Minerals in rock sections; the practical methods of identifying minerals in rock sections with the microscope, especially arranged for students in technical and scientific schools . Fig. 55.—Hornblende, showing twinning between crossed nicols, in amphibole-biotile-granite. (From Cohen.) Usual Appearance in Sections : Both in ciystals and more orless irregular grains, Figs. 55 and 56, the habit varying with thechemical composition as follows :6 82 CHARACTERS OF MINERALS. Trcniolitc () and Actuioliic ((M^Fe).jCa varieties), in longcolumnar to needle-like individuals, with no terminal plane
Minerals in rock sections; the practical methods of identifying minerals in rock sections with the microscope, especially arranged for students in technical and scientific schools . Fig. 55.—Hornblende, showing twinning between crossed nicols, in amphibole-biotile-granite. (From Cohen.) Usual Appearance in Sections : Both in ciystals and more orless irregular grains, Figs. 55 and 56, the habit varying with thechemical composition as follows :6 82 CHARACTERS OF MINERALS. Trcniolitc () and Actuioliic ((M^Fe).jCa varieties), in longcolumnar to needle-like individuals, with no terminal planes orfrayed out ends. May be in dense aggregates. Pargasitc in well developed crystals. Coviinon green HoDihlende (aluminous varieties) in crystals,compact grains or shreds. Basaltie Hornbleiuie (iron rich, aluminous varieties) in prismaticcrystals of varying length, which may often show magmaticresorption (to augite and magnetite) around outer zone orthroughout whole crystal. Crystals are simple in form of prismatic habit, with prism angle124° 30. Cross-sections are acutely rhombic, generally with. Fig. 56. • Hornblende, section parallel to c axis, showing prismatic cleavage, inhornblende-diorite. (From Cohen.) acute angles truncated, hence six-sided (pyroxene being eight-sided). Longitudinal sections are lath-shaped and fibrous struc-ture may be noticed. Skeleton crystals may also occur, beingvery fine in certain pitchstones. Zonal structure and parallel growth may be noticed in theamphiboles. Tivinning.—Frequent, parallel to ortho pinacoid (m P55 , 100).Twins dual, less often multiple. Fig. 55. Intergrowths withpyroxene and biotite occur. Color.—From colorless (tremolite), through green (actinolite,pargasite and hornblende) to brown (basaltic hornblende). Yellowin some \arieties and bluish in the soda varieties. Ineiex of Refraction.—n = to (, in the basaltichornblende), hence /-r//^/distinct and surface rough. AMPHIBOLE. 83
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