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 . —rt ^). Interferencecolors upper first or lower second Order, red, purple, blue, etc. Extiudion parallel toprisms. Ax. pi. || 00 Poo (loo), Bx„. || c, 2E = 44°. Optical character ( + )• Distinguished from : {a) Apatite by higher order interference colors and by elongation || c^ (apatitehas elongation || a)- {/>) Tremolite by always parallel extinction and small size of axial Fig. 46.—Sillimanite agg
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 . —rt ^). Interferencecolors upper first or lower second Order, red, purple, blue, etc. Extiudion parallel toprisms. Ax. pi. || 00 Poo (loo), Bx„. || c, 2E = 44°. Optical character ( + )• Distinguished from : {a) Apatite by higher order interference colors and by elongation || c^ (apatitehas elongation || a)- {/>) Tremolite by always parallel extinction and small size of axial Fig. 46.—Sillimanite aggregate, showing cross fracture, in mica-schist. (FromCohen.) ((-) Andalusite, see under the latter mineral. Remarks : Found especially in clay-rich contact rocks, gneisses and schists, oftenoccurring with iolite (cordierite). Crystals may appear in tjands. It is insoluble inhydrochloric acid. H., 6 to 7. Sp. gr., TOPAZ. Anisotropic. Biaxial. Orthorhombic. Composition: A1(A1(,) )SiO^. <- = c. Usual Appearance in Sections : Colorless crystals of short prismatic habit, grainsor rod-like radiating aggregates. Index of refraction about the same as that of calcite(«== ), hence ;y//</ medium. Cleavage perfect, parallel to base, butdoes not show as many cracks. Fluid inclusions abundant. Crossed Nicols : Double refraction weak (y — a = to i), about the sameas that of quartz. Interference colors middle first order, white, yellow, etc. Extinc-tion parallel to cleavage. In convergent light. Ax. pi. || 00 Poo (010), Bx^. || c, axia
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