Generally, shales contain large amounts of clay minerals and quartz, but are otherwise somewhat various in their composition and appearance. Some types of the rock contain significant amounts of organic material and may be utilized as a source of oil. These organic-rich shales tend to be very darkly colored. Aranaceous shale, however, is a kind of shale that contains appreciable amounts of sand and may exhibit a wide variety of hues. For instance, an arenaceous shale deposit in Sweden is a greenish-gray color, while in Kansas occurrences of the rock are red, and in China examples have been found that are a purplish hue. The red and purple coloring of shales is typically an indication of the presence of hematite or limonite, and mineral constituents that contain large amounts of ferrous iron are usually responsible for green, black, and blue shales.
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