Grain size (or particle size) is the diameter
of individual grains of sediment
, or the lithified
particles in clastic rock
s. The term may also be applied to other granular material
s. This is different from the crystallite
size, which refers to the size of a single crystal
inside a particle or grain. A single grain can be composed of several crystal
s. Granular material can range from very small colloidal particles
, through clay
, and cobbles
, to boulder
Krumbein phi scale
Size ranges define limits of classes that are given names in the Wentworth scale (or Udden–Wentworth scale) used in the United States
. The Krumbein ''phi'' (φ) scale, a modification of the Wentworth scale created by W. C. Krumbein
in 1934, is a logarithmic scale
computed by the equation
is the Krumbein phi scale,
is the diameter
of the particle or grain in millimeters (Krumbein and Monk's equation)
[PetroWiki: Estimating permeability based on grain size](_blank)
: 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 (comprising granule, pebble, cobble, and boulder in the table above).
ISO 14688-1:2002, establishes the basic principles for the identification and classification of soils on the basis of those material and mass characteristics most commonly used for soils for engineering purposes. ISO 14688-1 is applicable to natural soils ''in situ'', similar man-made materials ''in situ'' and soils redeposited by people.
An accumulation of sediment can also be characterized by the grain size distribution. A sediment deposit can undergo sorting when a particle size range is removed by an agency such as a river or the wind. The sorting can be quantified using the Inclusive Graphic Standard Deviation:
: is the Inclusive Graphic Standard Deviation in phi units
: is the 84th percentile of the grain size distribution in phi units, etc.
The result of this can be described using the following terms:
* Feret diameter
* Martin diameter
* Orders of magnitude (volume)
* Soil texture
* Substrate (biology)
* Unified Soil Classification System (USCS)
* R D Dean & R A Dalrymple, ''Coastal Processes with Engineering Applications'' (Cambridge University Press, 2002)
* W C Krumbein & L L Sloss, ''Stratigraphy and Sedimentation'', 2nd edition (Freeman, San Francisco, 1963).