Determination of the Feldspars. — Wine/tell. 
27 
Orthoclase 
Microciine 
Anorthoclase 
Albite 
Oligoclase 
Andesine 
Labradorite.. . 
Anorthite 
2V 
2E 
69^ 
IIQ° 
to 125^ 
88«* 
45*^ 
65^ 
to 75^ 
77° 
88« 
155- 

88*^ 
77"" 
77 
3Q. 
2H 
80^ to 85° 
90*^ to loo*^ 
85° to 89° 
85" 
Sign 
+ 
± 
+ 
Refraction and Double Refraction. 
The feldspars all possess low refraction and double refrac- 
tion, both being about the same as for quartz. By these char- 
acters, therefore, it is sometimes difficult to distinguish them, 
when pure, from quartz. When other diagnostics are not 
available resort may be had to the Becke method of distinc- 
tionf of comparative refraction. This consists in the follow- 
ing very delicate process: 
Flam norm«l 
to OJO, JIO , tlo 
Fig. 25.— Projection of the principal indices of Albito (n^ K OliKOclase (n'^ ) 
Andesine (n^ ), Labradorite (n* ),and Anorthite (n^ ). on the plane 010. 
*The values of 2 V from microciine to anorthite inclusive are taken 
from Fouque, Bui. M. Soc. France, 1894, p. 428. 
t Uber die Bestimmbarkeit der Gesteinsgemengtheile auf Grund 
ihres Lichtbrechungs Vermogens. Wien. Acad., 1893, I. 
