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 . e only primary tourmaline and is found in grani-toid rocks. Tourmaline in other rocks results from fumarole action ; hence occursin pegmatite, tin and copper veins, clay deposits, also (light colored) in contact rocksand crystalline schists. The hemimorphic terminations may sometimes be is not acted on by acids. H., 7 to 7-5- Sp. gr., 3 to It can beseparated from powdered rock by sp. gr. sol
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 . e only primary tourmaline and is found in grani-toid rocks. Tourmaline in other rocks results from fumarole action ; hence occursin pegmatite, tin and copper veins, clay deposits, also (light colored) in contact rocksand crystalline schists. The hemimorphic terminations may sometimes be is not acted on by acids. H., 7 to 7-5- Sp. gr., 3 to It can beseparated from powdered rock by sp. gr. solutions combined with magnetic methods. 70 CHARACTERS OF M/XERALS. ANDALUSITE. AmsotroiIC. Biaxial. Orthorhombic. Composition: Al(A10)SiO^. <= a. Elongation || a. Usual Appearance in Sections : In short, rounded, prismatic crystals, with almostsquare cross-section. Colorless or at times pale reddish and spotted. Index of refrac-tion medium (;/^), hence ;r//ty well marked and surface rough. Cleavage,parallel to almost square prism, may show. Pleochroism only marked in colored vari-eties, being reddish || c (the direction of elongation or cleavage). Carbonaceous inclus-. FlG. 45.—Chiastolite, showing characteristic carbonaceous inclusions. (FromCohen.) ions are characteristic, arranged as in macroscopic specimens {Chiastolifc), Fig. halos may surround inclusions. Crossed Nicols : Double refraction weak (y—a = ). Interference colorsmiddle 1st order, white to yellow. Extinction in general parallel to c axis in longi-tudinal sections, symmetrical in cross sections. In convergent light Ax. pi. || 00 Poo(010), Bx„. II c ; axial angle very large {2E~^ 180°) ; optical character (—). Alteration : Often takes place to dense aggregate of mica, when the pseudomorphmay be hard to recognize. Distinguished from: (rt) SiLLlMANlTE by much weaker double refraction, less elongated crystals and byelongation || a^ (Sillimanite elong. || c^). (b) DiOPSlDE by weaker
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