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 . arnet when colorless by octahedral shape of crystals (garnet forms being noand 211), when brown (picotite) from melanite garnet by common zonal colorationof the latter, but may be only possible by chemical reactions. Furthermore spinel mayhave green color and is never decomposed. (<J) Perovskite by the lower index of refraction and the absence of reaction for Ti. (r) Chromite chemically or by density or hardness
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 . arnet when colorless by octahedral shape of crystals (garnet forms being noand 211), when brown (picotite) from melanite garnet by common zonal colorationof the latter, but may be only possible by chemical reactions. Furthermore spinel mayhave green color and is never decomposed. (<J) Perovskite by the lower index of refraction and the absence of reaction for Ti. (r) Chromite chemically or by density or hardness. Remarks : Found in gneiss, granulite, Iherzolite and in regions of contact meta-morphism and secondary bedding formations (picotite), olivine-basalt and are insoluble in hydrochloric acid. H., 8. Sp. gr., to GARNET. Isotropic. Isometric. Composition : ^\ R^ (SiO J3. R is Ca, Mg, Fe or Mn ; Ris Al, Fe, or Cr, rarely Ti. Usual Appearance in Sections: Irregular grains, Fig. 13, orsimple crystals, showing forms ocO (i 10) and 2O2 (211), alone orin combination, Fig. 14 b. Zonal structure not infrequent, espe-cially in the titanium varieties. Fig. Fig. 34.—Garnet, with zonal strucuiic, in gneiss (From Cohen.) 5 2 CHA RA C TKRS OF MINE RA LS. Color. — Colorless, or nearly so, to yellowish, reddish orbrownish. Judex of Refraction. —// = 56, hence relief high andsurface very rough. Fracture. — Irregular cracks occur, but no cleavage noticed. Crossed Nicols: As garnets are isotropic, sections remain darkduring complete rotation. Optical anomalies may however occur,but are generally confined to, titanium free, lime garnets and man-ganese garnets. The effect being to divide the crystal sym-metrically into different areas, dodecahedral structure. Alteration : Garnets are usually fresh, but may be found alteredto chlorite or hornblende. Distinguished From : Spinel and Perovskite. — See under thelatter. Remarks : Found principally in gran
Size: 1688px × 1480px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, bookpublishernewyorkdvannostran
