<Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (November 2010) (Learn how and when to remove this template message) </Td> </Tr> <P> Amphibole (/ ˈæmfɪboʊl /) is the name of an important group of generally dark - colored, inosilicate minerals, forming prism or needlelike crystals, composed of double chain SiO 4 tetrahedra, linked at the vertices and generally containing ions of iron and / or magnesium in their structures . Amphiboles can be green, black, colorless, white, yellow, blue, or brown . The International Mineralogical Association currently classifies amphiboles as a mineral supergroup, within which are two groups and several subgroups . </P> <P> Amphiboles crystallize into two crystal systems, monoclinic and orthorhombic . In chemical composition and general characteristics they are similar to the pyroxenes . 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 and by the smaller angle of extinction (Z angle c) on the plane of symmetry . Amphiboles are the primary constituent of amphibolites . </P> <P> Amphiboles are minerals of either igneous or metamorphic origin; in the former case occurring as constituents (hornblende) of igneous rocks, such as granite, diorite, andesite and others . Calcium is sometimes a constituent of naturally occurring amphiboles . Those of metamorphic origin include examples such as those developed in limestones by contact metamorphism (tremolite) and those formed by the alteration of other ferromagnesian minerals (hornblende). Pseudomorphs of amphibole after pyroxene are known as uralite . </P>

The most common member of the amphibole group