andesite and others. Those of secondary origin have either been developed in Limestone s by contact Metamorphism (tremolite) or have resulted by the alteration of other ferromagnesian minerals such as 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 meta Silicate formula RSiO3. AMPHIBOLE GROUPS Orthorhombic Series
Monoclinic Series
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 Russia n variety containing little Iron as Kupfferite . Hornblende is an important constituent of many igneous rocks. It is also an important constituent of Amphibolite s 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 Greenschist s. 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 with glaucophane occurring in Blueschist s, 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 . Aenigmatite and its variety cossyrite are rare minerals forming constituents of igneous rocks of the nepheline-syenite and phonolite groups. Pargasite is a rare magnesium rich amphibole usually found in ultramafic rocks. It is formed only at extreme depths within the Mantle and is a typical constituent of Kimberlite , Lamproite and Lamprophyre goup rocks. It is hard, dense, black and usually Idiomorphic , with a red-brown Pleochroism in petrographic thin section. SEE ALSO
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