Fig. 74.—Drawings showing forms of crystals of common minerals: a and b, garnet (Isometric); c and d, feldspars (Monoclinic); e, f, and g, quartz (Trigonal); h, i, and j, calcite (Hexagonal); k, augite (Monoclinic); l. hornblende (Monoclinic); m, pyrite (Isometric).
Amphibole. A number of species closely related in composition, crystal form, and properties are here included. They are silicates of lime and magnesia usually with aluminum and iron. Most common by far are those which crystallize in the monoclinic system with prismatic faces and two good prismatic cleavages meeting at about 24 degrees. Color, commonly brown to black, but sometimes green or white. Hardness varies from 5 to 6, and specific gravity from 3 to 3.4. Hornblende, the most common species, is a dark colored silicate of lime, magnesia, aluminum, and iron. It is one of the few most common of all mineral species, especially in igneous and metamorphic rocks. Tremolite is a white to light gray silicate of lime and magnesia found especially in metamorphic limestones. Actinolite is a green silicate of lime, magnesia, and iron especially common in certain metamorphic rocks. One kind of jade is an amphibole similar to tremolite and actinolite in composition, while the other kind is a pyroxene (see [below]). Jade is and has been highly prized in the east (especially in China) where it has been carved into many objects of exceptional variety and beauty. Jade is probably the toughest (not hardest) of all minerals because of its wonderful microscopically fibrous structure. In color it is white, gray, and green.
Apatite. Crystallizes in the hexagonal system with a six-sided prism usually capped at each end by a six-sided pyramid (see [Figure 75g]). Composition, a phosphate of lime. Color variable, but mostly white, green, or brown. Hardness of 5, or just enough to scratch soft glass. Specific gravity, 3.2. No good cleavage. Tiny crystals are widely disseminated through many common rocks—igneous, metamorphic, and sedimentary. In certain metamorphic limestones excellent crystals a foot or more in length have been found. Apatite, mostly in uncrystallized form, is the source of most of our phosphate fertilizers.
Azurite. An azure-blue hydrous carbonate of copper which crystallizes commonly in small monoclinic crystals. Hardness, nearly 4, and specific gravity, nearly 4. Commonly occurs in veins deposited by underground water. One of the great ores of copper, especially in Arizona, Chile, and Australia.
Barite. A sulphate of barium crystallizing in orthorhombic prisms usually of tabular habit. White to light color shades. Hardness, 3.5; specific gravity, 4.5, which is notably higher than the average of light-colored minerals. Three good cleavages parallel to principal crystal faces. A common and widely distributed mineral, especially in many vein deposits associated with certain ores. Used in ground form to give weight to certain kinds of paper and cloth, and a barium compound used for refining sugar is made from it.
