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Amphibole ( /ˈæmfɪbl/) is a group of inosilicate minerals, forming prism or needlelike crystals,[1] 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.[2]

Photomicrographs of a thin section containing an amphibole crystal. In cross-polarized light on left, plane-polarized light on right.

Amphiboles crystallize into two crystal systems, monoclinic and orthorhombic.[3] 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.[4] 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.[citation needed] Amphiboles are the primary constituent of amphibolites.[5]

In rocks

Amphiboles are minerals of either igneous or metamorphic origin. Amphiboles are more common in intermediate to felsic igneous rocks than in mafic igneous rocks, because the higher silica and dissolved water content of the more evolved magmas favors formation of amphiboles rather than pyroxenes.[6] The highest amphibole content, around 20%, is found in andesites.[7] Hornblende is widespread in igneous and metamorphic rocks and is particularly common in syenites and diorites. Calcium is sometimes a constituent of naturally occurring amphiboles. Amphilotes of metamorphic origin include those developed in limestones by contact metamorphism (tremolite) and those formed by the alteration of other ferromagnesian minerals (such as hornblende as an alteration product of pyroxene).[8] Pseudomorphs of amphibole after pyroxene are known as Amphiboles crystallize into two crystal systems, monoclinic and orthorhombic.[3] 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.[4] 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.[citation needed] Amphiboles are the primary constituent of amphibolites.[5]

In rocks

Amphiboles are minerals of either igneous or metamorphic origin. Amphiboles are more common in intermediate to felsic igneous rocks than in mafic igneous rocks, because the higher silica and dissolved water content of the more evolved magmas favors formation of amphiboles rather than pyroxenes.[6] The highest amphibole content, around 20%, is found in andesites.[7] Hornblende is widespread in igneous and metamorphic rocks and is particularly common in syenites and diorites. Calcium is sometimes a constituent of naturally occurring amphiboles. Amphilotes of metamorphic origin include those developed in limestones by contact metamorphism (tremolite) and those formed by the alteration of other ferromagnesian minerals (such as hornblende as an alteration product of pyroxene).[8] Pseudomorphs of amphibole after pyroxene are known as uralite.[9]

History and etymology

The name amphibole (Ancient Greek ἀμφίβολος - amphíbolos literally meaning 'double entendre', implying ambiguousness) was used by René Just Haüy to include tremolite, actinolite and hornblende. The group was so named by Haüy in allusion to the protean variety, in composition and appearance, assumed by its

Amphiboles are minerals of either igneous or metamorphic origin. Amphiboles are more common in intermediate to felsic igneous rocks than in mafic igneous rocks, because the higher silica and dissolved water content of the more evolved magmas favors formation of amphiboles rather than pyroxenes.[6] The highest amphibole content, around 20%, is found in andesites.[7] Hornblende is widespread in igneous and metamorphic rocks and is particularly common in syenites and diorites. Calcium is sometimes a constituent of naturally occurring amphiboles. Amphilotes of metamorphic origin include those developed in limestones by contact metamorphism (tremolite) and those formed by the alteration of other ferromagnesian minerals (such as hornblende as an alteration product of pyroxene).[8] Pseudomorphs of amphibole after pyroxene are known as uralite.[9]

History and etymology