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Stable below 575°. 



Stable between 575° and 

 870°. Inverts on cool- 

 ing to a-quartz. 



Exists below 117°. Usu- 

 ally finely intergrown 

 aggregates. 



Exists only above 117°. 

 Inverts on cooling to 

 a-tridymite. 



Exists below 140°. 



Exists only above 140°. 

 Inverts on cooling to 

 a-tridymite. 



Resembles natural min- 

 eral in every respect. 



Polysynthetic twinning 

 not uncommon. Ex- 

 tinction angles small. 





Obtainable only as fine 

 powder. Plane of optic 

 axes normal to cleav- 

 age direction. 





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