66 CORUNDUM, ITS OCCURRENCE AND DISTRIBUTION. 
MII^ERAIiS ASSOCIATlilD WITH, AND AXiTERATIO:N^ PROD- 
UCTS OF, CORUIN^DUM. 
Ore deposits seldom show on the surface and in their upper i)or- 
tions the same mineralogical and physical characteristics that they 
do at greater depths, and their outcrops are apt to be stained a 
brownish color, due to the oxidation of iron from the deposit. These 
same phenomena are observed in connection with corundum deposits, 
especially those in which the mineral is associated Avith j^eridotite or 
other basic magnesian rocks. Both the alteration products of corun- 
dum and the minerals associated with corundum will vary accord- 
ing to the type of rock in which the corundum occurs and to the 
extent of alteration that the inclosins: rock has under£>one. Thus 
in granite, syenite, or gneiss the associated minerals Avould l)e radi- 
cally different from those occurring with corundum in peridotite^. 
Moreover, the minerals resulting from the alteration of the mineral 
constituents of the original rock will be more numerous and very 
different in the peridotites (the basic magnesian rocks) from those in 
the first-named, more acid rocks. Although corundum is one of the 
most stable minerals at the earth's surface, as is well illustrated by 
the fragments of this mineral that are found in the stream and river 
gravels exhibiting no sign of chemical alteration at their surface, yet, 
on the other hand, conditions have existed that have brought about 
extensive alterations in the corundum even when found in place, this 
being illustrated by the finding of man}^ undoubted pseudomorphs 
after corundum. 
These associated minerals of corundum are readily divided into 
groups as follows : 
1. Primary or original minerals. 
2. Minerals that are the result of contact metamorphism. 
3. Minerals that have been produced by regional metamorphism. 
4. Minerals that are due to the alteration of the foregoing minerals. 
5. Minerals that are the direct result of the alteration of corundum. 
1. The primary or original minerals which accompany corundum 
include those minerals that are constituents of the rock in which the 
corundum occurs, together with any primary accessory minerals oc- 
curring in this rock. Thus there will be a great variation in the 
primary minerals according to the type of rock in which the corun- 
dum occurs. Where corundum occurs in gneiss and granite, beyond 
the mineral constituents of the rock, there would be usually garnet, 
magnetite, pyrite, zircon, and rarely monazite and sodalite. 
AVhere corundum occurs in the peridotites and other basic mag- 
nesian rocks, the rock constituents are entirely different, consisting 
mostly of olivine, magnesia amphiboles, pyroxenes, and rarely pin 
gioclase feldspar, while the accessory primary minerals are usually 
