This article is about the mineral. For the logical fallacy, seeequivocation. For the ambiguous grammatical construction, seeamphibology. For the study of amphibians (amphibiology), seeamphibian.
Amphibole (/ˈæmfəboʊl/AM-fə-bohl) is a group ofinosilicateminerals, forming prism or needlelike crystals,[1] composed of double chainSiO 4tetrahedra, linked at the vertices and generally containingions ofiron and/ormagnesium in their structures. ItsIMAsymbol is Amp.[2] Amphiboles can be green, black, colorless, white, yellow, blue, or brown. TheInternational Mineralogical Association currently classifies amphiboles as a mineral supergroup, within which are two groups and several subgroups.[3]
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 andorthorhombic.[4] In chemical composition and general characteristics they are similar to thepyroxenes. 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.[5] Amphiboles are the primary constituent ofamphibolites.[6]
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 hasapical 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 toI-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.[7]
Double-chain inosilicate structure looking up the [100] axis. Silicon ions are hidden by apical oxygen ions.
Side view (along [010]) of double chain inosilicate backbone. Apical oxygens are at the bottom.
Amphibole structure looking along the [001] axis. Silicon ions are emphasized. Two "I-beams" are outlined in green.
Mineral assemblage of igneous rocksHornblende diorite from the Henry Mountains, Utah, USAmphibolite from Warrensburg, Adirondack Mountains, New York State, USA
Amphiboles are minerals of eitherigneous ormetamorphic origin. Amphiboles are more common inintermediate tofelsic igneous rocks than inmafic igneous rocks,[8] because the highersilica and dissolved water content of the moreevolved magmas favors formation of amphiboles rather than pyroxenes.[9] The highest amphibole content, around 20%, is found inandesites.[10]Hornblende is widespread in igneous and metamorphic rocks and is particularly common insyenites anddiorites. Calcium is sometimes a constituent of naturally occurring amphiboles. Amphiboles of metamorphic origin include those developed inlimestones by contact metamorphism (tremolite) and those formed by the alteration of other ferromagnesian minerals (such as hornblende as an alteration product of pyroxene).[11]Pseudomorphs of amphibole after pyroxene are known asuralite.[12]
The nameamphibole derives fromGreekamphíbolos (ἀμφίβολος,lit.'double entendre'), implying ambiguity. The name was used byRené Just Haüy to include tremolite,actinolite andhornblende. 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-chainsilicate formula RSi4O11.[13]
Four of the amphibole minerals are commonly calledasbestos. These are: anthophyllite, riebeckite, the cummingtonite/grunerite series, and the actinolite/tremolite series. The cummingtonite/grunerite series is often termedamosite or "brown asbestos", andriebeckite is known as crocidolite or "blue asbestos". These are generally called amphibole asbestos.[14] Mining, manufacture and prolonged use of these minerals can cause serious illnesses.[15][16]
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.[18]
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).[18]
Hornblende is highly variable in composition, and includes at least five solid solution series: magnesiohornblende-ferrohornblende (Ca2[(Mg,Fe)4Al]Si7AlO22(OH)2), tschermakite-ferrotschermakite (Ca2[(Mg,Fe)3Al2]Si6Al2O22(OH)2), edenite-ferroedenite (NaCa2(Mg,Fe)5Si7AlO22(OH)2), pargasite-ferropargasite (NaCa2[(Mg,Fe)4Al]Si6Al2O22(OH)2) and magnesiohastingstite-hastingsite (NaCa2[(Mg,Fe)4Fe3+]Si6Al2O22(OH)2). 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.[12]
Glaucophane to riebeckite form yet another solid solution series, which also extends towards hornblende and arfvedsonite.[19]
There isnot 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.[20] However, there is a solid solution series between hornblende and tremolite-actinolite at elevated temperature. Amiscibility gap exists at lower temperatures, and, as a result, hornblende often contains exsolution lamellae of grunerite.[21]
Hornblende is an important constituent of many igneous rocks. It is also an important constituent ofamphibolites formed by metamorphism ofbasalt.[22]
Actinolite is an important and common member of the monoclinic series, forming radiating groups ofacicular crystals of a bright green or greyish-green color. It occurs frequently as a constituent ofgreenschists. The name (fromGreekἀκτίς, ἀκτῖνος/aktís, aktînos, a 'ray' andλίθος/líthos, a 'stone') is a translation of the oldGerman wordStrahlstein (radiated stone).[13][23]
Glaucophane,crocidolite,riebeckite andarfvedsonite form a somewhat special group of alkali-amphiboles. The first two are blue fibrous minerals, with glaucophane occurring inblueschists 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 asnepheline-syenite andphonolite.[13][24]
^"Health Effects of Asbestos".Agency for Toxic Substances and Disease Registry. Centers for Disease Control. 10 December 2018. Retrieved6 November 2020.
![Double-chain inosilicate structure looking up the [100] axis. Silicon ions are hidden by apical oxygen ions.](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aAmphibole_100.jpg)
![Side view (along [010]) of double chain inosilicate backbone. Apical oxygens are at the bottom.](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aAmphibole_010.jpg)
![Amphibole structure looking along the [001] axis. Silicon ions are emphasized. Two "I-beams" are outlined in green.](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aAmphibole_001.jpg)
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