fenner: forms of silica 473 



These show a biaxial, positive figure, with the acute bisectrix 

 normal to the plate. Three determinations of the value of the 

 optic axial angle gave for 2V an average of 35f ? - The index 

 of refraction, as determined by the immersion method, is 1.475. 

 When tridymite, prepared at a high temperature as described, 

 is in turn heated with a little tungstate of soda at temperatures 

 below 870°, the transformation is reversed and quartz is formed. 

 The relation of the two is therefore enantiotropic. 



Inversion of tridymite to cristobalite 



Tridymite is stable at all temperatures from 870° ± 10° up 

 to 1470° ± 10°, where the inversion to cristobalite takes place. 

 Like the inversion of quartz into tridymite that of tridymite 

 into cristobalite, even in the presence of tungstate of soda, is 

 very slow near the inversion point, but proceeds more rapidly 

 at higher temperatures. At 1570° the reaction is complete in 

 a few hours. At temperatures below 1470° cristobalite is recon- 

 verted into tridymite. The relations between tridymite and 

 cristobalite are therefore analogous to those between quartz and 

 tridymite. 



As cristobalite also passes thru a low-temperature inversion- 

 point in cooling to room temperatures the grains are not iso- 

 tropic, but posses a faint birefringence. 



The grains have been too small to determine the optical 

 character with certainty, but some of maximum birefringence 

 were found to give a fairly satisfactory interference figure, appar- 

 ently perpendicular to the optic normal. The acute bisectrix 

 was then found to be a and the mineral may provisionally be 

 considered negative. The index of refraction as determined by 

 the immersion method is close to 1.485. 



From 1470° no further change was found to take place up to 

 the melting point. The latter was considered by Day and 

 Shepherd to be about 1600? l No further information has been 

 obtained with regard to this. 



Silica glass, when devitrified without a flux, passes into cristo- 



l The Lime-Silica Series of Minerals, Am. Jour. Sci., 4th ser., 22: 265-302. Oc- 

 tober, 1906. 



