Name. 
Chemical 
comp . and 
reactions. 
Specif. 
Grav. 
Cleavage. 
Usual co77ibi- 
fiations Cffo7'-77i 
of sections. 
Twitis. 
Optical deter 
77iinants. 
Double I'e- 
f7'action. 
b. Oligo- 
clase. 
Si O2 = 
62-66%. 
A little K 
(Aba Ahj 
Ab. Aiij^.) 
2.62- 
2.65 
(2.63.) 
Most per- 
fect 1 0, 
also i-i as 
albite. 0: 
i 1-93° 
28^ at the 
right. 
As albite. 
Always 
polysyn- 
thetic, of 
the albite 
type; also 
ofpericlin- 
ic type. 
Very similar 
to albite. In 
cleavage 
plates II i-i the 
axial points 
lie farther out 
of the field of 
view than in 
albite. c in- 
clined to the 
obtuse angle 
0: i-i. 
As albite. 
Extinction 
li 0 in- 
clined to 
angle 0: 
i-i +1° 
10^; 11 i-i 
inclined 
2°-4°. 
c. Ande- 
site. 
R A1 Si^ 
O12, R= 
Na2 and 
Ca. (Aba 
An^ to Ab^ 
AiiiO 
i 
2.65 
do 
As albite. 
As albite. 
Often with 
the albite 
and peri 
cline» type 
combined. 
Individu 
als twin- 
ned 11 i— 1 
are again 
twinned 
according 
to Carls 
bad type 
or in i-i or 
0. 
Similar to or- 
thoclase but 
the axial 
plane is more 
than I f in- 
clined to ob- 
tuse angle 0: 
i-i. Disp. 
=p<v. 
As above. 
Extinction 
II 0 inclin- 
ed to an- 
gle 0: i-i 
— 1° ^7^ to 
-2° 19" 
II i-i— 4° 
50^ to 8° 
d. Labra- 
dorite. 
RAl Sia 
Oio (Abi 
Aiij to Ab^ 
Ana ) At 
tacked by 
!HC1. 
2.68- 
2.70 
As ortho - 
clase in 
i-i often 
with a 
play of 
colors. 
Usually in 
large grains, 
rarely crys- 
tals, as ortho- 
clase. 
In surface j| 
i_i, lateral dis- 
appearance of 
one axis and 
signs of lem- 
nisci axial 
point not vis 
ible; 11 0 lat- 
eral disap- 
pearance of 
the other axis, 
axial point 
also not visi- 
ble. pfv. 
do 
Acute bisec 
trix ^^c at 
right angles to 
2-i. pfv. 
Plates 1 0 and 
i-i show lat 
eral disap- 
pearance of 
one or other 
axis Axial 
point on mar- 
gin of field. 
As ortho- 
clase. Ex- 
tinction, 
in 0 — 4° 
30^ to-6° 
54"; i-b 
— 1 6° 40'' 
to 21° 12C 
e. B)down- 
ite. 
(A mix- 
ture?) Si 
O2 = 49- 
45% (Aby 
I An3_An.) 
, More easi- 
4y decom- 
! posed by 
;HC1. 
2.70- 
2.73 
( 2 . 71 ) 
do 
do 
do 
As Lab- 
radorite. 
Extinction 
II 0=-i4. 
5° to 20° 
1 i-i=-27° 
to 32°. 

f. Anor- 
thite. 
[An] 
Ca AI2 Si^ 
Og SiOs 
== 45 — 
43%. Sol- 
uble in 
HCl with- 
out forma- 
tion of gel- 
1 atinous 
Isilica. 
2.73- 
(2.75.) 
Complete 
0 and i-i. 
P: M= at 
right = 
94° 10°. 
As albite. 
As albi- 
bite. 
As albite. ' 
Extinction 
110 ^- 36 ° 
to 42° II 
i-i— 37° to 
43°- 
