Argillaceous rocks (or argillites) are clayey sedimentary rocks of fine- grained sediments composed of particles that are less than 0.0625 mm in size. Rocks or sediments made of or largely composed of clay-size particles or clay minerals. Mudstones, clays, shales, marls, and silts are all examples. Most argillaceous rocks contain material of two types: clay minerals and rock flour, the latter consisting of very fine particles of quartz, felspar, and other rock-forming minerals. The great majority of argillaceous rocks have been deposited in water, whether in the sea, in estuaries, or in lakes. There are also terrestrial deposits in this category, such as loess and clays of glacial origin.
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