Editing Hornblende (section)

==Occurrence==
[[File:Horndio.jpg|thumb|Hornblende [[diorite]] from the [[Henry Mountains]], Utah, US]]
Hornblende is a common constituent of many [[igneous rock|igneous]] and [[metamorphic rocks]] such as [[granite]], [[syenite]], [[diorite]], [[gabbro]], [[basalt]], [[andesite]], [[gneiss]], and [[schist]]. It crystallizes in preference to pyroxene minerals from cooler magma that is richer in silica and water.{{sfn|Nesse|2000|p=279}}

It is the principal mineral of [[amphibolite]]s, which form during medium- to high-grade [[metamorphism]] of [[mafic]] to [[intermediate composition|intermediate]] igneous rock (igneous rocks with relative low silica content) in the presence of pore water. Much of the pore water comes from the breakdown of [[mica]]s or other hydrous minerals. However, hornblende itself breaks down at very high temperatures.{{sfn|Nesse|2000|p=279}} Hornblende alters easily to [[chlorite group|chlorite]], [[biotite]], or other mafic minerals.{{sfn|Nesse|2000|p=286}}

A rare variety of hornblende contains less than 5% of [[iron oxide]], is gray to white in color, and is named [[edenite]] from its locality in Edenville, [[Orange County, New York]].

''Oxyhornblende'' is a variety in which most of the iron has been oxidized to the [[ferric]] state, {{chem2|Fe^{3+}|}}. Charge balance is preserved by the substitution of oxygen ions for hydroxide. Oxyhornblende is also typically enriched in titanium. It is found almost exclusively in volcanic rock and is sometimes called ''basaltic hornblende''.<ref>{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology. |date=1997 |publisher=American Geological Institute |location=Alexandria, Virginia |isbn=0922152349 |edition=Fourth |chapter=basaltic hornblende}}</ref>{{sfn|Nesse|2000|p=285–286}}