Grain size

Size ranges define limits of classes that are given names in the Wentworth scale (or Udden–Wentworth scale named after geologists Chester K. Wentworth and Johan A. Udden) used in the United States. The Krumbein phi (φ) scale, a modification of the Wentworth scale created by W. C. Krumbein^[1] in 1934, is a logarithmic scale computed by the equation

${\displaystyle \varphi =-\log _{2}{\frac {D}{D_{0}}},}$ 

where

${\displaystyle \varphi }$  is the Krumbein phi scale,

${\displaystyle D}$  is the diameter of the particle or grain in millimeters (Krumbein and Monk's equation)^[2] and

${\displaystyle D_{0}}$  is a reference diameter, equal to 1 mm (to make the equation dimensionally consistent).

This equation can be rearranged to find diameter using φ:

${\displaystyle D=D_{0}\cdot 2^{-\varphi }\,}$ 

In some schemes, gravel is anything larger than sand (comprising granule, pebble, cobble, and boulder in the table above).

ISO 14688-1:2017, establishes the basic principles for identifying and classifying soils based on those material and mass characteristics most commonly used for soils for engineering purposes. ISO 14688-1 applies to natural soils in situ, similar man-made materials in situ and soils redeposited by people.^[3]

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:^[4]

${\displaystyle \sigma _{I}={\frac {\phi 84-\phi 16}{4}}+{\frac {\phi 95-\phi 5}{6.6}}}$ 

where

${\displaystyle \sigma _{I}}$  is the Inclusive Graphic Standard Deviation in phi units

${\displaystyle \phi 84}$  is the 84th percentile of the grain size distribution in phi units, etc.

The result of this can be described using the following terms:

Diameter (phi units)	Description
${\displaystyle \sigma _{I}}$  < 0.35	very well sorted
0.35 < ${\displaystyle \sigma _{I}}$  < 0.50	well sorted
0.50 < ${\displaystyle \sigma _{I}}$  < 1.00	moderately sorted
1.00 < ${\displaystyle \sigma _{I}}$  < 2.00	poorly sorted
2.00 < ${\displaystyle \sigma _{I}}$  < 4.00	very poorly sorted
4.00 < ${\displaystyle \sigma _{I}}$	extremely poorly sorted

• Deposition (geology)
• 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).
• Udden, J. A. (1914). "Mechanical composition of clastic sediments". Geological Society of America Bulletin. 25 (1): 655–744. Bibcode:1914GSAB...25..655U. doi:10.1130/GSAB-25-655.
• Wentworth, C. K. (1922). "A Scale of Grade and Class Terms for Clastic Sediments". The Journal of Geology. 30 (5): 377–392. Bibcode:1922JG.....30..377W. doi:10.1086/622910. JSTOR 30063207. S2CID 128682870.