THE MINERAWGRAPHY OF THE FELDSPARS 283 



It is sincerely hoped that the writer has already made clear that 

 the feldspars are solid solutions and mixtures of solid solutions of 

 three or more end members or " minals " as it was suggested that they 

 be called.^ It follows that the physical properties of the feldspars 

 are continuously varying in proportion to the change in chemical 

 composition. To make use of these properties for identification, 

 since they are multiple component systems, they should be plotted 

 as surfaces and not as curves. 



Here, in Part II, the subject is carried further and additional 

 aspects which were omitted before, because of the lack of space, are 

 presented and discussed. 



As our knowledge of the feldspar group, outside of crystallog- 

 raphy, is to a very large degree confined to the plagioclase series, 

 about one-half of the present paper is devoted to the potash-soda 

 series. This system includes orthoclase, microcline, albite, "bar- 

 bierite," anorthoclase, perthite, etc. It is not a little surprising 

 to find that there is much uncertainty regarding the relationships 

 between the different members of the system, as well as the corre- 

 spondence between chemical composition and physical properties. 

 Because the physical properties, including optical behavior, are a 

 means of determining by the microscope the actual composition, 

 this matter is presented in detail, chiefly by the aid of diagrams. 

 But before the relation of physical properties to the composition 

 can be thoroughly appreciated, it is necessary to understand the 

 thermal-diagrams of the potash-soda series. There are a number 

 of divergent opinions regarding the thermal-diagram, hence these 

 ideas will first receive attention. 



The potash-lime, barium, and strontium-bearing feldspars are 

 treated next. Other topics are twinning, and aventurine feldspars. 



THE POTASH SODA SERIES 

 THE DIAGRAM OF VOGT AND WARREN 



The thermal diagram of the perthite feldspars in Part I (Fig. 4, 

 p. 221) is only an approximation at the best. It is Warren's^ 



^Ibid., p. 218. 



= C. H. Warren, "A Quantitative Study of Certain Perthitic Feldspars," Proc. 

 Amer. Acad. Arts and Sci., LI (No. 3, 1915), 148. 



