Editing Amphibole (section)

==Mineralogy==
[[File:Thin section microscopy Siilinjärvi R636 6335 amphibole.jpg|thumb|[[Optical mineralogy|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>{{cite book |last1=Klein |first1=Cornelis |last2=Hurlbut |first2=Cornelius S. Jr. |title=Manual of mineralogy : (after James D. Dana) |date=1993 |publisher=Wiley |location=New York |isbn=047157452X |page=491 |edition=21st}}</ref> In chemical composition and general characteristics they are similar to the [[pyroxene]]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.{{sfn|Klein|Hurlbut|1993|pp=474-475,478,491}} Amphiboles are the primary constituent of [[amphibolite]]s.{{sfn|Klein|Hurlbut|1993|pp=590}}

===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>{{cite book |last1=Nesse |first1=William D. |title=Introduction to mineralogy |date=2000 |publisher=Oxford University Press |location=New York |isbn=9780195106916 |pages=277–279}}</ref>
<gallery>

File:Amphibole 100.jpg|Double-chain inosilicate structure looking up the [100] axis. Silicon ions are hidden by apical oxygen ions.
File:Amphibole 010.jpg|Side view (along [010]) of double chain inosilicate backbone. Apical oxygens are at the bottom.
File:Amphibole 001.jpg|Amphibole structure looking along the [001] axis. Silicon ions are emphasized. Two "I-beams" are outlined in green.
</gallery>

===In rocks===
[[File:Mineralogy igneous rocks EN.svg|thumb|upright=1.4|Mineral assemblage of igneous rocks]]
[[File:Horndio.jpg|thumb|Hornblende diorite from the Henry Mountains, Utah, US]]
[[File:Amphibolite (Precambrian; Warrensburg, Adirondack Mountains, New York State, USA) 1.jpg|thumb|Amphibolite from Warrensburg, Adirondack Mountains, New York State, USA]]
Amphiboles are minerals of either [[igneous rock|igneous]] or [[metamorphism|metamorphic]] origin. Amphiboles are more common in [[intermediate rock|intermediate]] to [[felsic]] igneous rocks than in [[mafic]] igneous rocks,<ref>{{Cite journal|last1=Peters|first1=Stefan T. M.|last2=Troll|first2=Valentin R.|last3=Weis|first3=Franz A.|last4=Dallai|first4=Luigi|last5=Chadwick|first5=Jane P.|last6=Schulz|first6=Bernhard|date=2017-03-16|title=Amphibole megacrysts as a probe into the deep plumbing system of Merapi volcano, Central Java, Indonesia|url=https://doi.org/10.1007/s00410-017-1338-0|journal=Contributions to Mineralogy and Petrology|language=en|volume=172|issue=4|pages=16|doi=10.1007/s00410-017-1338-0|bibcode=2017CoMP..172...16P|s2cid=132014026|issn=1432-0967}}</ref> because the higher [[silica]] and dissolved water content of the more [[igneous differentiation|evolved]] magmas favors formation of amphiboles rather than pyroxenes.{{sfn|Nesse|2000|p=279–280}}  The highest amphibole content, around 20%, is found in [[andesite]]s.<ref>{{cite book |last1=Levin |first1=Harold L. |title=The earth through time |date=2010 |publisher=J. Wiley |location=Hoboken, N.J. |isbn=978-0470387740 |page=62 |edition=9th}}</ref> [[Hornblende]] is widespread in igneous and metamorphic rocks and is particularly common in [[syenite]]s and [[diorite]]s. Calcium is sometimes a constituent of naturally occurring amphiboles. Amphiboles of metamorphic origin include those developed in [[limestone]]s by contact metamorphism ([[tremolite]]) and those formed by the alteration of other ferromagnesian minerals (such as hornblende as an alteration product of pyroxene).{{sfn|Klein|Hurlbut|1993|p=496-497}} [[Pseudomorph]]s of amphibole after pyroxene are known as [[uralite]].{{sfn|Nesse|2000|p=285}}