476 fenner: forms of silica 



verting it into cristobalite, was certainly a factor affecting the 

 inversion point of the resultant product, but it could not be 

 considered that the whole effect was due to this cause. Such 

 wide variations in the inversion point of a mineral appear very 

 remarkable. A certain analogy is suggested to the behavior 

 of antimony upon freezing. 2 



Relation of chalcedony to other forms of silica 



The relation of chalcedony to quartz has been the subject of 

 considerable discussion. As optical methods appear to have 

 been pursued as far as possible without obtaining conclusive 

 evidence, the writer has endeavored to attain a solution by 

 working along other lines, and certain results of significance 

 have been obtained. As quartz shows a sharp break at 575°, 

 chalcedony should, if it is identical with quartz, show the same 

 break. The region on both sides of 575° has been carefully ex- 

 plored by means of heating-curves, but so far chalcedony has 

 failed to give any trace of a break. 



A second class of experiments has had for their object the 

 determination of the form into which chalcedony would invert 

 when heated with sodic tungstate to temperatures near to but 

 lower than 870? A number of experiments have resulted in 

 the formation of tridymite and of new quartz crystals. 



On the supposition that chalcedony is the same mineralogi- 

 cally as quartz it is difficult to interpret these results, but on the 

 supposition that it represents a different phase the results are 

 perfectly consistent. The evidence therefore points to the exis- 

 tence of still another form of silica in chalcedony. 



Recapitulation of inversions 



The various inversions of silica may be briefly recapitulated. 



At some unknown, but probably rather low temperature 

 chalcedony passes into a-quartz (tetartohedral hexagonal), (the 

 relation between the two may be monotropic), 



575° a-quartz <=± /3-quartz (hemihedral hexagonal), 



2 A. L. Day and R. B. Sosmari, Am. Jour. Sci., 4th ser., 29: 126. February, 

 1910. 



