OXIDMS 



355 



handed ; if from northwest to southeast the crystal is left-handed. 

 Again if the edges of the /one mxsz ascend to the right around the 

 crystal, like a right-handed screw, the crystal is right-handed; 

 if to the let't. left-handed. Still a fourth method of distinguishing 

 right and left crystals is by the corrosion figures which often appear 

 on crystals as the result 

 of some solvent, or are 

 produced artificially by 

 treatment with hydro- 

 fluoric acid ; these fig- 

 ures are pointed at one 

 end and broad at the 

 other ; those on the right 

 are minor images of those 

 on the left. 



As the trapezohedral 

 faces are a key to both 

 the twinning of quartz 

 and its right- or lei't- 

 handedness, it should be 

 remembered that four of 

 these forms are possible. 

 The positive forms occur 

 the more often, and are 

 situated below r, the 

 positive right under the right-hand corner, and the positive left 

 directly under the left-hand corner. The left-handed forms hold 

 the same relation to the rhombohedral faces z. 



Twinning in quartz is very frequent, though it is not always to be 

 recognized unless the trapezohedral faces are present. 



1. Interpenetrating twins occur, where the twinning axis is 

 normal to the prism edge. In such twins x will occur on adjacent 

 pri-m faces, modifying the upper right-hand corner; they may not 

 appear on all six prism faces, but if they do appear on any two ad- 

 jacent faces it is sufficient to establish the twinned nature of the 

 crystal. In all such twins the rhombohedral faces are complex in 

 nature, portions which are bright are r and portions which may be 

 dull are z ; these areas are quite irregular and separated by curved 

 and jagged boundaries, plainly shown in the photographs. Whether 

 the twin consists of two right- or two left-handed individuals may be 

 determined from the relation of x to the bright patches of the rhom- 



FIG. 424. Quartz Twinned, composed of Right- 

 hand Individuals. St. Gothard, Switzerland. 



