fenner: forms of silica 479 



. T = absolute temperature 



L = latent heat, expressed in work units, 

 = 15 X 4.1833 X 1.02 X 10 4 gm. cm.* 

 = 15 X 42670 gm. cm. 

 v 2 — Vi = increase in volume in cc. per gm. 

 By substitution 

 dT 1143 X 0.057 



~dp 15 X 42670 



= 0.000102° for 1 gm. pressure per cm 2 . 



= 0.10537° per atmosphere. 

 Therefore 9.5 atmospheres will raise the inversion point 1°, 

 and a column of rock 10,000 feet deep will raise the temperature 

 88? 



This is probably as close an approximation as we can attain 

 with our present knowledge. In order to make a more precise 

 calculation the specific volume of tridymite at high tempera- 

 tures (up to and beyond 870°), and the energy-change involved 

 in the transformation of quartz into tridymite must be deter- 

 mined. With this information available it should be possible 

 to arrive at conclusions of great value regarding the conditions 

 of temperature and pressure under which quartziferous rocks 

 have crystallized. 



Summary 



The results of the investigation may be concisely stated 

 as follows: Either tridymite or cristobalite may, under certain 

 conditions, form at temperatures below their inversion-points. 

 The favorable conditions, we believe, are those which induce 

 rapid crystallization, such as the reactions of vapors or the sud- 

 den cooling of a melt. Either mineral, when formed under such 

 circumstances, will probably persist to ordinary temperatures 

 unchanged as regards the outer form. Quartz, on the other 

 hand, will probably never be deposited at a temperature greater 

 than 870° (i.e., neglecting the rather slight effect of pressure) 



* 1 gm. cal. = 4.1833 X 10 7 ergs. (Landolt-Bornstein tables, p. 810.) 

 1 erg. = 1.02 X 10 -8 kg. metres = 1.02 X 10 _s gm. centimetres. 

 /. 1 gm. cal. = 4.1833 X 1.02 X 10 4 gm. cm. 



