Difference between revisions of "Amphibole" - New World Encyclopedia

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[[Image:Amphibole.jpg|thumb|250px|Amphibole ([[Hornblende]])]]
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[[Image:Amphibole.jpg|thumb|250px|Samples of [[hornblende]], a common type of amphibole.]]
  
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'''Amphibole''' defines an important group of generally dark-colored rock-forming [[Silicate minerals|inosilicate]] [[mineral]]s. They are of either igneous or metamorphic origin; in the former case occurring as constituents ([[hornblende]]) of [[igneous rock]]s, such as [[granite]], [[diorite]], [[andesite]] and others. Those of metamorphic origin include examples such as those developed in [[limestone]]s by contact [[metamorphism]] ([[tremolite]]) and those formed by the alteration of other ferromagnesian minerals ([[hornblende]]). Pseudomorphs of amphibole after pyroxene are known as [[uralite]].
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'''Amphibole''' defines an important group of generally dark-colored, rock-forming, silicate [[mineral]]s. Some are constituents of [[igneous rock]]s, and others are part of [[metamorphic rock]]s. [[Hornblende]] is the most common subgroup of minerals in amphibole.
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== Etymology ==
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The name amphibole ([[Ancient Greek|Greek]] ''αμφιβολος/amfibolos'', meaning "ambiguous") was used by [[René Just Haüy|RJ Haüy]] to include tremolite, actinolite, and hornblende. This term has since been applied to the whole group.
  
 
== Characteristics ==
 
== Characteristics ==
  
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Amphiboles are composed of double chain SiO<sub>4</sub> tetrahedra, linked at the vertices and generally containing [[ion]]s of [[iron]] and/or [[magnesium]] in their structures. Amphiboles crystallize into two crystal systems, [[monoclinic]] and [[orthorhombic]]. In chemical composition and general characteristics they are similar to the [[pyroxene]]s. The chief differences between amphiboles and pyroxenes are that (i) they contain essential hydroxl ions (OH) 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 [[amphibolite]]s.
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Amphiboles are composed of double chain SiO<sub>4</sub> tetrahedra, linked at the vertices and generally containing [[ion]]s of [[iron]] and/or [[magnesium]] in their structures. Amphiboles crystallize into two crystal systems, [[monoclinic]] and [[orthorhombic]].
  
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The name amphibole ([[Ancient Greek|Greek]] ''αμφιβολος/amfibolos'' meaning 'ambiguous') was used by [[René Just Haüy|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 be built on the general double-chain [[silicate]] formula RSi<sub>4</sub>O<sub>11</sub>.
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In chemical composition and general characteristics they are similar to the [[pyroxene]]s. The chief differences between amphiboles and pyroxenes are that (i) they contain essential hydroxl ions (OH) 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 [[amphibolite]]s.
  
 
==Amphibole groups==
 
==Amphibole groups==
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Numerous subspecies and varieties of amphibole are known, the more important of which are shown below. The formula of each is built on the general double-chain [[silicate]] formula RSi<sub>4</sub>O<sub>11</sub>.
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'''Orthorhombic Series'''
 
'''Orthorhombic Series'''
 
* [[Anthophyllite]] (Mg,Fe)<sub>7</sub>Si<sub>8</sub>O<sub>22</sub>(OH)<sub>2</sub>
 
* [[Anthophyllite]] (Mg,Fe)<sub>7</sub>Si<sub>8</sub>O<sub>22</sub>(OH)<sub>2</sub>
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'''Monoclinic Series'''
 
'''Monoclinic Series'''
 
* [[Tremolite]]    Ca<sub>2</sub>Mg<sub>5</sub>Si<sub>8</sub>O<sub>22</sub>(OH)<sub>2</sub>
 
* [[Tremolite]]    Ca<sub>2</sub>Mg<sub>5</sub>Si<sub>8</sub>O<sub>22</sub>(OH)<sub>2</sub>
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== See also ==
 
== See also ==
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* [[list of minerals]]
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* [[Amphibolite]] rock, and Amphibolite Facies
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* [[Mineral]]
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* [[Igneous rocks]]
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* [[Igneous rock]]
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* [[List of minerals]]
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* [[Metamorphic rock]]
 
* [[Metamorphism]]
 
* [[Metamorphism]]
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* [[Rock (geology)]]
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== Notes ==
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<references/>
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*{{1911}}
  
 
==References==
 
==References==
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* Hurlbut, Cornelius S.; Klein, Cornelis, 1985, ''Manual of Mineralogy'', 20th ed., John Wiley and Sons, New York ISBN 0-471-80580-7
 
* Hurlbut, Cornelius S.; Klein, Cornelis, 1985, ''Manual of Mineralogy'', 20th ed., John Wiley and Sons, New York ISBN 0-471-80580-7
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*{{1911}}
 
  
 
[[Category:Physical sciences]]
 
[[Category:Physical sciences]]

Revision as of 20:42, 26 March 2007

Samples of hornblende, a common type of amphibole.

Amphibole defines an important group of generally dark-colored, rock-forming, silicate minerals. Some are constituents of igneous rocks, and others are part of metamorphic rocks. Hornblende is the most common subgroup of minerals in amphibole.

Etymology

The name amphibole (Greek αμφιβολος/amfibolos, meaning "ambiguous") was used by RJ Haüy to include tremolite, actinolite, and hornblende. This term has since been applied to the whole group.

Characteristics

Amphiboles are composed of double chain SiO4 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 are similar to the pyroxenes. The chief differences between amphiboles and pyroxenes are that (i) they contain essential hydroxl ions (OH) 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.

Amphibole groups

Numerous subspecies and varieties of amphibole are known, the more important of which are shown below. The formula of each is built on the general double-chain silicate formula RSi4O11.

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
  • Pargasite NaCa2Mg3Fe2+Si6Al3O22(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.

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.

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.

See also

Notes

  • This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.

References
ISBN links support NWE through referral fees

  • Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York ISBN 0-471-80580-7

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