Hematite.--One of the common and important iron oxides with less iron than magnetite and no water as has limonite. Crystallizes in hexagonal forms. Color, black, with metallic luster, when crystalline, otherwise usually dull red. Hardness, about 6; specific gravity, about 5. No cleavage. Red streak when rubbed on rough porcelain. Hematite is extremely widespread in rocks of all ages, especially in metamorphic and sedimentary rocks. Some occurs as crystals in igneous rocks, and some in vein deposits. It is the greatest ore of iron in the United States, especially in Minnesota, Michigan, Wisconsin, and Alabama. Kaolin. Commonly called "China clay." Composition, a hydrous silicate of aluminum. Crystallizes in scalelike monoclinic forms, but usually forms compact claylike masses. Hardness, a little over 2; specific gravity, 2.6. Color when pure, white. Usually feels smooth and plastic. Very abundant and widespread, especially forming the main body of clay and of much shale. Always of secondary origin, generally resulting from the decomposition of feldspar. It is the main constituent of chinaware, pottery, porcelain, tiles, bricks, etc. Limonite. An important oxide of iron in composition like hematite except for its variable water content. Never crystallized. Hardness, about 5; specific gravity, nearly 4. Color, light to dark brown to nearly black. Leaves a characteristic yellowish-brown streak when rubbed on rough porcelain. Exceedingly common and widely distributed, always as a mineral of secondary origin as a product of weathering of various iron-bearing minerals. Where accumulated in considerable deposits it is an iron ore of some importance. Magnetite. One of the three important oxides of iron containing no water, and richer in iron than hematite. (See Figure 75e.) Commonly crystallizes in isometric octahedral forms alone or combined with twelve-faced forms. Hardness, 6; specific gravity, 5. Color, black with metallic luster. Leaves black streaks on rough porcelain. Characteristically highly magnetic. Wide-spread as crystals in nearly all kinds of igneous rocks, and as large segregation masses in certain igneous rocks. Also very common in metamorphic rocks, in many cases forming lenses and beds as ore deposits. Occurs in some strata and sands. It is an important ore of iron. Malachite. A light-green hydrous carbonate of copper. In almost every way, except difference in color and slight difference in composition, it is very much like azurite (see above).