366 P. Eskola — Silicates of Strontium and Barium. 



the ratio of the percentages of BaO and SrO corresponds, 

 to a considerable degree, with the ratio of the percentages 

 of K 2 and CaO, so that it may be presumed that strontia 

 is present in the plagioclase and baryta in the potash 

 feldspar. A chemical investigation of the feldspars 

 from the rocks in question might prove this to be true. 



In the present study it was not intended to make any 

 exhaustive experimental investigation of the strontium 

 and barium feldspars, but a few experiments were made 

 to elucidate their behavior. 



As both the compounds, SrO.Al 2 3 .2Si0 2 and BaO. 

 Al 2 3 .2Si0 2 , were found to have very high melting 

 points (far above 1700°) and to crystallize but poorly 

 when the pure mixtures of the respective oxides were 

 heated up, I applied the vanadate flux method to produce 

 better crystals. In both cases the feldspar mixtures 

 were powdered with about 1/3 of their masses of 

 Sr(V0 3 ) 2 and Ba(V0 3 ) 2 , respectively, and kept over 

 night at about 1400°. The vanadates were then washed 

 out with very dilute cold HC1. 



Artificial barium feldspar, BaO.Al 2 3 .2Si0 2 . — The 

 barium feldspar thus obtained formed minute crystals, 

 not more than 0.05 mm in length. They were elongated 

 parallel to a and tabular parallel to ay, so that most 

 crystals placed themselves on that plane and, in conver- 

 gent light, showed the trace of the optical normal. The 

 terminal faces show an apparent bilateral symmetry, like 

 orthorhombic domes and, as the extinction is almost 

 always parallel so far as can be determined, the crystals 

 give entirely the appearance of orthorhombic symmetry, 

 in agreement with the earlier results of Fouque and 

 Michel-Levy, and of Dittler. I found, however, that this 

 is only apparent and due to the fact that most of the 

 crystals are Carlsbad contact twins, so that light has to 

 pass through the two individuals with opposite extinction 

 angles, the individuals being tabular parallel to 010, 

 which is also the composition plane. The other forms 

 are (110), which is the zone of elongation, and (001) and 

 (101). In simple crystals, or in outstanding simple 

 parts of the twins, there may be observed extinction 

 angles of two or three degrees, and the optical orientation 

 is thus: || b; a A c = 3°. Basal cleavage is not very 

 well developed, though its trace may be seen occasionally. 

 All these characters agree with those found in natural 



