54 HILLEBRAND, MERWIN AND WRIGHT. [April 25. 



The first and second minerals are regarded as hydrated acid 

 hexa vanadates — CaHaVgOi^-SHgO — the third as a normal hexa- 

 vanadate, CaaVgOi^-ii PHgO. 



The reasons for specific separation of hewettite and metahewet- 

 tite are set forth in detail. The two minerals are so sensitive to 

 changes in atmospheric humidity that their water content varies 

 within wide limits at different times of the year. The removal of all 

 or nearly all the water does not result in breaking down of the 

 crystal structure, and until this has occurred the water is wholly or 

 in great part taken up again when opportunity is offered. 



The importance is emphasized of bringing all minerals that be- 

 have in this way to a definite maximum water content before analyz- 

 ing them and of following carefully the course of dehydration under 

 prescribed conditions. Detailed directions are given for such tests 

 and for avoiding several sources of error. 



Attention is also called to two fairly constant associates of meta- 

 hewettite. One of these (also a constituent of carnotite ores) is a 

 gray hydrous silicate of aluminum, trivalent vanadium, and potas- 

 sium. The other is elemental selenium, the existence of which as a 

 mineral species seems now for the first time established. 



Acknowledgments. 



Our thanks are due primarily to Mr. D. Foster Hewett for the 



material from Peru and to Messrs. T. V. F. Curran, R. H. McMillen 



and A. G. McNaughton, also to Mr. Frank L. Hess and Dr. W. T. 



Schaller of the U. S. Geological Survey, for North American 



material and for valuable information. Dr. Wm. Blum and Mr. E. 



C. McKelvy, of the Bureau of Standards, were of material assistance 



in arranging the thermostat and electric furnace used for the water 



determinations. 



Bureau of Standards and Geophysical 



Laboratory of the Carnegie Institution, 

 Washington, D. C., April, 1914. 



