fenner: forms of silica 477 



870° ± 10°/3 -quartz <=± /3-tridymite (holohedral hexagonal), 



1470° ± 10° /3-tridymite <=± /3-cristobalite (isometric). 



The above are all stable forms, each with a certain range of 

 temperature, but on cooling /3-tridymite or ,8-cristobalite, meta- 

 stable forms result, as follows: 



115°-120° /3-tridymite ^± a-tridymite (biaxial, perhaps ortho- 

 rhombic) 



180°-270° (inversion point easily displaced and not definitely 

 determinable) */3-cristobalite +± a-cristobalite (crystal character 

 unknown, mineral may belong to any system but isometric). 



There are thus apparently at least seven separate and dis- 

 tinct crystalline forms of silica. 



It may be pointed out as a curious fact, whose significance 

 is not known, that with rising temperature each form passes 

 successively into one possessing a higher grade of crystallo- 

 graphic symmetry. Chalcedony, about which our knowledge 

 is deficient, must, of course, be excluded from this general 

 statement. 



Geological Application 



A brief examination may be made into the geological signifi- 

 cance of the results which have been attained. Since the exis- 

 tence of tridymite and cristobalite became known the problem 

 of the conditions under which they have been formed has been 

 rather a baffling one. It is believed that from the results of 

 this investigation answers may be given to some of the questions 

 involved. 



Ordinarily cristobalite forms crystalline deposits in small 

 cavities in igneous rocks. These rocks bear evidence of having 

 been subjected to corrosive agencies. The process of deposition 

 may have involved a reaction between some volatile compound 

 of silicon, such as the chloride or fluoride, with the vapor of 

 water, or the decomposition of silicates or alumino-silicates by 

 halogen acids, with the removal of the volatile halide and re- 

 crystallization of the silica. The difficulty which arises is to 

 explain the deposition of cristobalite, whose field of stability lies 

 above 1470°, in cavities in rocks which would certainly be in a 



