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(arenites, sandstones), rocks in which 50 percent or more of the mineral grains and rock fragments are between 0.05 mm and 1 mm. Loose psammites are called sand, and cemented ones are called sandstones.
There are three basic types of psammites: monomineralic (usually quartz), oligomictic (quartz-feldspar and others), and polymineralic (arkoses and graywackes). Psammites are formed chiefly by the physical weathering of rocks and subsequent transport and deposition of fragments. Thick beds of psammites of extremely varied mineral composition are usually found near mountain regions. On plains psammites generally occur in thin layers and consist almost exclusively of grains of quartz and other minerals that resist weathering. Some psammites are useful minerals (non-ore-building materials) and others contain accumulations of petroleum and gas or, more rarely, diamonds, gold, platinum, and other placer deposit metals.