12S Ford — Kjfect of the Presence of Alkalies in Beryl. 



Art. IX. — The Efect of the Presence of Alkalies in Beryl 

 upon its Optical Properties ; by W. E. Ford. 



TiiK attention of the writer was called to the subject of the 

 effect of the presence of alkalies in beryl upon its optical prop- 

 erties by a note in an article by Lacroix* on the tonrinaline- 

 bearin^ pegmatite veins of Madagascar. A description is 

 given there of a rose-colored beryl which possessed unnsuallv 

 high values for its indices of refraction and speciiic gravity, 

 and which on qualitative examination showed the presence of 

 alkalies, notably csesium. The assumption naturally followed 

 that these abnormal values for the refractive indices and 

 specific gravity were due to the presence of the alkalies in ^he 

 mineral. This seemed a point of sufficient interest and import- 

 ance to merit further investigation. 



The role played by the alkalies in beryl was determined in 

 this laboratory, a number of years ago, through a series of 

 analyses by Fenfield,f and later by the same investigator and 

 Sperry.:}: Specimens of the beryls rich in alkalies upon which 

 these analyses were made are preserved in the Brush collec- 

 tion, and in two cases yielded material suitable for optical 

 investigation. It was possible to make an orientated prism and 

 to measure the indices of refraction of the beryl from Willi- 

 mantic, Conn., analyzed by Penfield,:}: and also of the caesium 

 beryl from Hebron, Maine, analyzed first by Fenfield,f and then 

 later by Wells. § In addition to these, two other beryls were 

 investigated, first the pink beryl from San Diego Co., Cali- 

 fornia, in which the w)-iter determined qualitatively some 

 years ago the presence of alkalies, |j and second, the rose beryl 

 from Madagascar, presumably the same as described by Lacroix. 

 Specimens of the Madagascar beryl of gem quality were 

 generously placed at the writer's disposal by Dr. G. F. Kunz. 



An analysis was first made of the pink beryl from Mesa 

 Grande, San Diego Co., California, the material being furnished 

 by an irregular fragment of gem quality which was presented 

 to the Brush collection by Mr. E. Schernikow. The analysis 

 showed a total of 1'48 per cent of alkali oxides, a smaller 

 amount than had been anticipated and, further, gave no test for 

 the presence of caesium. As the specimen analyzed showed no 

 crystal faces it was impossible to cut an orientated prism from 

 it by means of which the two indices of refraction could be 

 determined. A prism was, however, cut at random from a 

 fragment of the material and the value of the index for the 

 ordinary ray obtained. There was in the Brush collection a 

 small crystal group of faintly pink beryl frotn the neighboring 



* Bull. Soc. Min., xxxi, 218, 1908. f This Journal, xxviii, 25, 1884. 

 X Ibid, xxxvi, 317, 1888. i^ Dana's System of Mineralogy, 6th ed., p. 407. 

 I This Journal, xxii, 22, 217, 1906. 



