288 HAROLD L. ALLING 



THE DIAGRAM OF MAKLNEN 



Makinen, in his recent and valuable paper/ offers a thermal dia- 

 gram that may well be considered as an intermediate diagram (such 

 as the writer suggested in Part I, p. 225). It is perhaps to be inserted 

 close to the front plane in the figure. Makinen makes the misci- 

 bility gap (Mischungsliicke) line DE in Figure i, Part II, very short, 

 covering only 10 per cent composition from about 66 per cent Ab 

 to 74 per cent Ab, which he says is greatly exaggerated. He places 

 the eutectic point at 70 per cent albite while Vogt placed it at 58 

 albite.^ 



Watts^ has made a similar suggestion that the eutectic point 

 should be nearer to albite than Vogt placed it. Here again we must 

 remember that artificial mixtures of natural feldspars were melted 

 and from such thermal data the conclusions were drawn. It seems 

 that these apparently conflicting ideas can be reconciled by stating 

 that a single binary diagram is not sufiicient to express the crystal- 

 lization of all potash-soda feldspars, but that a series of diagrams is 

 necessary, grading from a simple eutectiferous system to a system 

 of solid solutions with a minimum. Makinen points out* that peg- 

 matitic feldspars have a restricted range in composition compared 

 with porphyritic or plutonic feldspars on the one hand, and greater 

 compositional freedom than drusy and adularious^ feldspars on the 

 other. This can be interpreted to mean that the diagram for adu- 

 larious feldspars should show a eutectiferous system with the solu- 

 bility lines close to the sides, indicating very limited solubility and 

 consequently very restricted compositional freedom. The eutectic 

 point for adularious feldspars is not known, but a reasonable position, 

 it seems to the writer, would be about 50 per cent albite. Thus in a 

 three-dimensional model (Fig. 2, Part II) this eutectic at 50 per 

 cent Ab could be connected with the minimum point of the solid 



' Eero Makinen, "tjber die Alkalifeldspate," Sonderabd. aus. Geol. Foreningens. 

 Forhandl., XXXIX, H. 2 (February, 191 7), 149. 



2 The variation in the position of the eutectic point may well be due to the shift 

 brought about by metastable and labile conditions. See C. H. Gulliver, Metallic 

 Alloys, 1913, pp. 165-67, and J. V. Elsden, Principles of Chemical Geology, 1910, 

 pp. 114-16, 153. 



3 A. S. Watts, "The Feldspars of the New England and North Appalachian States," 

 U.S. Bur. Mines Bull. g2, 1916. 



4 0/'. cit., pp. 132-33, fig. A-E, Fig. 1-3, Fig. 2, Part II. 

 s Die Feldspdte der Adulardrusen. 



