THE MINERALOGRAPHY OF THE FELDSPARS 301 



laria from Eggerhorn, Switzerland/ as "in the process of inverting 

 from soda orthoclase to soda microcline" in view of the two sets of 

 extinction angles and the variability of the microclinic twinning. 

 Since that time several additional feldspars have been found that 

 apparently exist in this transitional stage, although it is quite likely 

 that such feldspars are metastable systems and not in equiHbrium. 

 Foerstner^ has already suggested the possibility that certain anortho- 

 clases are mixtures of the two series, "monoclinic" and "tricHnic," 

 and hence are representatives of "transitional" feldspars. 



Thus it may be seen that there are two general ideas expressed 

 by these diagrams. First, that of Warren, where the dimorphism is 

 shown by a single line (Cp), and second, by Makinen by an area, a 

 spherical triangle, FGH. 



As the writer understands Warren's diagram, the solubihty line 

 iN holds when orthoclase does not invert (transform) to microcline. 

 In the event of this change in modification, ipJsN graphically indi- 

 cates the decrease in solubility. These two possibilities are com- 

 bined in a single diagram which may be confusing, unless this duality 

 is kept in mind. 



Harker^ has given a diagram showing changes during cooling in 

 a binary system where limited solubility prevails in the soHd state 

 and where one component inverts to a lower temperature form. 

 While Harker did not state in definite terms that this illustrated the 

 behavior of the potash-soda series of feldspars, he believed that it 

 can be so regarded if a single line conception is entertained. 



The suggestion of Makinen, that the transformation of orthoclase 

 to microcline should be indicated on the diagram by an area, means 

 that within a restricted range of temperature and composition, FGH, 

 both orthoclase and microcline can be in equilibrium. H is an "inva- 

 riant" point, in the language of the phase rule. This means that 

 any change in temperature or composition or both destroys one of 

 the phases. At Z?^ there are three phases present : Z?^ as soda ortho- 

 clase'', H as soda microcline,^ and / as potash albite.^ The phase 



^ Specimen 989. 



^ Op. ciL, pp. 348, 188. 3 Op. cit., p. 256, Fig. 83. 4 OrggAbji. s MiesAbjj. 

 * K-feldspariiAbgg. The noncommittal "K-feldspar" is used because it is not 

 definitely known whether it is microcline or orthoclase. 



