Amphibole Group - Orthorhombic Secction, Antothopyllity and Amphibole

The minerals of the Amphibole Group crystallize in either the orthorhombic, monoclinic or triclinic systems, but the crystals of the different species are closely similar in many respects. Chemically they form a series parallel to that of the Pyroxene Group (page 245), being metasilicates with calcium, magnesium and ferrous iron as important bases, and also with manganese and the alkalies. Certain molecules that are present in some varieties contain aluminum and ferric iron.

Orthorhombic Secction
Antothopyllity
An orthorhombic amphibole, corresponding to the orthorhombic pyroxene group, statute - bronzite - hypersthene. An iron magnesium metasilicate, (Mg,Fe)SiOa. Rarely in distinct crystals. Cornmonly lamellar or fibrous. Perfect prismatic cleavage. Color gray to various shades. of green and brown. A comparatively rare

mineral, occurring in the crystalline schists and thought to have been derived from the metamorphism of chrysolite.

Amphibole
Composition. The amphiboles consist of a series of minerals analogous in many ways to the pyroxenes. They are chiefly metasilicates of calcium and magnesium with ferrous iron replacing the magnesium. Other molecules are at times introduced, in which are the aIkalies, aluminum and ferried iron. The more important varieties of amphibole with the formulas assigned to them follow.

• Tremolite, CaMg₃(Si0₃)₄
• Actinolite, Ca(Mg,Fe)₃(SiO₃)₄
• Hornblende, CaMg₃(Si0₃)₄ with Na₂Al₂(Si0₄)₃ & Mg₂ Al₄ (Si0₆)₂. Ferrous iron is isomorphous with the magliesium and ferried iron with the aluminum.

These varieties forro an isomorphous series and all gradations between them occur.

Crystallization. Monoclinic. Crystals prismatic in habit; the prism faces make angles of 55° and 125° with each other (compare the 87° and 93° angles of pyroxene). The prism zone shows, in addition to the prism faces, usually those of the clinopinacoid and sometimes also those of the orthopinacoid. Prism zone frequently vertically striated and imperfectly developed. When the prism faces are distinct, the cross section of the crystal, when viewed in a direction parallel to the vertical axis, does not have the rectangular shape shown by the crystals of pyroxene. The termination of the crystals is almost always formed by the two faces of a low clinodome.

Structure. In crystals. Often bladed and frequently in radiating columnar aggregates. Sometimes in silky fibers. Coarse to fine granular. Compact.

Physical Properties. Perfect prismatic cleavage at angle of 125°, often yielding a splintery surface. H. = 5-6. G. = 3-3.3. Vitreous luster. Often with silky sheen in the prism zone. Color varying from white and light green in tremolite, to green in actinolite, through dark green to black in hornblende. Color deepens with increase in the amount of iron present Transparent to opaque. .

Tests. Fusible 3-4. Chemical tests same as for pyroxene, which see. Told from pyroxene by its better prismatic cleavage, by the difference in the prismatic angle and by the characteristic presence on the crystals of the low clinodome.