364 P. Eskola — Silicates of Strontium and Barium. 



ture. This conclusion, however, is not very dependable, 

 as our experience is that strontium replacing calcium 

 changes the optical properties of crystals very little. 

 More weighty evidence proving positively that the 

 amount of the strontium compound entering into solid 

 solution in the diopside must be very small, is that the 

 product consisted of more than one phase, the other 

 phases besides the diopside being present as a fine 

 crystalline mass. 



The preparation of the composition BaO.Mg0.2Si0 2 , 

 on cooling from a melt heated not above 1370°, also gave 

 several phases, and no double compound analogous to 

 diopside was formed. 



This negative result is very remarkable, especially in 

 the case of strontium, which in other ways showed such 

 close similarity with calcium. 



The Strontium and Barium Feldspars. 



Former investigations. — From melts of the corre- 

 sponding oxides Fouque and Michel-Levy 22 believed they 

 had prepared strontium, barium, and lead analogs of 

 anorthite, oligoclase and labradorite. 



Their different "feldspars" had the following specific 

 gravities : 



Sr Ba Pb 



Oligoclase 1.619 2.906 3.196 



Labradorite 2.862 3.333 3.609 



Anorthite 3.043 3.573 4.093 



The products were not well crystallized and their crystal 

 system could not be determined with certainty. The 

 barium feldspar, BaAl 2 Si 2 8 , always formed microlites 

 showing parallel extinction with negative elongation, and 

 the crystals, being usually rectangular in cross section, 

 seemed to be orthorhombic. Later investigation by E. 

 Dittler, 23 on artificial barium feldspar, confirmed this 

 result and also established it to be biaxial. Ginsberg, 24 



B2 F. Fouque et A. Michel Levy : Sur la production artificielle de f eldspaths 

 a base de baryte, de strontia et de plomb, correspondant a 1 'oligoclase, 

 a labradore et a 1 'anorthite; etudes des proprietes optiques des ces miner - 

 aux, Bull. soc. min. France 3, 124. 1880. 



23 E. Dittler, Tscherm. Min. Petr. Mitt,, 30, 122, 1911. 



24 A. S. Ginsberg, Ann. de 1 'Inst. Polyteeh. Pierre le Grand a, Petrograde, 

 1915, XXIII. 



