VII. Amphibole and Pteoxene.
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19. Common Characteristics. — These minerals have the*same composition. They are silicates of two or more of the fol-lowing bases: lime, magnesia, oxide of iron, and alumina. Karely,certain other bases occur. Lime and magnesia are generallypresent in the different varieties, of which there are very many.Amphibole (am'phi-bole) and pyroxene (pyr'ox-ene) are oftendistinguished with difficulty unless in crystals, the shapes ofwhich are unlike. In their different varieties they vary in colorfrom white through shades of green to black. They may be incrystals or crystalline masses, or in grains, leaves, or fibers.Some of the fine, fibrous, or silky kinds of both are called asbes-tos. These minerals vary in hardness from 5 to 6£, and aretherefore not as hard as quartz. They are from three to threeand a half times as heavy as water. 120. Amphibole. — This is often called hornblende, but we con-fine this name to the dark green or black varieties; hornblendecontains iron and alumina. It frequently takes the place ofmica in granite and allied rocks. It often looks like mica in the
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