356 HAROLD L. ALLING 



THE ZONING OF FELDSPARS 



It was Hoepfner^ who first called attention to zonal structures in 

 plagioclase. Rosenbusch^ early explained it as due to periodic 

 interruptions during crystallization and to selective decomposition. 

 Michel-Levy^ considered zonals as "the result of a submicroscopic 

 twin lamination after the albite and pericline laws. " Later Rosen- 

 busch showed that in many cases the kernel of a zonal is more basic 

 than that of the shells. He explained this on the basis that there 

 exists an isomorphous lamination in which an original, basic, central 

 crystal is surrounded by shells of other plagioclase which gradually 

 become more and more acid. The great majority of zonals can be 

 fully understood by following the crystallization of a melt by means 

 of a thermal diagram. The phenomenon is not confined to feldspars 

 nor to isomorphous minerals; it is also found in many alloys.'' 



In deep-seated rocks zonals are not common, nor do they occur 

 in glassy rocks. Yet in certain porphyritic rocks zonal feldspars are 

 frequently found. This simple observation has an important genetic 

 significance. In Part I, page 215, the statement is made that the 

 degree of homogeneity is a function of the rate of chill. Bowen^ 

 says: "There is a certain definite rate of cooling which gives maxi- 

 mal zoning With a somewhat quicker rate of cooling, the 



range of zoning is not so great owing to a moderate degree of under- 

 cooling, and when the undercooling is very great there is no zoning 

 at all." This relation is graphically expressed by Figure 7 which, of 

 course, is to be interpreted qualitatively and not quantitatively. 

 We can make use of this observation in arriving at the conditions 

 under which rocks of this kind solidified. 



' C. Hoepfner, "tjber das Gestein des Mte. Tajumbia in Peru," N. Jahrb.f. min. 

 u. geol., II (1881), 164-92; J. Blumrich, Tscher. Min. u. Petro. Mitt., XIII (1892), 

 239, 258; A. Pelikan, ibid., XVI (1896), i. 



^ Rosenbusch-Iddings, Microscopical Physiography of the Rock Making Minerals, 

 pp. 307, 326. 



3 A. Michel-Levy, Camp. Rendu, XCIV (1882), 93, 178. 



t Edgar Bain, "Cored Crystals and Metallic Compounds," Chem.and Met.Eng., 

 Vol. XXVIII, No. 2 (January 10, 1923), 65-69; Walter Rosenhain, ibid., No. 10 

 (March 7, 1923), 442-46; Trans. Amer. Inst. Min. and Met. Eng., June, 1923. 



5 N. L. Bowen, Jour. Geol. SuppL, XXIII (No. 8, 1915), 33. 



