Amphibole
Encyclopedia
Silicate minerals
The silicate minerals make up the largest and most important class of rock-forming minerals, constituting approximately 90 percent of the crust of the Earth. They are classified based on the structure of their silicate group...
mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s, composed of double chain tetrahedra
Tetrahedron
In geometry, a tetrahedron is a polyhedron composed of four triangular faces, three of which meet at each vertex. A regular tetrahedron is one in which the four triangles are regular, or "equilateral", and is one of the Platonic solids...
, linked at the vertices and generally containing ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s of iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
and/or magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
in their structures.
Mineralogy
Amphiboles crystallize into two crystal systems, monoclinic and orthorhombic. In chemical composition and general characteristics they are similar to the pyroxenePyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
s. 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. In optical characteristics, many amphiboles are distinguished by their stronger pleochroism
Pleochroism
Pleochroism is an optical phenomenon in which a substance appears to be different colors when observed at different angles with polarized light.- Background :Anisotropic crystals will have optical properties that vary with the direction of light...
and by the smaller angle of extinction (Z angle c) on the plane of symmetry. Amphiboles are the primary constituent of amphibolite
Amphibolite
Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
s.
In rocks
Amphiboles are minerals of either igneousIgneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
or metamorphic
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
origin; in the former case occurring as constituents (hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
) of igneous rocks, such as granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
, diorite
Diorite
Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar , biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory...
, andesite
Andesite
Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite,...
and others. Calcium is sometimes a constituent of naturally occurring amphiboles.(C. Michael Hogan. 2010) Those of metamorphic origin include examples such as those developed in limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
s by contact metamorphism (tremolite
Tremolite
Tremolite is a member of the amphibole group of silicate minerals with composition: Ca2Mg5Si8O222. Tremolite forms by metamorphism of sediments rich in dolomite and quartz. Tremolite forms a series with actinolite and ferro-actinolite. Pure magnesium tremolite is creamy white, but the color grades...
) and those formed by the alteration of other ferromagnesian minerals (hornblende). Pseudomorphs of amphibole after pyroxene are known as uralite.
History and etymology
The name amphibole (GreekAncient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...
αμφιβολος - amphibolos meaning 'ambiguous') was used by René Just Haüy
René Just Haüy
René Just Haüy – 3 June 1822 in Paris) was a French mineralogist, commonly styled the Abbé Haüy after he was made an honorary canon of Notre Dame. He is often referred to as the "Father of Modern Crystallography." -Biography:...
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 two series. The formulae of each will be seen to be built on the general double-chain silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
formula RSi4O11.
Chemical formulae
Orthorhombic series- AnthophylliteAnthophylliteAnthophyllite is an amphibole mineral: 7Si8O222, magnesium iron inosilicate hydroxide. Anthophyllite is polymorphic with cummingtonite. Some forms of anthophyllite are lamellar or fibrous and are used as asbestos...
(Mg,Fe)7Si8O22(OH)2 - HolmquistiteHolmquistiteHolmquistite is a lithium magnesium aluminium inosilicate mineral with chemical formula: Li23Al2Si8O222. It crystallizes in the orthorhombic crystal system as prismatic crystals up to 10 cm or as massive aggregates...
Li2Mg3Al2Si8O22(OH)2
Monoclinic series
- TremoliteTremoliteTremolite is a member of the amphibole group of silicate minerals with composition: Ca2Mg5Si8O222. Tremolite forms by metamorphism of sediments rich in dolomite and quartz. Tremolite forms a series with actinolite and ferro-actinolite. Pure magnesium tremolite is creamy white, but the color grades...
Ca2Mg5Si8O22(OH)2 - ActinoliteActinoliteActinolite is an amphibole silicate mineral with the chemical formula .-Etymology:The name actinolite is derived from the Greek word aktis , meaning "beam" or "ray", because of the mineral's fibrous nature...
