Amphibole

File:Tremolite-121232.jpg

Amphibole (tremolite)

Amphibole (Template:IPAc-en Template:Respell) is a group of inosilicate minerals, forming prism or needlelike crystals,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> composed of double chain Template:Chem tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is Amp.<ref>Template:Cite journal</ref> Amphiboles can be green, black, colorless, white, yellow, blue, or brown. The International Mineralogical Association currently classifies amphiboles as a mineral supergroup, within which are two groups and several subgroups.<ref>Mindat, Amphibole Supergroup</ref>

Mineralogy

File:Thin section microscopy Siilinjärvi R636 6335 amphibole.jpg

Photomicrographs of a thin section containing an amphibole crystal; under cross-polarized light on the left, and plane-polarized light on the right.

Amphiboles crystallize into two crystal systems, monoclinic and orthorhombic.<ref>Template:Cite book</ref> In chemical composition and general characteristics they are similar to the pyroxenes. The chief differences from pyroxenes are that (i) amphiboles contain essential hydroxyl (OH) or halogen (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.Template:Sfn Amphiboles are the primary constituent of amphibolites.Template:Sfn

Structure

Like pyroxenes, amphiboles are classified as inosilicate (chain silicate) minerals. However, the pyroxene structure is built around single chains of silica tetrahedra while amphiboles are built around double chains of silica tetrahedra. In other words, as with almost all silicate minerals, each silicon ion is surrounded by four oxygen ions. In amphiboles, some of the oxygen ions are shared between silicon ions to form a double chain structure as depicted below. These chains extend along the [001] axis of the crystal. One side of each chain has apical oxygen ions, shared by only one silicon ion, and pairs of double chains are bound to each other by metal ions that connect apical oxygen ions. The pairs of double chains have been likened to I-beams. Each I-beam is bonded to its neighbor by additional metal ions to form the complete crystal structure. Large gaps in the structure may be empty or partially filled by large metal ions, such as sodium, but remain points of weakness that help define the cleavage planes of the crystal.<ref name=Nesse2000p277t279>Template:Cite book</ref>

• Amphibole 100.jpg

  Double-chain inosilicate structure looking up the [100] axis. Silicon ions are hidden by apical oxygen ions.

• Amphibole 010.jpg

  Side view (along [010]) of double chain inosilicate backbone. Apical oxygens are at the bottom.

• Amphibole 001.jpg

  Amphibole structure looking along the [001] axis. Silicon ions are emphasized. Two "I-beams" are outlined in green.

In rocks

File:Mineralogy igneous rocks EN.svg

Mineral assemblage of igneous rocks

File:Horndio.jpg

Hornblende diorite from the Henry Mountains, Utah, US

File:Amphibolite (Precambrian; Warrensburg, Adirondack Mountains, New York State, USA) 1.jpg

Amphibolite from Warrensburg, Adirondack Mountains, New York State, USA

Amphiboles are minerals of either igneous or metamorphic origin. Amphiboles are more common in intermediate to felsic igneous rocks than in mafic igneous rocks,<ref>Template:Cite journal</ref> because the higher silica and dissolved water content of the more evolved magmas favors formation of amphiboles rather than pyroxenes.Template:Sfn The highest amphibole content, around 20%, is found in andesites.<ref>Template:Cite book</ref> Hornblende is widespread in igneous and metamorphic rocks and is particularly common in syenites and diorites. Calcium is sometimes a constituent of naturally occurring amphiboles. Amphiboles of metamorphic origin include those developed in limestones by contact metamorphism (tremolite) and those formed by the alteration of other ferromagnesian minerals (such as hornblende as an alteration product of pyroxene).Template:Sfn Pseudomorphs of amphibole after pyroxene are known as uralite.Template:Sfn

History and etymology

The name amphibole derives from Greek Template:Transliteration (Template:Wikt-lang, Template:Lit), implying ambiguity. The name was used by {{#invoke:Lang|lang}} to include tremolite, actinolite 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 two series. The formulae of each will be seen to be built on the general double-chain silicate formula RSi₄O₁₁.<ref name="EB1911">{{#if: |

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Four of the amphibole minerals are commonly called asbestos. These are: anthophyllite, riebeckite, the cummingtonite/grunerite series, and the actinolite/tremolite series. The cummingtonite/grunerite series is often termed amosite or "brown asbestos", and riebeckite is known as crocidolite or "blue asbestos". These are generally called amphibole asbestos.<ref>US Geological Survey, Asbestos, accessed 20 July 2015.</ref> Mining, manufacture and prolonged use of these minerals can cause serious illnesses.Template:Sfn<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Mineral species

