130 Ford — Effect of the Presence of Alkalies in Beryl. 



values of tlie index for the extraordinary ray and the birefrin- 

 gence, as determined from the prism, while not exactly correct, 

 mnst be very nearly so and conse(]nently thev have l)een 

 qnoted, tlie fignres being enclosed in each case in a parenthesis. 



Above are given the various analyses with the corresponding 

 determinations of the indices of refraction and specific gravity. 

 The indices of refraction were all measured in sodium light. 



From the consideration of the above table it is (juite clear 

 that the replacement of beryllium oxide in beryl by the various 

 alkali oxides raises both the specific gravity and the values for 

 the indices of refraction in the mineral. There is also shown 

 an increase in the amount of birefringence as the percentage 

 of the alkali oxides rises. The relationship that exists in 

 minerals between their cotnposition and their optical constapts 

 is at best exceedingly complicated. And in the case of beryl 

 where one oxide (BeO) is replaced in varying amounts by four 

 other oxides (Li„0, Na,0, K^O, and Cs„0), possessing widely 

 differing molecular weights, it is impossible in our present state 

 of knowledge to derive any law that shall accurately coordinate 

 the chemical and optical factors. It is, however, a recognized 

 fact that, in general, where a mineral varies in its composition 

 by the isomorphous replacement of one element or oxide by 

 another, the values of the indices of refraction and the 

 specific gravity are raised, when the replacing element or oxide 

 has a higher atomic or molecular weight than the one replaced 

 and vice versa. The results of the present investigation of 

 beryl are fully in accord with this principle. The effect of 

 such variation in composition upon the birefringence of the 

 mineral is, however, apparently not necessarily a corresponding 

 one, although, in the case of beryl, it also rises in amount with 

 increase in the values of the indices of refraction. 



An interesting fact might be noticed in this connection. 

 Ordinarily because of their comparative low molecular weights 

 (excepting in the case of the rare oxide of caesium) the intro- 

 duction of alkali oxides into a mineral serves to lower its 

 indices of refraction and birefringence. The opposite is true 

 in bei-yl and apparently so because the oxide replaced, BeO, 

 has a lower molecular weight than any of the alkali oxides, the 

 respective values being as follows: BeO, 25"1 ; Li„0, 30'06 ; 

 Na.O, 62-10; K,0, 94-30; Cs,0, 281-76. It could, 'therefore, 

 be reasonably assumed that the introduction of any of the 

 alkalies would cause the rise of the values for the refractive 

 indices, but it \\\ probable, however, that the presence of 

 caesium oxide with its high molecular weight would have the 

 predominating influence. 



Laboratory of Mineralogy, 



Sheffield Scientific School, Yale University, 



New Haven, Conn., June, 1910. 



