356 



MINERALOGY 



bohedral faces, as x will be under the bright areas in right- and under 

 the dull areas in left-handed crystals. 



2. Twins occur in which the face x modifies both upper or both 

 lower corners of the prism face; these are twinned by reflection 



over a plane perpendicular to the 

 prism face and parallel to the verti- 

 cal axis. These are termed Bra- 

 zilian twins and are often repeated, 

 interpenetrating and quite irregu- 

 lar, but the twins are always 

 formed by the union of right and 

 left individuals. In some cases 

 the plane of reflection may pass 

 through opposite prism edges, when 

 four x faces will lie adjacent to a 

 single and alternate prism edge. 



3. Twins occur in which the 

 twinning axis is perpendicular to 

 the rhombohedral face 1122; after 

 a revolution of 180 around this 



axis the vertical crystallographical axes of the two individuals will 

 lie at 84 33'. This type of twins is usually flattened parallel to the 

 prism face and they are known as the Japanese twins, as the most 

 beautiful specimens are ob- 

 tained from that country. 



Quartz is the most com- 

 mon of all minerals. It is 

 distributed universally and 

 occurs under the most varied 

 conditions. It is one of the 

 essential minerals of granite, 

 mica schist, and gneiss, while 

 quartzite and sandstones 

 may be almost pure quartz. 

 In rock magmas SiO 2 takes 



FIG. 425. Quart'z Crystals from near 

 Rome, Italy. 



FIG. 426. Quartz twinned on 1122. 



Alaska. 



the part of an acid, and for 

 this reason quartz is never 

 found in the basic and dark- 

 colored igneous rocks, as quartz is formed only in those cases where 

 there is an excess of Si0 2 over the basic oxides. It is usually near 

 the last to crystallize or separate in the solidification of a rock 



