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 . Fig. 71. — Plagioclase, showing Ijroad lamellee, in gabbro. (From Cohen. of microcline, Fig. 72. In addition the polysynthetic crystals maybe twinned like orthoclase after Carlsbad and Baveno laws. IIBRARTyWIVERSITY CF CALIFOI^NIA R 102 CHARACTERS OF MINERALS. The general characters are the same as in orthoclase with thefollowing differences : Indices of Refraction :;/= Albite. n = Oligoclase, Ab^An,.n
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 . Fig. 71. — Plagioclase, showing Ijroad lamellee, in gabbro. (From Cohen. of microcline, Fig. 72. In addition the polysynthetic crystals maybe twinned like orthoclase after Carlsbad and Baveno laws. IIBRARTyWIVERSITY CF CALIFOI^NIA R 102 CHARACTERS OF MINERALS. The general characters are the same as in orthoclase with thefollowing differences : Indices of Refraction :;/= Albite. n = Oligoclase, Ab^An,.n = I-abradorite, = surface of anorthite appears shghtly rougher than that oforthoclase. Cleavages, parallel to base (OP, 001) and brachy pinacoid(co P CO, 010), never intersect at right angles, as is the case in sec-tions of orthoclase parallel to b axis. This is due to the triclinic. Fig. 72. — Plagioclase, showing crossed lamellae, in olivine-gabbro. (From Cohen.) system of crystallization, but the divergence from a right angle issmall (93° 36 to 94° 10). Inclusions at times may be quite important, as the vitreous in-clusions of oligoclase in andesites, etc., and the iron ore inclusionsand other microlites in labradorite. The arrangement of theseinclusions may be zonal or in parallel orientation. Double refraction is a little stronger than for orthoclase {y — to (anorthite)), hence producing slightly brighterinterference colors in sections of the same thickness. Extinction takes place in all sections unsymmetrically withrespect to crystallographic, twinning or cleavage lines (as theseminerals are triclinic) ; hence extinction angles are always observed. Convergent Light: All plagioclases show the emergence of abisectrix,^ more or less oblique, on brachy pinacoid (00 P oc, 010) * For the positions of the optic axes, bisectrices, etc., relativ
Size: 1589px × 1573px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, bookpublishernewyorkdvannostran
