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Grain size refers to the physical dimensions of particles of Rock or other Solid . This is different from the Crystallite size, which is the size of a single crystal inside the solid (a grain can be made of several single Crystal s).

Grain sizes can range from very small Colloidal Particles , through Clay , Silt , Sand , and Gravel , to Boulder s.

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 , is a Logarithmic Scale computed by the equation:
:\phi=-\log_2(\hbox{grain size in mm}).

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).


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