II. Double-refractive minerals. 
A. U^iiaxial optically. 
I. Tetragonal system, a. Double refraction positive. 
Name. 
Chemical 
comp, and 
reactions. 
Specific 
graviiy. 
Cleavage. 
Usual combi- 
Jiations and 
form o f sec's. 
Twins. 
Character cena 
amount ofdou 
ble refraction. 
Polariza- 
tion col ■ 
ors. 
I . Leucitc 
K, (Al,) 
Si, 0,.,. 
2 45 - 2-5 
Incom- 
plete pris 
matic i-i 
and 0. 
Grains and 
(generally) 
crystals i. 
4-2. Sec 
tions octa- 
gons fre 
quently with 
rounded an 
gles, more 
rarely hex- 
agons. 
2-i, polysyn- 
thetic twin 
striations in 
these planes, 
crossing at 
right or 
oblique an - 
gles. 
Small individ- 
uals without 
twin-striations 
appear 'isotro- 
pous. In con 
verg. pol. 
light no evi- 
dent axis fig- 
ure. Double 
refraction fee- 
ble, positive. 
Not bril- 
liant, blu- 
ish gray. 
2. Rutile 
(Nigrine 
Sage- 
nite.) 
Ti 0 ^. 
7 "z-reac- 
tion with 
phosphor- 
ous bead. 
Insoluble 
in acids. 
4 - 2 - 4-3 
(4-277) 
1 
I 
and 
i-i 
i 
I. i-i, 
I 
granular; 
generally, 
however, in 
'minute elon- 
gate needles 
and crystals. 
The prisms 
are striate 
j parallel to 
'axis c. 
1 
i 
Very frec|uent 
and character- 
istic in the 
plane i-i; 
‘knee shaped’ 
with an angle 
ot ii_4°-2 5"; 
also tissue of 
fine needles 
which cut 
each other at 
1 an angle of 
about 60° 
(sagenite)., also 
cordate twins 
in 3-i, fre- 
quent. 
Crystals usu- 
ally too small 
for determi- 
nation with 
condenser. 
Double re- 
fraction 
strong, posi- 
tive. 
None par- 
ticularly 
brilliant. 
3. Zircon 
Si 0, 
H2 SO, 
decompo- 
ses; other 
acids with- 
out effect. 
1 
i 
1 
1 
4-4-4 - 7 
i 
i 
j Incom- 
plete I 
1 and I. 
j 
i 
I, i-i, 
3 - 3 ; 
numerous 
combina- 
i tions; al- 
imost solely 
i in minute 
ibut sharply 
■ defined crys- 
‘tals. 
! 
i 
Very rare 
I-i. 
Positive, 
very strong. 
Very bril- 
liant, 
emerald- 
green, hya- 
cinth-red 
and irrid- 
escent. 
