2. Minet'als crystallizing iti 
a. Double refrac- 
Na??ie. 
Chemical 
comp . and 
reactions. 
Specific 
Gravity. 
Cleavage. 
IJnsual combi- 
nations Cfform 
of sections. 
Tuvins. 
Char, and 
amount of 
did. refract 
I . Quartz. 
Si 0 .,. 
Insoluble 
except in 
H FI. 
! 
i 
2.65. 1 
! 
1 
i 
Incom- 
plete in R. 
Sections 
full of 
cracks due 
to coil’ 
; choidal 
dracture. 
Oranular or 
crystals R.— R 
or iR. R.-.R. 
Usually in 
larger indi. 
viduals. Sec- 
tions are reg- 
ular hexagons 
iwith two par- 
1 allel sides 
With parallel 
axes. Rarely 
twinned when 
forming a 
component of 
rocks. 
i 
! 
Feeble. 
i 
1 
i 
j longer than 
ithe others, or 
■ in rhombs, 
never as mi 
croliths. 
I . Tridy 
mite. 
As above. 
2.282 
2.326. 
jlnc’mpl’te 
iil O. 
In very mi' 
nute thin 
plates O and 
I. 
Frequent. 
Twinning 
plane a sur> 
face of ^ and 
Ve.iv fee- 
ble. ' 
On account of the minute size of the 
crystals found in rocks accurate deter 
mination is usually impossible. Micro 
jscopic tridymite behaves in parallel 
[polarized light as though hexagonal. 
