A popular guide to minerals : with chapters on the Bement Collection of minerals in the American Museum of Natural History, and the development of mineralogy . Fig. 287 Fig. 288 Twinning of crystals, pseudosymmetry, inclusions, etc., are de-tected in a beautiful way, in thin sections examined in parallel polar-ized light, through the difference in brightness or color in the differ-ent sections of the compound crystal. If the section is perpendicu-lar to the twinning plane, a symmetrical extinction direction to thesame is observed (Fig, 289, hornblende) ; if the twinning is re-peated (polysynth
A popular guide to minerals : with chapters on the Bement Collection of minerals in the American Museum of Natural History, and the development of mineralogy . Fig. 287 Fig. 288 Twinning of crystals, pseudosymmetry, inclusions, etc., are de-tected in a beautiful way, in thin sections examined in parallel polar-ized light, through the difference in brightness or color in the differ-ent sections of the compound crystal. If the section is perpendicu-lar to the twinning plane, a symmetrical extinction direction to thesame is observed (Fig, 289, hornblende) ; if the twinning is re-peated (polysynthetic), alternating black and light stripes are seen 84 A POPULAR GUIDE TO MINERALS (Fig. 290) or contrasted colors; if the crystal is variously con-structed of individuals, these may be seen, as in Fig. 291, or, ifmore regularly united, as in Fig. 292, (labradorite, twinned in twoways).
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectmineral, bookyear1912
