Wright — Determination of Feldspars. 363 



By shading half the microscopic field in this manner, the 

 edges of the small grains on the object glass will appear to be 

 unequally lighted near the dark shadow. If the mineral grain 

 be more highly refracting than the liquid, its edge next the 

 shadow will appear brighter than the edge opposite; on the 

 other hand, if the liquid have higher refracting power, the 

 phenomena are reversed and the light band appears on the 

 farther side from the shadow, while the adjacent edge appears 

 darker than the rest of the grain. In case both mineral and 

 liquid have equal refractive indices for light waves of medium 

 length, the opposite edges of the mineral grain appear bril- 

 liantly colored — bright red on one side and blue along the 

 farther side. The occurrence of the red and blue bands along 

 opposite edges of the mineral is, therefore, a sufficient criterion 

 that the refractive indices of mineral and liquid are equal for 

 light waves of medium length. The dispersive power of 

 liquids is often strong, and care should be taken that the colors 

 which appear are actually red and blue and that both edges of 

 the grain are equally bright. 



By trial, the approximate refractive index of the feldspar 

 fragment is thus found and the variety determined. 



Since in this method cleavage fragments alone are used, 

 extinction angles on (001) P and (010) M can be measured in 

 the same powder preparation and a still more accurate deter- 

 mination made. The determination of the refractive index, 

 moreover, relieves the ambiguity which exists in the angles 

 obtained from certain members of the feldspar series. 



In practice, this method has proved particularly valuable in 

 the study of rocks containing both plagioclase and orthoclase. 

 In the thin section it is not always a simple matter to recog- 

 nize small amounts of orthoclase when abundant acid plagio- 

 clase is present, while by the refractive index method the two 

 can be distinguished readily and accurately. Similar conditions 

 obtain in specimens containing both orthoclase and nephelite, 

 in which case the method applies to equal advantage. 



Geophysical Laboratory, Carnegie Institute, Washington, D. C. 



