X rays and crystal structure . Quartz isthus referred to three equal axes making equalangles with each other, just as is calcite. Quartz, * See W. H. Bragg, Proc. Roy. Soc, A, Vol. 89, p. IiG. 53. ANALYSIS OF CRYSTALS 161 howev^er, unlike calcite, possesses neither planesnor centre of symmetry, having axes of symmetryalone. There is a single axis of three-fold sym-metry, with three axes of twofold symmetry inter-secting in it, these being shown in the quartz has no centre or plane of symmetry, Basal plane(III) Prism face(ZV\) f\^ (100) l\ (PI2) (MO) (511) .^ZV K K i{ yv /\
X rays and crystal structure . Quartz isthus referred to three equal axes making equalangles with each other, just as is calcite. Quartz, * See W. H. Bragg, Proc. Roy. Soc, A, Vol. 89, p. IiG. 53. ANALYSIS OF CRYSTALS 161 howev^er, unlike calcite, possesses neither planesnor centre of symmetry, having axes of symmetryalone. There is a single axis of three-fold sym-metry, with three axes of twofold symmetry inter-secting in it, these being shown in the quartz has no centre or plane of symmetry, Basal plane(III) Prism face(ZV\) f\^ (100) l\ (PI2) (MO) (511) .^ZV K K i{ yv /\ A A e- -e- 10° 20° 30° 40° 50° 60° Fig. 54. it follows that the crystals may be of two kinds,right- and left-handed, these kinds being related toeach other as are an object and its image in a mirror. The spectra yielded by various faces of quartzare given in Fig, 54. This crystal is more complicated than any as yetmeasured, and it is somewhat difficult to express B. R. 162 ANALYSIS OF CRYSTALS these results in terms of the crystal structure. Thisbecomes more simple if we give up the axes to whichquartz is generally referred by crystallographers, andemploy a set of hexagonal axes.
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