AMPHIBOLE (Gk. apcpipoaoc, ancphibolos, doubtful, ambiguous; alluding to its being easily confounded with augite). An important group of rock-making minerals closely allied to the Pyroxene (q.v.) group. The amphiboles are metasilieates, principally of calcium, magnesium, or iron, and sometimes also of manganese, so dium, and potassium. The group is subdivided according to the forms of crystallization. Those that crystallize in the orthorhombic system in clude Anthophyllite and its variety Gedrite; the monoclinic section includes the typical mineral Amphibole with its varieties, as well as Glance> phane, Crocidolite, and certain other minerals; while the triclinic section includes -Enigmatite. All of these minerals have a common prismatic cleavage of from 54° to 56°, and also agree in their optical characters and chemical composi tion.
The most important member of the group is the mineral Amphibole, which gives its name to the series. The several varieties of amphibole are divided into two groups, according as they do or do not contain aluminum. The non-aluminous
varieties include Tremolite, a calcium magnesium silicate that is usually white to dark gray in color, and is found both in crystals and massive; Actinolite, a calcium magnesium and iron sili cate of varying shades of green; Grtinerite, an iron silicate which is of a brown color that oc curs in fibrous masses. The aluminous varieties include the several varieties of Hornblende, which comprises the dark green and black va rieties, known as common Hornblende (black), Pargasite (green and blue), and Edenite (white, gray, and pale green). These minerals are found in crystalline metamorphic limestones, granitic, and schistose rocks, and in volcanic or igneous rocks. Nearly every member of the group has several varieties, each of which, besides having a separate name, differs from the type by some slight variation in color, optical properties, or chemical composition. Many varieties of amphi bole have been cut as gem-stones.