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. 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.—Fre
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. 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. Fig. 57.—Hornblende, cross-section. Cleavage.—Perfect, parallel to prism of ° 30. Generallyappears in thin sections as sharp cracks crowded close together,Figs. 56 aud 57. More perfect than in pyroxene. Some of the long prisms (actinolite and tremolite) may showtransverse parting. Inclusions.—The iron ores, apatite, etc., may be found in horn-blende. Polarized Light: PlcocJiroisui.—All colored amphiboles show pleochroism, whichin general is stronger thedarker the color of the variety(actinolite and pargasite showbut little). The absorptionis very marked in the horn-blendes, being greatest in thegeneral direction of the cleav-age lines in longitudinal sec-tions. Marked differences in absorption are also characteristic of the mineral species biotite,tourmaline and allanite. Pleochroic halos (brownish) surroundinginclusions may be Nicols: Double Refraction.—Quite strong, but a little weaker than inpyroxene
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