The more common amphiboles are classified as shown in the following table:Template:Sfn

Amphibole classification (After Nesse 2000Template:Sfn)
Group	W	Template:Chem2	Template:Chem2	Template:Chem2	Mineral	Symmetry	Comment
Iron-magnesium		Template:Chem2	Template:Chem2	Template:Chem2	Anthophyllite	Orthorhombic	Orthoamphibolite
		Template:Chem2	Template:Chem2	Template:Chem2	Gedrite
		Template:Chem2	Template:Chem2	Template:Chem2	Cummingtonite-Grunerite	Monoclinic	Low-Ca-clinoamphibolite
Calcic		Template:Chem2	Template:Chem2	Template:Chem2	Tremolite-actinolite		Ca-clinoamphibole
	Template:Chem2	Template:Chem2	Template:Chem2	Template:Chem2	Hornblende
	Template:Chem2	Template:Chem2	Template:Chem2	Template:Chem2	Kaersutite
Sodic-calcic	Template:Chem2	Template:Chem2	Template:Chem2	Template:Chem2	Richterite		Na-Ca-clinonamphibole
	Template:Chem2	Template:Chem2	Template:Chem2	Template:Chem2	Katophorite
Sodic		Template:Chem2	Template:Chem2	Template:Chem2	Glaucophane-riebeckite		Na-clinoamphibole
	Template:Chem2	Template:Chem2	Template:Chem2	Template:Chem2	Eckermanite-arfvedsonite

Other species

Orthorhombic series

• Holmquistite, Li₂Mg₃Al₂Si₈O₂₂(OH)₂

Monoclinic series

• Pargasite, NaCa₂Mg₃Fe²⁺Si₆Al₃O₂₂(OH)₂
• Winchite, (CaNa)Mg₄(Al,Fe³⁺)Si₈O₂₂(OH)₂
• Edenite, NaCa₂Mg₅(Si₇Al)O₂₂(OH)₂

Series

Certain amphibole minerals form solid solution series, at least at elevated temperature. Ferrous iron usually substitutes freely for magnesium in amphiboles to form continuous solid solution series between magnesium-rich and iron-rich endmembers. These include the cummington (magnesium) to grunerite (iron) endmembers, where the dividing line is placed at 30% magnesium.Template:Sfn

In addition, the orthoamphiboles, anthophyllite and gedrite, which differ in their aluminium content, form a continuous solid solution at elevated temperature. As the amphibole cools, the two end members exsolve to form very thin layers (lamellae).Template:Sfn

Hornblende is highly variable in composition, and includes at least five solid solution series: magnesiohornblende-ferrohornblende (Template:Chem2), tschermakite-ferrotschermakite (Template:Chem2), edenite-ferroedenite (Template:Chem2), pargasite-ferropargasite (Template:Chem2) and magnesiohastingstite-hastingsite (Template:Chem2). In addition, titanium, manganese, or chromium can substitute for some of the cations and oxygen, fluorine, or chlorine for some of the hydroxide. The different chemical types are almost impossible to distinguish even by optical or X-ray methods, and detailed chemical analysis using an electron microprobe is required.Template:Sfn

Glaucophane to riebeckite form yet another solid solution series, which also extends towards hornblende and arfvedsonite.Template:Sfn

There is not a continuous series between calcic clinoamphiboles, such as hornblende, and low-calcium amphiboles, such as orthoamphiboles or the cummingtonite-grunerite series. Compositions intermediate in calcium are almost nonexistent in nature.Template:Sfn However, there is a solid solution series between hornblende and tremolite-actinolite at elevated temperature. A miscibility gap exists at lower temperatures, and, as a result, hornblende often contains exsolution lamellae of grunerite.Template:Sfn

Descriptions

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.<ref name="EB1911"/>

Hornblende is an important constituent of many igneous rocks. It is also an important constituent of amphibolites formed by metamorphism of basalt.Template:Sfn

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 ἀκτίς, ἀκτῖνος/aktís, aktînos, a 'ray' and λίθος/líthos, a 'stone') is a translation of the old German word Strahlstein (radiated stone).<ref name="EB1911"/>Template:Sfn

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.<ref name="EB1911"/>Template:Sfn

Pargasite is a rare magnesium-rich variety of hornblendeTemplate:Sfn 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 automorphic, with a red-brown pleochroism in petrographic thin section.<ref name="Handbook">

Template:Cite book</ref>

See also

• List of minerals
• Classification of silicate minerals