Elements of mineralogy, crystallography and blowpipe analysis from a practical standpoint .. . h four are not known to be representedamong the crystals of minerals and two are each represented by asingle species. All that was said, p. 23, as to series may be repeated here with-out essential alteration. CLASS OF THE DIHEXAGONAL PYRAMID. 27. No. 6. Holohedral, Liebisch. No. 13. Normal Group, Dana. Symmetry of the Class. Very few minerals crystallize in forms of this highest class ofhexagonal symmetry. Beryl furnishes the best example. Asshown in Fig. 117 the forms are symmetrical to one horizont
Elements of mineralogy, crystallography and blowpipe analysis from a practical standpoint .. . h four are not known to be representedamong the crystals of minerals and two are each represented by asingle species. All that was said, p. 23, as to series may be repeated here with-out essential alteration. CLASS OF THE DIHEXAGONAL PYRAMID. 27. No. 6. Holohedral, Liebisch. No. 13. Normal Group, Dana. Symmetry of the Class. Very few minerals crystallize in forms of this highest class ofhexagonal symmetry. Beryl furnishes the best example. Asshown in Fig. 117 the forms are symmetrical to one horizontalplane and to six vertical planes at thirty degrees to each other aswel as to the lines of ir*isection of these planes. * Also called ^ombohedral, Sechsgliedrige, Drei-und-Einaxige and Monotrimetric. t A portion of lexagonal system the rhombohedral division is referred by Miller to three oblique axes and this method is followed by Groth. Even when fouraxe a e used the value of the third horizontal intercept can always be calculated fromthe other two. 34 CR YSTALLOGRAPHY. Fig. 117. Fig. II.
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