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 . n.— ;/ = , hence no relief and generallysmooth surface. Fracture.— May be noticed, but no cleavage observed. Inclusions.— Common, radially or zonally arranged, consistingof minerals or glass. Fig. 35. * The system of crystallization of leucite has been the subject of much habit is isometric. The consensus of opinion seems to be that leucite crystallizes inthe isometric system, but that the isome
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 . n.— ;/ = , hence no relief and generallysmooth surface. Fracture.— May be noticed, but no cleavage observed. Inclusions.— Common, radially or zonally arranged, consistingof minerals or glass. Fig. 35. * The system of crystallization of leucite has been the subject of much habit is isometric. The consensus of opinion seems to be that leucite crystallizes inthe isometric system, but that the isometric molecular arrangement, at least of thelarger crystals, cannot exist for the temperature and pressure at the earths molecular displacement takes place, giving rise to a more or less complicatedapparent twinning, and optical anomalies are noticed. The isotropic character returnsif the section is heated to 500°C. Iddings Roseultiiscti, p. 133. LEI CITE. 53 Crossed Nicols: The smaller crystals appear isotropic ; the largercrystals show characteristic intersecting systems of twin lamellae,Fig. 36. Double Refraction.—Very weak (/— « = ). In thin. Fig. 35.—Leucite, with radial and tangential inclusions, Vesuvius Lava. (FromCohen.) sections it may be necessary to use a sensitive color-plate to provedouble refraction. Interference Colors.— Very low ist order, dark gray, etc. Alteration : Quite frequent to fibrous or granular zeolites.
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