THE COLLECTION OF MINERALS 
semi-metals and represent the combination of oxygen with 
metallic bases. They form one of the most important classes 
of minerals, and among them are found many of the valuable 
ores, while Quartz, the oxide of silicon, is the most widely dis- 
tributed of all the rock-making minerals. In this section, He- 
matite, Corundum, Spinel, Magnetite, Franklinite and Rutile 
present some of the most brilliant phases of the mineral world 
(Desk Cases 6 and 7, Wall Case I).. 
Quartz, including the chalcedonic forms of silica (Desk Cases 
4 and 5, Wall Cases F, G, H, P), 1s the most prized, the mioss 
beautiful and the most varied of all minerals. In color, form 
and physical constitution, it assumes so many aspects, and the 
changes from one form to another are so gradual that its com- 
plete illustration is almost impossible. It crystallizes in the 
hexagonal system and usually has the form of e six-sided prism 
terminated by a pyramid. In the Bement collection its great 
diversity of development is well shown. Quartz occurs in easily 
visible particles in granite and many other igneous and meta- 
morphic rocks, in sandstones and quartzites and, as a deposit 
from aqueous solution, in cavities and crevices in all kinds of 
rocks. It is the common vein material of mountainous and 
ore-bearing regions. The absorption of iron and other metallic 
oxides produces many colors, especially in the various chalce- 
donic forms of silica. 
Following the Oxides are the Carbonates, which form a very 
important group. The carbonates of lime and magnesia consti- 
tute the limestones, while the carbonates of iron and copper are 
valuable ores. On account of their beauty, many Carbonates,— 
for example, Marbles, Malachite, Azurite and Mexican Onyx, 
—take a conspicuous position in a mineral cabinet. The carbon- 
ates have been formed by the combination of carbon dioxide 
with various bases, and the union in many instances has been 
brought about through the agency of water carrying the carbon 
dioxide in solution. This has dissolved the base of the salt, the 
whole being re-deposited afterward from a more or less saturated 
fluid on a further change of conditions. 
In this series the Calcite, Aragonite, Malachite, Azurite, Sider- 
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