72 



ALBERT JOHANNSEN 



Caf 



one for each class, the corners being formed by quartz, potash- 

 feldspar, albite, anorthite, and the feldspathoids. But these 

 tetrahedrons may be subdivided into orders. Based on the old 

 classifications, these orders depend upon the proportions of the albite 

 to the anorthite molecule; consequently the divisions must be made 

 by planes all of which cut the quartz-potash-feldspar-feldspathoid 

 edge but separate across the central plane of the double tetrahedron. 



as shown by the dotted 

 lines in the figure, or by 

 Fig. ii, which is a hori- 

 zontal section through the 

 center. That is, the edge 

 Qu-Kf-Foids remains 

 common to all of the di- 

 visions, the plagioclase 

 corner simply having been 

 changed. Now while the 

 triangles formed by the 

 intersections of 

 these planes with 

 Naf the tetrahedron 

 IO (Fig. 10) are not 

 all equilateral, the 

 relative position of any rock plotted on an equilateral triangle on the 

 basis of the three components represented by its corners and reduced 

 to ioo will be the same as the same rock plotted with four components 

 within the solid tetrahedron. Consequently the different orders also 

 may be represented simply by a series of double equilateral triangles 

 (Figs. 20-23 or 24-26) whose right-hand corners vary with the kind 

 of feldspar. It would, of course, be possible to make 20 or 100 or 

 more different orders based upon variations of 5 or 1 or some other 

 percentage in the albite content, but this is neither desirable nor 

 necessary. Here the divisions have been made (1) albite (Ab I00 An 

 to Ab 9S An s ), (2) oligoclase and andesine, (3) labradorite and 

 bytownite, (4) anorthite (Ab s An 9S to Ab An I00 ), giving four orders. 

 In other words, the dividing points between albite and anorthite 

 are 100-95-50-5-0 of the albite molecule. 



Fig. 11. — A section through the central plane of Fig 



