DetcrJiii nation of the Feldspars. — 
Winchell. 
43 
Albite 
Between ooi and the ob- o°-2o'^ 
Near oio. 
tuse angle ooiAoio. 
Oligoclase AbiAn, . 
o°-y^° 
Between ooi and ooi 
Aoio acute. 
Oligoclase AbsAn, . 
d'-o" 
Andesine 
Near ooi, between ooi 
and the obtuse angle 
ooiAoio. 
o«-7^ 
I n o I o. 
' 
Labradorite 
Between ooi and ooi A 
o'^-iS'^ 
Near oio. 
Between 
010 obtuse. 
OOI and 
acute. 
OOI Aoio 
Labradorite (basic) 
Between ooi and ooi A 
o''-32<-"' 
Near oio. 
Between 
010 obtuse. 
OOI and 
acute. 
oo I A o I ( ) 
Anorthite 
Between ooi and ooi A 
o'^-SS" 
Near oio. 
Between 
010 obtuse. 
oo I and 
acute. 
ooi Aoio 
note that the zone ooi :oio always contains a plane nearly per- 
pendicular to the bisectrix //g. Compare p. 33. 
In making- use of the crystallographic zones of the feld- 
spars it is evident, therefore, that the zone perpendicular to 
OIO is the most reliable and has the widest application. When 
the albite twinning line is visible it alone leads to characteristic 
results b^ the shortest, most rapid and easiest way. The task- 
is facilitated and the distinction between albite and certain 
andesines is assured, when several instances occur of the com- 
bination of Carlsbad and albite twinning, which is very fre- 
quently the case. The method of the positions of equal lumi- 
nosity is a convenient means of searching out the different 
groupings of the twinned lamellae, as well as the sections 
properly oriented. 
After the zone perpendicular to the brachypinacoid, which 
is susceptible of universal and almost exclusive use with 
microlites of small size, the zone 001:010 affords similar ad- 
vantages for every case of the variolytes and the porphyrytes, 
or andesytes, in which plagioclase takes arborescent or spher- 
ulitic forms. 
The anorthites cannot be confounded with any other feld- 
spar. As to the oligoclases, it should be remembered that 
their properties are very near those of the anorthoclases. It is 
convenient then to resort to the determination of the indices 
of refraction, or to the measurement of the angle of the optic 
axes, which in the anorthoclases is nuich smaller about the 
acute negative bisectrix (p. 27). 
