THE MINERALOGRAPHY OF THE FELDSPARS 355 



cent of orthoclase but beyond that amount the cooled material con- 

 sists of two phases. The maximum separation of the potash-rich and 

 the lime-rich feldspars occurs at about 45 per cent Or, 55 per cent 

 An. It is not the intention of the writer to convey the idea that the 

 results of these thermal studies prove that this percentage fixes the 

 position of the eutectic point, but only that it is probably in this 

 neighborhood. An artificial mixture of 40 per cent orthoclase, 20 

 per cent albite, 40 per cent anorthite, when held at 1250° C. for 

 several hours and then cooled rapidly showed a pseudohomogeneous 

 solid solution, a crypto-oranite (to parallel Brogger's "cryptoper- 

 thite"). This sample when "annealed" at a lower temperature for 

 a longer time became a micro-oranite, consisting of two phases. 

 This separation became more perfect as the time of annealing was 

 extended. Similar treatment of anorthoclase gave no such results. 

 The interpretation of these observations is that the viscosity of the 

 lime-free feldspars prevents extensive separation of the two phases. 

 But with increased lime the viscosity of the melt is proportionally 

 lower, allowing a greater separation.^ 



The petrogenetic significance of this is that magmas which con- 

 tain potential oranitic feldspar give rise to two-phase systems con- 

 sisting of orthoclase-rich and plagioclase-rich feldspar. This applies 

 to many granites, syenites and monzonites. The presence of such 

 potential oranitic feldspars in the common rocks is often concealed 

 by recasting the chemical analysis into the norm, and while it is not 

 so easy to secure the mode of an igneous rock, the latter alone shows 

 the real distribution of the feldspar components into their respective 

 phases. Such a procedure is necessary to obtain a true conception 

 of the nature of the feldspars in many rocks. 



The reader may observe that the writer is extending the meaning 

 of the term "oranite" to cover molten feldspar mixtures. We are 

 so much in the habit of considering our minerals as solid substances 

 (the usual definition) that we lose sight of the manner of their crys- 

 tallization. To give to these feldspathic melts a definite name 

 emphasizes their existence, and leads us to be on the lookout for the 

 end-products of such processes. 



' A. L. Day and E. T. Allen, "The Isomorphism and Thermal Properties of the 

 [Plagioclase] Feldspars," Carnegie Inst. Pub. 31 (1905). 



