474 fenner: forms of silica 



balite, even at temperatures below 1470? From this inter- 

 mediate stage the transformation to tridymite can be completed 

 only with the assistance of a flux. Below 870°, even with tung- 

 state of soda present, the glass is transformed first to tridymite 

 and only very slowly to quartz. Precipitated silica behaves in 

 the same manner as silica glass and is probably to be considered 

 as strictly amorphous and analogous to the fused material. 



The mutual relations of the three crystalline minerals and the 

 amorphous forms have been repeatedly confirmed. Quartz has 

 been converted into tridymite and into cristobalite; tridymite 

 into quartz and into cristobalite; and cristobalite into quartz 

 and into tridymite. Moreover in numerous cases where, in 

 conformity with Ostwald's principle, intermediate stages have 

 been reached, it has been found without exception that by longer 

 heating or by the use of a catalytic agent the appropriate stable 

 form can be attained. 



Preparation of quartz in aqueous solutions 



Quartz may be prepared with facility by heating either silica 

 glass or amorphous, precipitated silica in a steel bomb with water 

 and sodic carbonate at 400° to 500° for two or three days. 



In a slightly varying form of experiment artificial tridymite 

 was employed and quartz crystals were obtained as usual. Arti- 

 ficial cristobalite likewise recrystallized as quartz. In no case 

 has the presence of tridymite or cristobalite in preparations 

 made in aqueous solution been detected, but, on the contrary, 

 when the higher forms of silica have been used they have recrys- 

 tallized as quartz. 



Other inversions 



The inversions of silica thus far described are characterized 

 by a complete change in the outer form of the mineral, and can 

 only be produced slowly and with difficulty. There is, however, 

 a second class of inversions which take place almost instantane- 

 ously. The outer form of the mineral is retained, but the inter- 

 nal structure must necessarily be modified. These are the trans- 

 formations of a- into /3-quartz, of a- into /3-tridymite, and of 