Ca2(Mg,Fe)5Si8O22(OH)2 - CummingtoniteCummingtoniteCummingtonite is a metamorphic amphibole with the chemical composition 7Si8O222, magnesium iron silicate hydroxide.Monoclinic cummingtonite is compositionally similar and polymorphic with orthorhombic anthophyllite, which is a much more common form of magnesium-rich amphibole, the latter being...
Fe2Mg5Si8O22(OH)2 - GruneriteGruneriteGrunerite is a mineral of the amphibole group of minerals with formula Fe7Si8O222. It is the iron endmember of the grunerite-cummingtonite series. It forms as fibrous, columnar or massive aggregates of crystals. The crystals are monoclinic prismatic. The luster is glassy to pearly with colors...
Fe7Si8O22(OH)2 - HornblendeHornblendeHornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2 - GlaucophaneGlaucophaneGlaucophane is a mineral belonging to the amphibole group, chemical formula []Na2]Mg3Al2][2|Si8O22].-Name:It is named from its typical blue color . Glaucophane crystallizes in the monoclinic system....
Na2(Mg,Fe)3Al2Si8O22(OH)2 - RiebeckiteRiebeckiteRiebeckite is a sodium-rich member of the amphibole group of silicate minerals, chemical formula [][Na2][32][2|Si8O22]. It forms a series with magnesioriebeckite. It crystallizes in the monoclinic system, usually as long prismatic crystals showing a diamond-shaped cross section, but also in...
(or Crocidolite) Na2Fe2+3Fe3+2Si8O22(OH)2 - ArfvedsoniteArfvedsoniteArfvedsonite is a sodium amphibole mineral with composition: [Na][Na2][4Fe3+][2|Si8O22]. It crystallizes in the monoclinic prismatic crystal system and typically occurs as greenish black to bluish grey fibrous to radiating or stellate prisms....
Na3Fe2+4Fe3+Si8O22(OH)2 - RichteriteRichteriteRichterite is a sodium calcium magnesium silicate mineral belonging to the amphibole group. If iron replaces the magnesium within the structure of the mineral, it is called ferrorichterite; if fluorine replaces the hydroxyl, it is called fluororichterite. Richterite crystals are long and prismatic,...
Na2Ca(Mg,Fe)5Si8O22(OH)2 - PargasitePargasitePargasite is a complex inosilicate mineral of the amphibole group with formula NaCa24AlO222.It was first described for an occurrence in Pargas, Finland in 1814 and named for the locality....
NaCa2Mg3Fe2+Si6Al3O22(OH)2 - Winchite (CaNa)Mg4(Al,Fe3+)Si8O22(OH)2
Descriptions
On account of the wide variations in chemical composition, the different members vary considerably in properties and general appearance.Anthophyllite
Anthophyllite
Anthophyllite is an amphibole mineral: 7Si8O222, magnesium iron inosilicate hydroxide. Anthophyllite is polymorphic with cummingtonite. Some forms of anthophyllite are lamellar or fibrous and are used as asbestos...
occurs as brownish, fibrous or lamellar masses with hornblende in mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
-schist
Schist
The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...
at Kongsberg
Kongsberg
is a town and municipality in Buskerud county, Norway. It is located at the southern end of the traditional region of Numedal. The administrative centre of the municipality is the town of Kongsberg....
in Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
and some other localities. An aluminous related species is known as gedrite
Gedrite
Gedrite is a silicate mineral of the amphibole group with formula: 2[3Al2]O222.Gedrite occurs in contact and medium to high grade metamorphic rocks in association with garnet, cordierite, anthophyllite, cummingtonite, sapphirine, sillimanite, kyanite, quartz, staurolite and biotite.Gedrite was...
and a deep green Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
n variety containing little iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
as kupfferite.
Hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
is an important constituent of many igneous rocks. It is also an important constituent of amphibolite
Amphibolite
Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
s formed by metamorphism of basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
.
