300 HAROLD L ALLING 



mixture of both of them, accordingly, tends to adjust itself to the 

 final static molecular state. Inability to attain this [condition] is 

 caused by the overpowering influence of the additional isomorphous 

 constituents."^ 



He shows that through the proper physical means this molecular 

 overbalance can be broken up. He believed that it was perfectly 

 possible to change triclinic feldspar into a monocHnic form by heat- 

 ing, and vice versa by cooling. This can be explained, as Makinen 

 points out, by dimorphism and not by Groth's theory of poly- 

 symmetry. It must be remembered, however, that most of these 

 physical properties are measured upon the cold specimen, and such 

 observations may or may not apply to heated feldspar. 



Here the matter of the dimorphism (and isomerism) of both the 

 potash and soda feldspars rests. 



Thermal diagrams and dimorphism. — Various investigators have 

 suggested diagrams that indicate the possible dimorphism of 

 KAlSijOs. It is not necessary to discuss them all in detail. A satis- 

 factory conception can be obtained by examining the accompanying 

 diagrams (Fig. i. Part II). These have been modified more or less 

 in detail by the writer to bring them up to date; the main facts, 

 however, remain. 



Warren^ has modified Vogt's original diagram to indicate the 

 inversion of orthoclase into microcline. This is shown by the line 

 Cp in Figure i. It will be seen that it interrupts the solubility line 

 iN in that microcline is less able to dissolve the soda feldspar com- 

 ponent. The consequences of this, as Warren points out, are that 

 many perthitic intergrowths are due to exsolution caused by the 

 decrease in solubiHty of the soda phase in the potash component on 

 its inversion. According to the diagram orthoclase and microcline 

 cannot exist together in equilibrium over any range in temperature. 

 Whether they can or cannot exist in this manner in nature it is impos- 

 sible at present to say. Although Makinen^ says that "no examples 

 [are] found in natural feldspars" the writer in Part I listed an adu- 



I H. Foerstner, op. ciL, p. 348. 



2 C. H. Warren, "A Quantitative Study of Certain Perthitic Feldspars," Proc. 

 Amer. Acad. Arts and Sci., LI (No. 3, 1915). 127, 154. 



3 Op. ciL, p. 149. 



