from the
Henry Mountains, Utah, US . The black sprays are hornblende amphibole. Hornblende is a common constituent of many
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. It is the principal mineral of
amphibolites, which form during medium- to high-grade
metamorphism of
mafic to
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
micas or other hydrous minerals. However, hornblende itself breaks down at very high temperatures. Hornblende alters easily to
chlorite,
biotite, or other mafic minerals. 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. ==Etymology==