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silt,predominantly quartz mineral particles that are between sand size and clay size, i.e., between 1-16 and 1-256 mm ( 1-406 – 1-6502 in.) in diameter. Silt, like clay and sand, is a product of the weathering and decomposition of preexisting rock. Hardened silt forms a sedimentary rock called siltstone, which tends to deposit in thin layers sometimes referred to as flagstone because it is hard, durable, and flat, breaking into nearly rectangular slabs.
the fine-grained, soft sediment on the bottom of bodies of water, not altered by diagenesis. Between 30 and 50 percent of the particles it contains are less than 0.01 mm in size. Occurring in a fluid state under natural conditions, it assumes the properties of a solid body when dry. On the bottom of seas and continental bodies of water (lakes, rivers, and ponds) there are silts consisting of fine-grained products of rock decomposition (terrigenous, clayey, and lime silts) and silts made up of microscopic shells or the skeletal remains of marine organisms (globigerina, diatomaceous, radiolarian, pteropod silts). Silts that have been enriched by volcanic ash (volcanic mud) form a separate category. Sometimes silts are enriched with organic matter (sapro-pel) whose decay causes hydrogen sulfide contamination or putrefaction (“putrid silt”).
Some silts, including lake, pond, and lagoon silts, are used as fertilizer and for mineral feed supplements for livestock; they are also used in medicine, for pelotherapy.