THE MINERALOGRAPHY OF THE FELDSPARS 369 



who recorded its physical properties. This ferric feldspar is, how- 

 ever, unstable and it is a very reasonable supposition that much of 

 the hematite in aventurine feldspars is derived from the decomposi- 

 tion of KFeSijOs. As this is a potash feldspar it is perhaps to be 

 expected that it would occur in orthoclasic or microclinic feldspars. 

 The gem orthoclase from Madagascar is perhaps unique, as Wherry 

 notes. The writer's suggestion is that perhaps it contains some 

 undecomposed KFeSi308. The following analyses of this feldspar 

 from Madagascar show, when recast, the probable presence of this 

 potash-ferric component. 



The question arises : Has the compound KFeSijOs any bearing on 

 the color of amazonstones ? 



While the work of the Geophysical Laboratory on the MgO— 

 AlaOj-SiOa system shows no compound MgAlaSiaOs, the writer asks 

 whether or not the existence of a magnesium feldspar, stable only 

 in small quantities in the presence of other feldspars, would not 

 explain the occurrence of MgO in many of the analyses ? 



BARIUM FELDSPARS 



In view of Eskola's' recent paper it is not necessary that a com- 

 plete discussion of the synthesis of strontium and barium feldspars 

 be given here. It is sufficient that a summary of the present status 

 of these minerals and their relationships to the more common feld- 

 spars be recorded. 



Descloizeaux^ found that some of the lime in plagioclase can be 

 replaced by barium without changing its triclinic character. The 

 work of Ginsberg^ on the CaAlaSizOg-BaAlaSiaOs system indicates 

 that anorthite will dissolve a limited amount of the barium feldspar 

 forming a solid solution, and likewise barium feldspar will form a 

 restricted range of solid solutions with anorthite. On this basis we 

 can conclude that the system is a eutectiferous one with limited 

 solubility. 



' Pentti Eskola, "Silicates of Strontium and Barium," Amer. Jour. Sci. (5), IV 

 (1922), 331-75, especially pp. 364-67. 



^ A. Descloizeaux, Tscherm. Min. Pelro. Milt., VII (1877), 99. 



3 A. S. Ginsberg, Ann. de Vlnst. Polytech. Pierre le Gi-and a Petrograde, XXIII 

 (1915)- 



