Dctertninatio)! o 
rT 
f the Feldspars. — W 
'/lelie/L 
15 
A 
r^^ 
(i:^i\ ^. 
71 
y 
\ 
/ooi/ 
010 
ilo 
110 
wo 
110 
010 
Uol 110 
/ 
/ 
\ 
201 
I / 
^2 01^ 
K/ 
V / 
^ 
^01 
V 
Fig, 1. 
Fig. 
I. F:g. 3. 
Simple Forms of Orthoclase. 
Cleavages. 
The feldspars all possess an easy cleavage parallel to the 
base (001) and another less evident parallel to the pinacoid 
010. There are also rudimentary, often irregular, and coarser 
cleavages parallel to the prism faces no and no, etc. The 
basal cleavage is always visible if the thin section be not too 
thick, nor parallel to the base. That parallel to 010 is parallel 
to the albite striations, and disappears in sections cut parallel 
to GIG. In orthoclase these cleavages form a right angle with 
each other. In all the other feldspars they are oblique. These 
cleavages are best observed in sections rather thin, and on low- 
ering the condenser. 
Twi/ifiing. ' 
The feldspars are all subject to twinning*. Orthoclase is 
especially frequent in the form of Carlsbad twins, but also 
shows the forms of Manebach (Four-la-Brouque) and Baveno 
(figs. 4, 5 and 6). 
The Manebach type (fig. 4) has the basal plane as composi- 
tion face, and the axis about which the crystal turns is a line 
perpendicular to the base 001 (Lacroix). The cleavages ggi, 
of one twin, are parallel to those of ooi of the other. The same 
is true of the cleavages gig. But their extinctions have oppo- 
site signs, only one of the twins being in the conventional po- 
sition (p. 14). 
In the Carlsbad form the twins are united by some plane, 
usually GIG, parallel to the vertical axis (fig. 5). One is turned 
)8o° from the position of the other about the common vertical 
axis. In a thin section of a Carlsbad twin the pinacoidal 
*The French word "made" might appropriately and conveniently be 
substituted for the word twinning. 
