II. b. 3. Triclinic 
a. Elongated columnar crystals, colorless or blue, 
Name. 
Che 77 i . 
Comp, and 
Reactions. 
Specif. 
Grav. 
Cleavage. 
Usual combina - 
tio?is and for?n 
of sections. 
Tivins. 
Optical deterrnmants. 
Coloj's 
of Polar 
Ration. 
D is them. 
(Cyanite) 
(Ag Si O5 
Insoluble. 
3-48- 
3.68 
■ 
Very 
Perfect. 
II • 
Perfect 
II i-i and 
0. 
Grains or col 
umns i-i . i-i 
with an angle 
of io 5 ° 15^, 
rarely termi 
nated. Cross 
section quad- 
rate (or hex- 
agonal of i^-i 
or i-G is ad- 
ded. 
1 
Frequent. 
Less so micro- 
scopic. Either 
multiples in 
i-i or 
(2) perpendic- 
ular to c axis, 
or 
(3) to b axis, or 
(4) parallel to 
0. 
Opt. axis plane makes 
an angle of 30° with 
the angle i-i : 0, with 
the angle i-i . 0 an 
angle of 60° 15^ and, 
like the acute bisec- 
trix = a stands at 
nearly a right angle 
with i-i, parallel to 
which plane sections 
show a biaxial inter- 
ferance ^figure with 
negative bisectrix. 
Very 
vivid. 
b. Broad tabular crystals or grains, colorless. 
Tricilnic 
Feldspars 
I. Micro- 
cline 
{fibrous 
^ ortho- 
clase.) 
As 
orthoclase 
1 
2.54- 
2-57 
Perfect 
II 0 . 
quite 
perfect, 
11 > 
i-T^. 
As orthoclase 
i-i . 0 . i^-i 
i-C predomi 
nating. 
Rare, |i i-i and 
at right angles 
to it. Almost 
always inter- 
lammellate 
with ortho- 
clase givingin 
sections || O 
the character- 
istic “lattice 
structure.” 
Also polysyn- 
thetic twins 
with albite, 
so that the 0 
surfaces are 
parallel in 
each. 
i 
Opt. axis plane, al- 
most -L 0 , its sec- 
tion with i-i forms 
with the obtuse angle 
0 . i-i 5-6° in the 
obtuse angle ac 
cleavage plates in i-i 
show one axis . evi- 
dently, the axial 
plane is somewhat 
oblique to i-i. 
Very 
.bril- 
liant. 
2. Plagio- 
clase. 
Feldspars 
(a) Albite 
[Ab] 
N a^ AI2 Sig 
Oxn 
Traces of 
Ca and K. 
Not at- 
tacked by 
acids. 
Si O2 
68 
2.61- 
2.63 
Complete 
II 0 and 
i-i 
incom- 
plete 
i-i^ and 
I. Angle 
0 : i-i at 
the right 
=" 93 ° 36" 
i-i . 0 . i^-i . 
i-i^ . I-i . I 
Very similar 
to orthoclase 
Opt. axis plane forms 
an angle of 96° 16^ 
with the c axis, with 
the perpendicular to 
i-i 16° 17^ Acute bi- 
sectrix = c. Disper- 
sion slight, p<7r'.cleav- 
age plates || i-i show 
a complete distorted 
interferance figure. 
Usual- 
ly. 
quite 
vivid, 
not so 
strong 
as 
quartz. 
In very 
thin 
sec- 
tions 
feeble 
blue- 
g^ay. 
Twins. 
Almost always twinned. 
( 1 ) Albite type i-i, and gen- 
erally polysynthetic. Often 
two such polysynthetic indi- 
viduals are again twinned ac- 
cording to the Carlsbad type. 
(2) Percline type and these 
again united on the Mane- 
bach type. 
(3) By a union of (i) and (2) 
there results a lattice arrange- 
ment reminding of micro- 
cline. 
