472 fenner: forms of silica 



frequently does not pass directly into the form which is most 

 stable under those conditions, but first into some less unstable 

 condition, and only after a lapse of time into the stable form. 

 Because of the sluggishness of transformation the writer has 

 not found it possible, without employing a catalytic agent, to 

 convert quartz into tridymite at any temperature, but at temper- 

 atures in the neighborhood of 1400°C. quartz gradually changes 

 into cristobalite. 1400° is still well below the temperature at 

 which cristobalite is stable, but in the passage from quartz to 

 tridymite ciistobalite is reached and there the process halts. 

 It is only by the addition of a small quantity of some catalytic 

 agent, such as tungstate of soda, that the reaction proceeds to 

 completion. A parallel case is afforded in the transformation 

 of silica glass into quartz at a temperature of 850? In the 

 course of a few hours it becomes converted into tridymite, but 

 this is only an intermediate stage, for after a much longer time 

 at the same temperature crystals of quartz make their appear- 

 ance, and this is in reality the stable form at that temperature. 

 It was only after much experimenting that such facts were fully 

 brought out and their significance appreciated. 



Inversion of quartz to tridymite 



When finely ground quartz is heated with tungstate of soda 

 it remains unchanged until a temperature of about 870° is reached. 

 At temperatures above 870° it is gradually converted into tri- 

 dymite. Near the inversion temperature the reaction proceeds 

 very sluggishly so that it is only after the lapse of several days 

 that identifiable tridymite appears. 



At higher temperatures the change proceeds more rapidly. 

 In several experiments conducted at about 1300° for 24 hours 

 the surface of the cake in the crucible showed comparatively 

 large crystals, easily visible to the naked eye. 



As tridymite, in passing from the temperature of formation 

 to ordinary temperatures, goes through a second inversion point 

 the crystals lose hexagonal symmetry as regards optical properties 

 but retain the outward form. The hexagonal plates therefore 

 are not isotropic but are divided into faintly birefringent fields. 



