THE MINERALOGRAPHY OF THE FELDSPARS 359 



Using the data and the graphic methods of Professors Nitikin and Koloulsky 

 , . . . , in spite of the fine precision in their work, the points of the curves 

 of the representative feldspars were not exactly upon the published curves of 

 either Michel-Levy^ or those of Nikitin. The deviation is to be explained by 

 the presence, in the plagioclase series, of the term KAlSijOg, triclinic, frequently 

 entering [into the composition] in notable quantities. The published data of 

 the feldspars, for example that of the extinction [angles] for a determined face, 

 should not be distributed upon a curve, but be spread more or less over an area, 

 according to the proportion of the potash [component] present. 



Fedorow^ said that " Anotlier question wliicli the law of Tschermalc 

 is not competent to explain^ is the presence of potassium often in 

 notable proportions in the majority of plagioclases."'' 



J. Schetelig,^ in spealiing of the discrepancy between the optical 

 and chemical determination of the composition of an albite says: 



The difference is due either to small errors in the exact orientation of the 

 sections and in the determination of the angles of extinction, or to the hitherto 

 not exactly known influence of the relatively large amount of the Or-component on 

 the optical properties of the plagioclases. [The writer's italics.] The tables of 

 F. Becke for computation of the composition of the plagioclase are based upon 

 careful investigation of pure mixtures of the Ab- and An-components, but the 

 influence of the Or-component is not taken into consideration. 



If there should be a series of thermal diagrams instead of a single 

 binary one to show the crystallization, of the potash-soda series so 

 that all potash-soda feldspars, no matter of what origin, could be 

 fully understood, then there should be a group of three dimensional 

 rnodels to show all possible feldspars consisting of three components. 

 While the phenomenon of undercooling is especially noticeable in the 

 case of the potash-soda series, we should not forget that the other 



' E. S. Larsen, in a review of Johannsen's recent Essentials for the Microscopical 

 Determination of Rock-forming Minerals (Eng. and Min. Journal-Press, Vol. 114, 

 No. 18 [October 28, 1922], p. 775), says: " It is regrettable .... that for the Michel- 

 Levy method the old diagram has been used. Determinations made by this diagram 

 are commonly in error as much or even greater than 10 per cent, whereas by using the 

 diagram published by Wright the determinations check with those made by other 

 methods." 



^ E. von Fedoroff (Fedorov, Fedorow), " Universalmethode and Feldspathstudien," 

 Zeitschr.f. Kryst. Min., XXIX, 604. 



3 This criticism is too severe. Tschermak would consider the potash feldspar to be 

 partially isomorphous with soda and lime components. 



'•As cited by Elvira Carrasco, "Contribution a I'etude des macles de feldspaths 

 au moyen de la methode de Fedoroff," Bull. Sac. Vaud. Sc. Nat., LII, 483-564. 



5 Adolf Hoel and J. Schetelig, Nephelin-Bearing Pegmatitic Dykes in Seiland, 1916. 



