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particle size (grain size) The diameter or volume of the grains in a sediment or sedimentary rock. The particle size can be determined by sieving, by measuring the settling velocity, or (for pebbles, boulders, and cobbles) by direct measurement of individual clasts. The smaller particles are normally defined by their volume diameters, i.e. the diameter of a sphere with the same volume as the particle. There are numerous particle-size classifications. Two commonly used ones are the Udden—Wentworth scale (Wentworth scale) and the British standard classification. In the Udden—Wentworth scale the sediment grades are: more than 256 mm, boulder; 64–256 mm, cobble; 2–64mm, pebble; 62.5–2000 μm, sand; 4–62.5 μm, silt; less than 4 μm, clay. In the British classification: more than 200mm, boulder; 60–200 mm, cobble; 2–60 mm, gravel; 600–2000 μm, coarse sand; 200–600 μm, medium sand; 60–200 μm, fine sand; 2–60 μm, silt; less than 2 μm, clay. See PHI SCALE.