^ 2. Crystals with 
i aa. Optical axis plane -L i-F; perfect 
Name. 
Chem. 
Comp and 
Reactions. 
Specif. 
Grav. 
Cleav- 
age. 
Usual combina- 
tions and fo 7 'm 
of sections. 
Twins. 
Optical 
determinants. 
Double 
refraction. 
I a. Or- 
thoclasc 
K,(Al,)Si, 
^16 • 
Not 
attached 
by acids. 
2.50-2.59 
( 2 - 57 ) 
1 
Very 
perfect 
Be- 
tween 
0 and 
i-i^ 
angle 
89° ArO' 
In grains or 
columnar, 
0 . i-F . I . 
2 -i . 2 -F . I . 
Sometimes tab- 
ular or more 
rarely, in min- 
ute crystals. 
1 F I . 0 . 
2-i . i-i . 
Frequent. 
(1) Oftenest 
according 
to Karlsbad 
type. Twin- 
ning plane 
1- i. 
(2) Baveno 
type. Twin 
ning plant 
2- i^. 
(2) Rarel) 
Manebach 
type. 
Optical axis 
plane inclined 
to 0 forming an 
an angle of tf 
I with vertical 
axis. 
C=b 
a : a = 5“ 
Real axial angle 
=69°. 
In sections || i-F 
or i-i a distorted 
bisxial inter- 
ferance figure 
visible in con- 
vergent pol. 
light. 
Negative, 
rather 
strong. 
' In sections 
1 i-i^ one 
direction of 
extinction 
varies from 
the angle 
0 . i-i 
(=a . a) 
by 5° 18^ 
Sections in 
zone 0 . i-i 
extinguish 
perpendic- 
ularly. 
1 b. 
Sani- 
dtne. 
As 
above. 
Full of 
rifts. 
In slender rods 
or large crystals, 
almost never 
granular. Sec- 
tiuns 11 0 and i-i 
elongate bands 
truncate or 
acute at the 
ends, II i-F dis 
torted hexagons 
with sides cor- 
responding to 
0 . I . i-i . 
In the columnar 
type, sections 
rectangular if 
perpendicular to 
0 . i-F , if 2-i^ 
is present 
octagonal. 
Twinning 
plane= 
0. 
If parallel i-F 
B=b 
a : a=50°. 
