478 fenner: forms of silica 



fluid condition at such a temperature, but in view of the repeated 

 instances of the formation of cristobalite at temperatures well 

 below the inversion point, which the writer has met in these 

 investigations, the difficulty appears to vanish. It is quite 

 probable that under such conditions as have been suggested 

 cristobalite would be deposited at temperatures below 1470? 

 With tridymite the case appears essentially the same. With 

 either mineral it is only when in contact with a flux for a con- 

 siderable length of time that the stable form* is necessarily 

 reached. 



Effect of pressure upon the quartz-tridymite inversion point 



All the inversion points which have been given have neces- 

 sarily been determined under atmospheric pressure. It may 

 be of interest to inquire into the effect which great pressure 

 would have in increasing the range over which quartz is stable. 



In the calculations which follow the numerical values which 

 have been employed are only roughly approximate and at the 

 best we can obtain merely the probable order of magnitude of 

 pressure-effects. 



For the purposes of the calculation we shall take the tempera- 

 ture of inversion under one atmosphere pressure as 870°, for the 

 volume change a value derived from the differences in volume 

 between quartz and tridymite at ordinary temperatures, or 

 0.057 cc. per gm., for the energy-change 15 calories, or approxi- 

 mately two-fifths of the energy involved in all the inversions 

 from a-quartz to fused silica, according to the investigations 

 of O. Mulert. 3 We may then employ the fundamental Clausius- 

 Clapeyron formula, arranged as follows 



dT T , 



— = T 0>2 - Vy) 



dp L 



dT 

 in which — is the increase in temperature for an increase of 

 dp 



pressure of 1 gm. per cm 2 . 



3 Zs. fur Anor. Chem., Bd. 75, Heft 2, p. 206. May, 1912. 



