Everything about Amphibole totally explained
» 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 SiO
4 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're 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 RSi
4O
11.
Amphibole groups
Orthorhombic 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.
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.
Further Information
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