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Sand is a naturally occurring, finely divided rock, comprised of particles ranging in size from 0.063 to 2 mm. An individual particle in this range size is termed a sand grain. The next smaller size class in geology is silt, which are particles below 0.063 mm down to 0.004 mm in size; the next larger size above sand is gravel, which ranges up to 64 mm (see grain size for standards in use).
The commonest constituent of sand in inland continental settings and non-tropical coastal settings, is silica (silicon dioxide), often in the form of quartz. However, the composition of sand varies according to local rock sources and conditions. Much of the fine white sand found in coral reef settings, for example, is ground-up coral (limestone) that has passed through the digestion of the parrot fish. Some locations have sands that contain iron or feldspar.
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