
THE COLLECTION OF MINERALS 
and rhodochrosite are related to calcite; cerussite is mainly formed by 
the alteration of galena through the action of water charged with carbon 
dioxide. 
The two copper carbonates Malachite and Azurite (Case 9) are 
attractive by reason of their rich colors and the unique shapes taken by 
the radiating, silky fibers of the one and the brilliant crystal masses of 
the other. Like cerussite they are alteration minerals which have 
resulted from the action of water charged with carbon dioxide on other 
copper ores. 
SILICATES 
The largest and from some points of view the most important divi- 
sion of the natural chemical compounds which constitute the minerals 
is that one which has for its basis the combinations of the two com- 
monest elements, silicon and oxygen. The oxygen salts composed of 
these two elements combined with the oxides of the metals give us the 
very numerous and varied groups of rock-forming minerals known as the 
Silicates. 
_ Broadly speaking the silicates are the minerals of the igneous or 
fire-formed rocks; they are essential constituents of granites, pegmatites, 
gabbros, diorites and gneisses, and some of them are to be found in 
crystalline limestones and as secondary minerals lining the cavities of 
lava, basalt and diabase. 
The Feldspars, shown in Case 10, are silicates of aluminum with 
some other metal. They are the commonest and most widely distrib- 
uted group of minerals in this division and constitute nearly 60 per cent 
of the mineral composition of igneous rocks. In the series exhibited, 
orthoclase, microcline and albite are especially beautiful and interesting, 
as is also labradorite with its brilliant and varied play of colors. 
The Pyroxenes (Case F) form another important group of silicates 
embracing a number of closely related minerals, all conforming to a 
characteristic crystal habit. Here may be seen marked differences of 
color due to variations in chemical composition as well as differences in 
. transparency from the clear gem-like diopside to the opaque black 
augite. 
Rhodonite (Case G) is a triclinic pyroxene containing manganese 
which gives to it a handsome rose color. The specimens of this suite are 
especially attractive. 
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