210 LARSEN AND WHERRY: LEVERRIERITE FROM COLORADO 



A similar change in the apparent indices of refraction, as 

 measured by the immersion method, has been noticed in a num- 

 ber of other minerals. In some it is clearly due to the fact that 

 the mineral has submicroscopic pores and that the apparent index 

 of refraction is the result of the combined effects of the mineral 

 and gas or water filling the pores; but if the pores are filled with 

 a liquid having the same index of refraction as the mineral, the 

 true index of the mineral can be measured. 2 For instance, the 

 halloysite ("isotropic kaolinite") 3 from Wagon-Wheel Gap, 

 Colorado, is clouded when first immersed, and the liquid can 

 be seen to penetrate the grains, making them clear and increasing 

 their index of refraction. The pores in the leverrierite are sub- 

 microscopic and the mineral appears clear, but the penetration 

 of the liquid can be followed by the change in index of refraction 

 in a zone from the border inwards. 



Chemical properties. Qualitative examination having shown 

 the material to be a hydrous aluminium silicate, it was analyzed 

 quantitatively in the laboratory of the National Museum. The 

 behavior of the water proved to be rather peculiar, so its relation 

 to the optical properties of the mineral was studied first. 



Behavior of the water. It is customary in mineral analysis 

 nowadays to determine water of two kinds. That given off by 

 heating to 105° or 110° is recorded as H 2 — , and that given off 

 above one of these temperatures is recorded as H 2 +. This repre- 

 sents a decided advance over the older plans of determining only 

 total water, or of drying the sample at 110° before weighing it out 

 for analysis, and for many purposes may be adequate. But our 

 knowledge of hydrous colloidal minerals, and even of many 

 containing water of crystallization, will never be complete unless 

 the actual temperature ranges over which the water is liberated 

 both above and below 110° are recorded. While the boiling 

 point of pure liquid water under a pressure of one atmosphere is 

 100°, the water held by a solid, even though not chemically 

 combined in the strictest sense, may vaporize at a far lower 



2 Surface tension and similar phenomena may modify this. 



3 Larsen, E. S., and Wells, R. C. Some minerals from the fluorite barite vein 

 near Wagon-Wheel Gap, Colorado. Proc. Nat. Acad. Sci., 2: 364. 1916. 



