AMPHIBOLE  , an important

group of rock-forming minerals, very similar in chemical composition and general characters to the pyroxenes, and like them falling into three series according to the system of crystallization . They differ from the pyroxenes, however, in having an angle between the prismatic cleavage of 56° instead of 87°; they are specifically lighter than the corresponding pyroxenes; and, in their optical characters, they are distinguished by their stronger pleochroism and by the wider angle of extinction on the plane of symmetry . They are minerals of either original or secondary origin; in the former case occurring as constituents (hornblende) of igneous rocks, such as granite, diorite, andesite, &c . Those of secondary origin have either been developed (tremolite) in limestones by contact-metamorphism, or have resulted (actinolite) by the alteration of augite by dynamo-metamorphism . Pseudomorphs of amphibole after pyroxene are known as uralite . The name amphibole (from the Gr. aw/,i3oXos, ambiguous) was used by R . J . Hauy to include tremolite, actinolite and hornblende; this term has since been applied to the whole group . Numerous sub-species and varieties are distinguished, the more important of which are tabulated below in three series . The formulae of each will be seen to conform to the general meta-silicate formula R"SiO3 .