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), is the most prized, the most 

 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 a 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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