358 MINERALOGY 



by running water form the stratified sandstones in which silica 

 deposited from solution may be the cementing agent. 



Varieties. Rock crystal is that clear, colorless, crystallized 

 variety to which the sciences of mineralogy and crystallography owe 

 so much. It has furnished convenient material to the scientist 

 and physicist for experimentation and for apparatus since historic 

 times, and it stands in its relation to crystallography as the frog 

 does to the biological sciences. It has indeed furnished the name 

 crystal, as the ancients believed it to be water which had been sub- 

 jected to such a low temperature as to be no longer capable of re- 

 turning to the liquid state. Nicolaus Steno in 1669 noted the 

 similarity of the angles between crystal faces on quartz while cut- 

 ting sections. 



Beautiful clear crystals of quartz occur in the calcareous sand- 

 stone of Herkimer County, New York, known from their brightness 

 as Herkimer County diamonds. These are in some cases chemi- 

 cally pure SiO 2 , others are colored dark with carbonaceous inclu- 

 sions, while others have cavities containing liquids in which bubbles 

 may be rolled back and forth like the bubble of a spirit level. Evi- 

 dently all these crystals have, from the character of the inclusions, 

 been formed from solution and at a low temperature. This variety 

 of quartz is in all cases considered to have been formed at a tempera- 

 ture below that of 575 C., for when quartz is heated to a tempera- 

 ture of 575 C., it passes over to another phase, /3-quartz, in which 

 the physical properties are different from a-quartz, the phase stable 

 below 575 C. 



Large crystals and thick sections of quartz in passing this 

 inversion temperature are shattered, crack and fall in pieces; 

 /8-quartz is hexagonal holoaxial, while a-quartz is trigonal holoaxial ; 

 and whenever the trigonal trapezohedral face x appears, that crystal 

 must have been formed at a temperature below the inversion point 

 and as a-quartz, since the trigonal trapezohedron. is not a possible 

 form in the hexagonal holoaxial type ; also the occurrence of two 

 rhombohedrons r and z unequal in development and luster would 

 indicate that the crystal had separated as a-quartz, since these faces 

 in the hexagonal holoaxial type should be similar in all respects. 

 Quartz of granite micropegmatites and some macropegmatites has 

 been formed at a temperature above 575, as is shown by its frac- 

 tured condition. Large clear crystals are found at the Hot Springs, 

 Arkansas ; these have been formed in several stages of growth as is 

 indicated by the internal crystalline outlines known as " phantoms," 



