II. Biaxial minerals. 
I. Rhombic System. 
a. Sections parallel O without interference figures. 
Name. 
Chemical 
comp . and 
reactions. 
Specipc 
Gravity. 
Cleavage. 
Unsual co 7 nbi- 
naiions Crform 
of sections. 
Twins. 
Optical deter 
minants. 
Double re- 
-frcition. 
I . Olivine 
(Crysolite) 
Si 0. 
+ Fe,,SiO,' 
Rather 
easily solu- 
ble in HCl 
with sepa- 
ration of 
gelatinous 
silica. 
3 - 2 - 3-4 
Complete 
in i-i, in 
complete- 
in i-I, con 
choidal 
fracture. 
In sections 
usually 
curved 
crevices. 
i 
1 
Tabular crys- 
tals I. I-i. 
i-i, hexagonal 
sections. 
I=i39°2^ 
[-4^76° 54C 
or large roun- 
ded grains. 
i 
1 
1 
Very rare 
I-i and in- 
terpene- 
trating. 
Acute bisec- 
trix at right 
angles to i-i. 
a=c 
b=a p<ov. 
c^=& 
Positive, 
strong. 
( 3 . sections parallel 0 with interference 
aa. disappearance of positive 
I . Sillima- 
nite . 
Al, SiO^. 
Insoluble. 
I 
3 - 23 - 3-24 
Parallel 
i-i ; frac- 
ture of 
thin nee- 
dles in 0. 
Columnar. 
1= iii° : 
Optical axis 
plane !| i-i. 
acute bisec- 
trix at right 
angles to 0; 
c^=C 
b=a 
Strong, 
positive. 
2. Stauro- 
lite. 
H2 R3 
(Al2)g Sig 
034- 
R=3Fe+ 
Mg. 
Insoluble. 
3 - 34 - 
3-37 
i-i com- 
plete; 
I 
incom- 
plete; frac- 
ture in 0. 
Rarely in ir- 
regular 
grains, crys- 
tals I. i-i. 0 
1=128° 42^. 
Interpene- 
trating 
twins, ver 
tical axes 
inclined or 
. vertical, 
rarely mi 
croscopic. 
As above, 
(dispersion 
feeble p'P>v). 
As above. 
Pleochro- 
ism strong 
C=dark 
brown 
a=b= 
light yel- 
low. 
3. Ensta- 
1 tile. 
Mg Si Og 
Soluble 
very 
slightly. 
3 I- 3-29 
( 3 - 153)- 
I 
complete ; 
i-i i-i lon- 
gitudinal 
sections 
seem fib- 
rous frac- 
ture. 0. 
Long column- 
ar I. i-i. i-i. 
m-i, 8 -sided 
cross sections 
with two pairs 
of longer 
sides, like mo- 
noclinic au- 
gite. 
When 1=^92° 
in front the 
optical axis 
plane=i-i 
acute bisec- 
trix at right 
angles to O, 
c^=C b=b 
a=a. 
Positive 
rather 
strong, 
feebler 
than iu 
monoclin- 
ic augite. 
