Minerals. ' table xvii. 
(or granular.) Cleavage in i-I, i-i, andO. 
Double 
refraction. 
Color^ etc. 
Structure. 
Association. 
Inclu- 
sions. 
Altera- 
tions. 
Occurrence. 
Remarks . 
Negative, 
strong. 
Extinction 
in sections 
il i -1 
c : 0 =^ 56 ° 
vfp 
Colorless 
or sky 
blue. 
/Ip= 1.72 
strong re- 
lief. If 
blue, with 
rather 
strong 
pleochro 
ism. 
C=blue 
a=white 
In long col- 
umns or ir- 
regular 
grains with 
crevices | 
or 4- to 
axis c. 
Often com- 
pletely or 
mottled 
blue. Rare 
ly in aggre- 
gates of 
needles. 
With 
quartz, 
mica, 
garnet, 
omphacite, 
hornblende 
rarely with 
orthoclase. 
Poor. 
Fluid 
inclu- 
sions. 
Rarely 
mar- 
gined, 
by a 
zone 
of al- 
tera- 
tion. 
Raie as primary 
accessory in 
crystalline slates 
granulite, eclo- 
gite and mica 
schists. 
If colorless, 
only to be dis- 
tinguished 
from silliman- 
ite by position 
of axis of 
elasticity. 
Cleavage parallel to 0. and i-i. 
Negative, 
rather 
strong. 
In plates 
parallel to 
i-i positive. 
Extinction 
not perpen- 
dicular but 
oblique in 
plates par- 
allel to 0 . 
Colorless 
Reliefnot 
strong, as 
in ortho- 
clase. 
i 
In rocks 
only as 
grains,often 
coalesced 
with 
quartz, as 
in graphic 
granite. 
.(a) With 
orthoclase, 
elaeolite, 
sodalite, 
augite, and 
hornblende 
(b) With 
quartz, 
orthoclase, 
biotite, 
hornblende 
&muscovite 
(c) With the 
above and 
garnet and 
, cyanite. 
Usual- 
ly very 
poor. 
Horn- 
blende 
biotite, 
zircon, 
apatite 
Simi- 
lar to 
that of 
ortho- 
clase. 
Primary essen- 
tial with ortho- 
clase in 
(a) elaeolite- 
syenite. 
(b) in various 
granites par- 
ticularly in 
graphic granite. 
(c) Crystalline 
slates as granu- 
lite and gneiss. 
Easily dis- 
tinguished 
from ortho- 
clase by 
extinction 
oblique to 
0 and 
twinning; 
from other 
tnclinic 
feldspars by 
latticed 
twinning 
and optical 
peculiarities. 
Positive, 
rather 
strong. In 
cleavage 
plates II 0 
extinction 
inclined 
with the 
angle 0 : 
i-i 
+3° 54"- 
+4° 51 /; 
+ 15° 33"- 
4-20° . 
Colorless 
clear, 
slight 
relief. 
^•537 
In large 
grains, rare- 
ly in crys- 
tals, often 
coalesced 
with ortho- 
clase and 
quartz In 
eruptive 
rocks in 
slender 
rods. 
With cal_ 
cite, quartz, 
mica and 
orthoclase, 
alsochlorite 
and more 
rarely 
hornblende 
I 
i 
1 
V ery 
poor, 
fluid 
inclu- 
sions. 
Rarely 
altered 
Be- 
comes 
cloudy 
as or- 
tho- 
cl a. sc. i 
i 
In granular 
limestone, fre- 
quent. In crys- 
talline slates. In 
many semi-crys- 
talline gneisses, 
phyllite, sericite 
slate. Rarely in 
erupt ives, in 
grains in dio- 
rites, in rods in 
andesites and 
porphyrys. 
All plagio- 
clase is char- 
acterized by 
the polysyn- 
thetic twin- 
ning. Triclinic 
feldspars can 
only be dis- 
tinguished by 
chemical 
means or de- 
tei mination of 
direction of 
extinction etc. 
Even then 
only in rather 
large grains. 
