20 CORUNDUM, ITS OCCURRENCE AND DISTRIBUTION. 
Rank of mmerah accordiruj to resistance to passage of X rays. 
1. Diamond. 
2. Coi'uiidum. 
3. Talc. 
4. Quartz. 
5. Rock salt. 
6. Calcite. 
7. Ceriissite. 
8. Realgar. 
Corundiiin is iiorinally uniaxial with negative double refraction. 
The mean index of refraction is high, being about l.T()5. For the 
various indices, see page 23. The double refraction of corundum is 
0.008 to 0.009, or about the same as quartz. Some varieties of corun- 
dum have been observed that are abnormally biaxial. 
CHEMICAL COMPOSITION OF CORUIS^DUM. 
Theoretically pure corundum contains only alumina, ALO.^ ; but 
with few exceptions, all the specimens that have been examined show 
the presence to a greater or less degree of other chemical compounds, 
the principal ones being silica (SiOo), water (H.,0), and ferric oxide 
(Fe^03). Water is almost always present in amounts from a trace 
to 2 per cent or more. The silica and ferric oxide also vary from 
nothing in some corundums to as much as 5 per cent in others. Of 
course this does not apply to emery, which is a mechanical mixture of 
corundum and magnetite; but it does apply to the corundum when 
separated from the mixture, and the impurity in this corundum is 
usually ferric oxide. The purest known form of corundum is the 
transparent crystallized variety, or what might be called the sapphire 
or gem variety. 
In the following table a few analyses are given of both American 
and foreign corundums : 
Analyses of corundum. 
Locality. 
AloO:,. 
FeaOy. 
SiO,. 
HoO. 
Insolu- 
ble resi- 
due. 
Total. 
Analyst. 
Hastings County, 
Ontario 
Per cent. 
96.92 
97. 51 
97. 82 
98.79 
95.51 
Per cent. 
Per cent. 
Per cent. 
2.43 
Per cent. 
1.36 
100.71 
100. 20 
99.62 
101.22 
98.58 
Wells. 
Sapphire from India ^ 
1.89 
1.09 
.75 
.88 
0.80 
1.21 
.90 
1.45 
Smith. 
Ruby from India _ ^ _ 
Do. 
Corundum Hill 
mine, North Car- 
olina - 
.78 
.74 
Emerson. 
Laurel Creek mine, 
Georgia 
Do. 
These analyses are of selected pure material, and not of the com- 
mercial product, which contains besides corundum varying quantities 
