226 Rogers — The Occurrence of Cristobalite in California. 



the kind. One usually thinks of the chemical composition 

 and crystal form taken together as being sufficient to 

 determine any mineral, but on account of the possibility 

 of paramorphism it is necessary to determine some physi- 

 cal property in addition. Of the various physical proper- 

 ties the index of refraction is in general (except for 

 opaque minerals of course) the most useful one in deter- 

 minative work. 



Fragments of the cristobalite have weak birefringence 

 and mottled interference colors. Some of the fragments, 

 however, are minute crystals of hexagonal tabular form 

 which remain dark between crossed nicols and suggest 

 that they may be associated tridymite, but repeated trials 

 show that the index of refraction is always greater than 

 1480. These hexagonal crystals are perfectly clear and 

 colorless without any hint of the mosaic effect and 

 are probably single uniaxial (tetragonal!) crystals of 

 a-cristobalite which have retained the hexagonal form of 

 the original tridymite from which they were formed. 

 This probably involves only a slight change in the space- 

 lattice. 



The conversion of the tridymite into cristobalite was 

 probably brought about by hot gases or mineralizers. 

 According to Le Chatelier 8 rapid crystallization is re- 

 sponsible for the formation of cristobalite. As evidence 

 of post-magmatic action the opaque zones in the andesine 

 crystals, the fibrous alteration product of the augite and 

 the opaque red hematite may be recalled. 



Associated with the cristobalite aggregates in the cavi- 

 ties are very thin tabular six-sided crystals which 

 prove to be kaolinite. They show several features out 

 of the ordinary and will be described at another time. 

 The kaolinite is later than the cristobalite (or original 

 tridvmite, at least) but the data are not sufficient to prove 

 whether it is a hypog'ene or supergene mineral. Long 

 acicular crystals of unknown identity occur in the cavities 

 with the kaolinite. 



Summary. 



Cristobalite is described from two localities in Cali- 

 fornia. Near Tuscan Springs in Tehama County it 

 occurs as distinct octahedral crystals in an auganite. At 

 Jamestown in Tuolumne County it occurs in an augite 

 andesite and proves to be paramorphous after tridymite. 



Dept. of Geology, Stanford University, California, Nov. 3917. 

 "Bull. Soc. Min., 40, 56, 1917. 



