# Particle size (grain size)

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Particle size (grain size)

Particle size, also called grain size, refers to the diameter of 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 crystallite size, which is the size of a single crystal inside the particles or grains. A single grain can be composed of several crystals. Granular material can 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" (&phi;) scale, a modification of the Wentworth scale created by W. C. Krumbein, (Krumbein & Sloss 1963) is a logarithmic scale computed by the equation:$phi=-log_2\left\{D/Do\right\},,$where:$phi$ is the Krumbein phi scale,:$D$ is the diameter of the particle,:$Do$ is a reference diameter, equal to 1 mm to make the equation dimensionally consistent.This equation can be rearranged to find diameter using &phi;::$D=Do imes 2^\left\{-phi\right\},.$

In some schemes "gravel" is anything larger than sand (&gt;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 &phi;).

ee also

*The Unified Soil Classification System (USCS)
*Soil texture

References

*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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