schaller: immense bloedite crystals 75 



than the latter and usually the ratio of film thicknesses is greater 

 than 3:1. The suggested explanation is that replacement con- 

 tinues only so long as space can be found for the necessary expan- 

 sion, so that where pyrite is the core mineral for chalcocite 

 deposition reaction would be inhibited much sooner than where 

 chalcopyrite is the nucleus. 



Essentially the same volume relations appear in using the 

 simplest possible expressions for complete reaction between cupric 

 sulfate and pyrite or chalcopyrite when the product is covellite — 

 equations (15) and (16). It seems safe to predict, therefore, 

 that chalcopyrite will be shown to be a more favorable nucleus 

 than pyrite for receiving secondary covellite. 



The volume relations required by equation (14) show no essen- 

 tial change, if sphalerite is transformed into chalcocite — ^that is 

 the ratio is nearly 1 : 1, a slight expansion being found by calcu- 

 lation. On the other hand, when sphalerite is converted to 

 covellite, if the reaction goes on molecularly according to the 

 simplest equation which may be written, there is a diminution 

 of volume in the ratio 12 : 11. Other things being equal, it would 

 seem that volume relations favor the deposition of covellite rather 

 than chalcocite where sphalerite comes into contact with copper 

 sulfate solution. 



Altho the equations which have been given as a basis for the 

 foregoing discussions are speculative, it is hoped that the sys- 

 tematic arrangement here presented may hasten experimental 

 work which is needed before the chalcocite problem can be 

 solved. 



MINERALOGY. — Immense bloedite crystals. Preliminary note. 

 Waldemar T. Schaller, Geological Survey. 



A recent find of bloedite by Mr. Hoyt S. Gale of the Geological 

 Survey, is remarkable for the immense size of the crystals. An 

 extensive deposit of soda forms a crust on Soda Lake, in Carriso 

 Plain, San Luis Obispo County, California. This deposit has 

 been described by Arnold and Johnson^ who give a detailed anal- 

 ysis of the saline crust which shows it to be a nearly pure sodium 



1 Bull. 380, U. S. Geological Survey, p. 369. 1908. 



