THE MINERALOGRAPHY OF THE FELDSPARS 



367 



mine the percentage of the Hme and potash members with anywhere 

 the same accuracy" as the soda member, "The most satisfactory 

 means .... is to consider that the value of the extinction angles 

 gives the percentage of the soda component only." From this and 

 the average amount of potash feldspar usually found in plagioclase 



Fig. 7. — Diagram suggesting that porphyritic feldspars are able to possess the 

 maximum amount of zonal structure while feldspars in obsidians, perlites, and plutonics 

 exhibit little or no zonal habit. This diagram is to be considered in a qualitative and 

 not in a quantitative manner. 



the other two components can approximately be ascertained. The 

 word "soda" (in italics) is in error. The truth is that extinction 

 angles give the lime component more accurately and by assuming 

 4 to 6 per cent potash feldspar, the amount of the soda component 

 is found by subtracting the sum of these from one hundred. 



SUNSTONE AND AVENTURINE FELDSPARS 



Viola^ and Andersen^ have investigated the aventurine feldspars 

 and the latter reached the conclusion that the included plates of 

 definitely oriented hematite (or magnetite, ilmenite, rutile, etc.) are 

 due to exsolution.^ This suggests that the original melt could hold 

 a compound of iron in solution while molten but the decrease in solu- 

 bility of the feldspar for the iron compound with falling tempera- 

 ture caused its separation. Furthermore potassium feldspars are 

 often pink while albite is more frequently white.'* This may mean 



' C. Viola, Zeitschr. f. Kryst., XXXIV (1901), 171. 



= 01af Andersen, Amer. Jour. Sci. (4), XXXIX (1915), 379. 



3 See Alfred Harker, op. cit., p. 257; Judd, Quar. Jour. Geol. Soc, XLI (1885), 

 374-89; J. H. L. Vogt, ibid., LXV (1909). 



■t " Flesh-colored albite occurs as well as flesh-colored orthoclase." — Iddings, Igneous 

 Rocks, Vol. II, p. 46. 



