90 CORUNDUM^ ITS OCCURRENCE AND DISTRIBUTION. 
There seems to be but little tendency for the alumina to unite with 
the magnesia to form the double silicates, except where the magnu) 
contains but little magnesia and some iron, where the mineral cor 
dierite is found, or with the magnesia alone to form the mineral 
spinel, MgO,Al203, except when there is an excess of MgO, when 
some spinel is formed ; but the greater portion of alumina separates 
as corundum. This would seem to show' that the affinity of mag- 
nesium for silica in the formation of a silicate molecule is stronger 
than for alumina to form the aluminate molecule. 
From what has been observed in nature and from the experiment- 
that have been made in the laboratory, therefore, it seems that the 
separation of alumina as corundum from molten magmas is depend- 
ent upon the composition of the chemical compounds that are the 
bases of the magmas, upon the oxides that are dissolved with the 
alumina in the magma, and upon the quantity of alumina itself, and 
that the following conclusions are justified: 
1. When the magma is a calcium-sodium-potassium silicate, nc^ 
alumina held in solution by such a magma will separate out as corun 
dum, except when the ratio of the alumina to the other bases is more 
than 1 : 1 and the ratio of the silica is less than 6. 
2. If magnesia and iron are present in the magma thus composed, 
corundum will not form unless there is more than enough alumina to 
unite with the magnesia and iron. 
3. Wlien the magma is composed of a magnesium silicate without 
excess of magnesia, all the alumina held by such a magma will sepa- 
rate out as corundum. 
4. Where there is an excess of magnesia in the magma just de- 
scribed, this excess of magnesia will unite with a portion of the 
alumina to form spinel and the rest of the alumina will separate 
out as corundum. 
5. Wliere there is chromic oxide present in a magma composed 
essentially of a magnesium silicate (as the peridotite rocks) and only 
a very little alumina and magnesia are present, these, uniting, sepa- 
rate out with chromic oxide to form the mineral chromite, and no' 
corundum or spinel is formed. 
6. When peridotite magmas contain, besides the alumina, oxides of 
the alkalies and alkali earths, as soda, potash, and lime, a portion of 
the alumina is used in uniting with these oxides and silica to form 
feldspar. 
7. There is a strong tendency for the alumina to unite with the' 
alkali and alkali-earth oxides to form double silicates like feldspars, 
whether such silicates form the chief minerals of the resulting rock 
or are present only in relatively small amount. There is, however, 
