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 . Orthoclase cleavage 65. — Basal. Fig. 66. — Clino pinacoid. formation with plagioclase (the orthoclase on the periphery).Also intergrown with quartz* forming pegmatite and micro-pegmatite, Fig. 67. Zonal structure often seen. Fig. 20, especially when decompo-. FlG. 67. — Micro-pegmatitic structure, in granophyric quartz-porphyry. (FromCohen.) .sition has commenced ; and in fresh crystals may be indicate
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 . Orthoclase cleavage 65. — Basal. Fig. 66. — Clino pinacoid. formation with plagioclase (the orthoclase on the periphery).Also intergrown with quartz* forming pegmatite and micro-pegmatite, Fig. 67. Zonal structure often seen. Fig. 20, especially when decompo-. FlG. 67. — Micro-pegmatitic structure, in granophyric quartz-porphyry. (FromCohen.) .sition has commenced ; and in fresh crystals may be indicated byzonal arrangement of inclusions. Tiviiiiiitig.—Very common, generalh after Carlsbad\ayN, Figs. 18 * See under quartz, p. 60. FELDSPAR GROUP. 97 and 69 ; the twinning boundary, dividing the section longitudinally,either being parallel to edges of crystal or bent or jagged. Twin-ning after Bavcno (twinning boundary diagonal, \\\\\\ the two partsextinguishing at the same time, but having a and c directions,crossed in the two portions) and Ma)iebacJi laws less common * Color.—Colorless or tinged by oxide of iron. Cloudy if decom-posed. Index of Refraction.—n = , hence no relief and surfacesmooth. Cleavage.—Varies and sometimes only seen in very thin sec-tions, but is an important character and should always be searchedfor. It occurs perfect, parallel to base (OP, 001), and almost as
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