Actinolite
Actinolite
Actinolite is an amphibole silicate mineral with the chemical formula .-Etymology:The name actinolite is derived from the Greek word aktis , meaning "beam" or "ray", because of the mineral's fibrous nature...
is an important and common member of the monoclinic series, forming radiating groups of acicular
Crystal habit
Crystal habit is an overall description of the visible external shape of a mineral. This description can apply to an individual crystal or an assembly of crystals or aggregates....
crystals of a bright green or greyish-green color. It occurs frequently as a constituent of greenschist
Greenschist
Greenschist is a general field petrologic term applied to metamorphic or altered mafic volcanic rock. The term greenstone is sometimes used to refer to greenschist but can refer to other rock types too. The green is due to abundant green chlorite, actinolite and epidote minerals that dominate the...
s. The name (from Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...
ακτις/aktis, a 'ray' and λιθος/lithos, a 'stone') is a translation of the old German
German language
German is a West Germanic language, related to and classified alongside English and Dutch. With an estimated 90 – 98 million native speakers, German is one of the world's major languages and is the most widely-spoken first language in the European Union....
word Strahlstein (radiated stone).
Glaucophane
Glaucophane
Glaucophane is a mineral belonging to the amphibole group, chemical formula []Na2]Mg3Al2][2|Si8O22].-Name:It is named from its typical blue color . Glaucophane crystallizes in the monoclinic system....
, crocidolite, riebeckite
Riebeckite
Riebeckite is a sodium-rich member of the amphibole group of silicate minerals, chemical formula [][Na2][32][2|Si8O22]. It forms a series with magnesioriebeckite. It crystallizes in the monoclinic system, usually as long prismatic crystals showing a diamond-shaped cross section, but also in...
and arfvedsonite
Arfvedsonite
Arfvedsonite is a sodium amphibole mineral with composition: [Na][Na2][4Fe3+][2|Si8O22]. It crystallizes in the monoclinic prismatic crystal system and typically occurs as greenish black to bluish grey fibrous to radiating or stellate prisms....
form a somewhat special group of alkali-amphiboles. The first two are blue fibrous minerals, with glaucophane occurring in blueschist
Blueschist
Blueschist is a rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures, approximately corresponding to a depth of 15 to 30 kilometers and 200 to ~500 degrees Celsius....
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
Nepheline
Nepheline, also called nephelite , is a feldspathoid: a silica-undersaturated aluminosilicate, Na3KAl4Si4O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites...
-syenite
Syenite
Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or...
and phonolite
Phonolite
Phonolite is a rare igneous, volcanic rock of intermediate composition, with aphanitic to porphyritic texture....
.
Pargasite
Pargasite
Pargasite is a complex inosilicate mineral of the amphibole group with formula NaCa24AlO222.It was first described for an occurrence in Pargas, Finland in 1814 and named for the locality....
is a rare magnesium-rich amphibole with essential sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
, usually found in ultramafic rocks. For instance, it occurs in uncommon mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
xenolith
Xenolith
A xenolith is a rock fragment which becomes enveloped in a larger rock during the latter's development and hardening. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption...
s, carried up by kimberlite
Kimberlite
Kimberlite is a type of potassic volcanic rock best known for sometimes containing diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an diamond in 1871 spawned a diamond rush, eventually creating the Big Hole....
. It is hard, dense, black and usually idiomorphic, with a red-brown pleochroism
Pleochroism
Pleochroism is an optical phenomenon in which a substance appears to be different colors when observed at different angles with polarized light.- Background :Anisotropic crystals will have optical properties that vary with the direction of light...
in petrographic
Petrography
Petrography is a branch of petrology that focuses on detailed descriptions of rocks. Someone who studies petrography is called a petrographer. The mineral content and the textural relationships within the rock are described in detail. Petrographic descriptions start with the field notes at the...
thin section
Thin section
In optical mineralogy and petrography, a thin section is a laboratory preparation of a rock, mineral, soil, pottery, bones, or even metal sample for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a...
.
See also
- List of minerals
- Classification of minerals - SilicatesClassification of minerals - SilicatesThis list gives an overview of the classification of minerals and includes mostly IMA recognized minerals and its groupings. This list complements the alphabetical list on List of minerals and List of minerals. Rocks, ores, mineral mixtures, not IMA approved minerals, not named minerals are...
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