- Particle size (grain size)
Particle size, also called grain size, refers to the
diameterof individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallitesize, which is the size of a single crystalinside the particles or grains. A single grain can be composed of several crystals. Granular materialcan range from very small colloidal particles, through clay, silt, sand, and gravel, to boulders.
Size ranges define limits of classes that are given names in the Wentworth scale (or Udden-Wentworth) used in the
United States. The Krumbein "phi" (φ) scale, a modification of the Wentworth scale created by W. C. Krumbein, (Krumbein & Sloss 1963) is a logarithmic scalecomputed by the equation:where: is the Krumbein phi scale,: is the diameterof the particle,: is a reference diameter, equal to 1 mm to make the equation dimensionally consistent.This equation can be rearranged to find diameter using φ::
In some schemes "gravel" is anything larger than sand (>2.0 mm), and includes "granule", "pebble", "cobble", and "boulder" in the above table. In this scheme, "pebble" covers the size range 4 to 64 mm (-2 to -6 φ).
Unified Soil Classification System(USCS)
*W C Krumbein & L L Sloss, "Stratigraphy and Sedimentation", 2nd edition (Freeman, San Francisco, 1963).
*J A Udden, "Mechanical composition of clastic sediments", "Bull. Geol. Soc. Am." 25:655–744 (1914).
*C K Wentworth, "A scale of grade and class terms for clastic sediments", "J. Geology" 30:377–392 (1922).
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