Siever, Raymond Formerly, Department of Geology, Harvard University, Cambridge, Massachusetts.
Last reviewed:October 2019
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Clastic sedimentary rocks containing silt- or clay-sized particles that are less than 0.0625 mm and/or clay minerals. The argillaceous rocks (lutites) include shales, argillites, siltstones, and mudstones. They are the most abundant sedimentary rock type, varying according to different estimates from 44 to 56% of the total sedimentary rock column. Claystone is hardened or cemented (indurated) clay, which consists dominantly of fine material of which at least a major proportion is clay mineral (hydrous aluminum silicates). Shale is a laminated or fissile claystone or siltstone, in general more consolidated than claystone. Mudstone is a claystone that is blocky and massive. The term argillite is used for rocks which are more indurated than claystone or shale but not metamorphosed to slate. All these argillaceous rocks are consolidated equivalents of muds, oozes, silts, and clays. Loess is a fine-grained, unconsolidated, windblown deposit. The term shale has been used by many authors generically to denote all of these types of rock. See also: Bentonite; Clay; Clay minerals; Loess; Sedimentary rocks; Shale
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