148 



WARREN. 



have been located in accordance with \'ogt's estimate, at 72% and 

 12% of orthochise respectively, which represents probably a fair 

 approximation to the truth. x\ssuniing that the diagram represents 

 in a general way the conditions of equilibrium for the crystallization 

 of these feldspars, it is evident that if we start with a melt having a 

 composition represented by a point lying well to the right of i, say 

 Or. 95% (composition .r), we should have, as the temperature fell to 

 ordinary temperatures, represented by the base line, assuming of 

 course that equilibrium was established throughout, as a final product, 

 a homogeneous mixed-crystal of the microcline type of the same 



Or. 1(10-; 9U 

 Ali.Oj 10 



composition with which we started. If we start with a mixture say 

 of the composition represented by a feldspar like No. 4 with about 80% 

 Or. (composition ij on the diagram), crystallization would begin when 

 the curve AE was reached and the composition of the first crystalliza- 

 tion would be that of the point t/o. Assuming a continuous adjust- 

 ment of equilibrium as crystallization progressed, the composition of 

 the solid phase would change continuously along the line Ai until a 

 composition yz was reached when the whole mixture would become 

 solid. With still further lowering of temperature, if we assume that 

 at some temperature, say at that represented by the point C, an inver- 

 sion of the orthoclase to microcline occurred, then when the temperature 



