AMERICAN MUSEUM GUIDE LEAFLETS 
shown in Cases 3 and 4, illustrate the very great variation in color 
which is a chracteristic of this mineral and which is due to the presence 
of a slight amount of such impurities as iron and manganese. Many of 
the fluorite specimens show bandings of color produced by slight changes 
in the composition of the mineral-forming solution. Among the 
oxychlorides in Case 4 will be found some beautiful examples of the rare 
copper minerals boleite and percylite. 

FLUORITE ON SMOKY QUARTZ FROM GRIMSEL, SWITZERLAND 
OXIDES 
Cases 4, C, 5, 6, D and 7 
Oxygen, one of the most energetic of the elements, also constitutes 
a large part of the atmosphere of the earth, and of water which is almost 
universally present on it. Consequently, as we would expect, the oxides, 
or compounds of oxygen with the metals, form a large and important 
division of minerals. But the element silicon ranks next to oxygen in 
abundance and combines readily with it. It is therefore quite 
obvious that the mineral quartz, which is the oxide of silicon, should 
be the commonest and most widely distributed of all minerals. 
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