536 AUSTIN F. ROGERS 



of i . 557 and a water content of 15.5 per cent, and hence should be 

 referrred to halloysite. 



Helmhacker 1 describes halloysite with a small botryoidal sur- 

 face from Banat, Hungary. The Rochlitz myeline also shows 

 colloform crusts in cavities. 



From the available evidence it seems clear that halloysite is 

 the amorphous equivalent of kaolinite. The recognition of this 

 may help in a clearer understanding of the constitution of clays. 

 The determination of the index refraction, which varies from 1 . 538 

 to 1.557, i s probably a safer means of distinguishing halloysite 

 from kaolinite than the absence of double refraction. The pres- 

 ence of aluminum hydrate in solid solution may, however, modify 

 the index of refraction. 



Allophane. Al 2 SiO s (H 2 0) a; (carolathine) . — Allophane has long 

 been recognized as one of the typical amorphous minerals. Its 

 colloform shape, optical isotropism, and absence of cleavage 

 and structure leave no doubt as to its amorphous character. 

 The index of refraction of a specimen from Bedford, Indiana, is 

 1.473=*= 0.003. 



Whether schrotterite and collyrite should be considered syno- 

 nyms of allophane is open to question. 



Stevensite. H 2 Mg 3 (Si0 3 ) 4 *(H 2 0) a ; (talc in part, lucianite?). — - 

 An alteration product of pectolite found at several localities in 

 New Jersey is probably the amorphous equivalent of crystalline 

 talc. This substance was first described by Leeds 2 who used 

 stevensite as a name for talc pseudomorphous after pectolite. 

 Glenn 3 has recently studied this mineral, and his work proves that 

 it should be considered a distinct mineral. Wherry made a 

 microscopic examination and shows that it is isotropic with an 

 index of refraction of about 1 . 50. Stevensite is also distinguished 

 from talc by its lower specific gravity and higher water content. 



Stevensite is probably not a monohydrate of talc, as Glenn 

 suggests, but the water content is evidently variable and reaches 

 over 19 per cent in the clay mentioned in the next paragraph. 



1 Min. u. pelr. Mitth., II (1879), 232. 



2 Am. Jour. Sci. (3), VI (1873), 22-23. 



3 American Mineralogist, I (1916), 44-46. 



