P. Eskola — Silicates of Strontium and Barium. 365 



however, states that crystals of BaAl 2 Si 2 O s obtained by 

 cooling of the pure melt were uniaxial, positive, and he 

 regards this form as presenting a nephelite analog. 



There is some evidence to the effect that some amount 

 of lime, in plagioclase feldspars, may be replaced by 

 baryta without changing the triclinic symmetry. Des- 

 cloizeaux 25 found this to be the case in a natural feldspar 

 from unknown locality, whose optical and crystallo- 

 graphic properties were similar to those of labradorite, 

 while its composition resembled that of oligoclase more 

 closely, with 7.30 BaO, 1.83 CaO, 7.45 Na 2 0, and 0.83 K 2 0. 

 Ginsberg 26 investigated the binary system CaAl 2 Si 2 8 - 

 BaAl 2 Si 2 8 in artificial products and found that the tri- 

 clinic anorthite may take up limited amounts of barium 

 feldspar in solid solution, whereby the optic axial angle 

 diminishes. The hexagonal BaAl 2 Si 2 8 , on the other 

 hand, may take up some lime feldspar. 27 



The relation of barium feldspar and orthoclase is far 

 better known. Penfield 2S described barium-bearing 

 orthoclase from Blue Hill, Delaware County, Pennsyl- 

 vania. Later Sjogren 29 and Strandmark 30 studied the 

 barium feldspar, or celsian, from Jakobsberg, Sweden, 

 and proved it to have the composition BaO.Al 2 3 .2Si0 2 . 

 The latter establishes its monoclinic symmetry and 

 crystallographic similarity to adularia. The crystals are 

 usuallv elongated parallel too. The angle a A o = 3°1' 

 in the acute angle p. <* = 1.5835 ; p = 1.5886 ; y = 1.5941. 

 Sp. g. 3.37. Strandmark also proved that the hyalo- 

 phanes are isomorphous mixtures of celsian and ortho- 

 clase. 



Rock analyses including determinations of BaO and 

 SrO commonly show small amounts of both of these 

 oxides. As Washington 31 has pointed out, the highest 

 amount of both of these oxides have been found in certain 

 highly potassic rocks of Wyoming, showing up to 1.10 

 per cent BaO and 0.30 per cent SrO. A study of the 

 analyses published in Washington's Tables 32 shows that 



23 A. Descloizeaux, Tscherm. Min. Petr. Mitt., 7, 99, 1877. 



26 A. S. Ginsberg, loc. cit. 



27 The thermal work of Ginsberg was done by the cooling curve method 

 and is subieet to criticism (cf. p. 332). 



28 S. L. Penfield, this Journal, 36, 326, 1888. 



29 Hj. Sjogren, Geol. Foren., Forh., 17, 578, 1895. 



30 J. F. Strandmark, Geol. Foren. Forh. 25, 289, 1903 ; 26, 97, 1903. 



31 H. S. Washington, J. Franklin Inst., 190, 767, 1920. 



32 H. S. Washington, U. S. Geol. Surv., Prof. Paper 99, 1917. 



