THE MINERALOGRAPHY OF THE FELDSPARS 305 



In the course of thermal studies of his own the writer has found 

 that a convenient test for the development of glass (or leucite) on 

 the margins of crushed fragments which have been subjected to high 

 temperature is to measure the indices of refraction by the immersion 

 method/ using the Becke test, with monochromatic light. This 

 will reveal the thinnest possible shell of glass upon an unaltered core. 



Such thermal experiments would suggest that the liquidus and 

 soHdus curves of some of the diagrams should not be so flat as those 

 of Makinen, etc. Makinen has shown that a perthite (MiesAbjoAuz) 

 when heated to 1200° C.^ exhibited incipient fusion and says (in 

 translation): "The perthite albite is almost entirely melted and 

 contained abundant blebs [bubbles]. The microcline [phase] was 

 so much altered that the extinction angle on the (001) face in the 

 proximity of the melted albite amounts to 9-10° and as for the rest 

 it amounts to 15° as the maximum." Here Makinen had the chance 

 of drawing a valuable conclusion. Consultation of the optical con- 

 stants of the feldspars would have at once suggested that anortho- 

 clase had been developed by re-solution. The writer has satisfied 

 himself through actual thermal experiments that the long heating 

 of the microcline-microperthite (hypoperthite) from Verona, Onta- 

 rio, Canada (approximately Mi4iAbs8Abi) ,^ at 1000° C. produced 

 anorthoclase of approximately the same composition as the sum of 

 the two phases of the original, 



[To be continued] 



