Grain size

Diameter of individual grains of sediment, or of lithified particles in clastic rocks

Wentworth grain size chart from United States Geological Survey Open-File Report 2006-1195: Note size typos; 33.1mm is 38.1 & .545mm is .594

Beach cobbles at Nash Point, South Wales

Grain size (or particle size) is 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 refers to the size of a single crystal inside a particle or grain. A single grain can be composed of several crystals. Granular material can range from very small colloidal particles, through clay, silt, sand, gravel, and cobbles, to boulders.

Krumbein phi scale

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).

International scale

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]

Sorting

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:

See also

• Deposition (geology)
• Feret diameter
• Martin diameter
• Orders of magnitude (volume)
• Soil texture
• Substrate (biology)
• Unified Soil Classification System (USCS)

References

1. Krumbein, W. C. (1934). "Size frequency distributions of sediments". Journal of Sedimentary Petrology. 2 (4). doi:10.1306/D4268EB9-2B26-11D7-8648000102C1865D.
2. PetroWiki: Estimating permeability based on grain size
3. "ISO 14688-1:2017 – Geotechnical investigation and testing – Identification and classification of soil – Part 1: Identification and description". International Organization for Standardization (ISO).
4. Folk, Robert L.; Ward, William C. (1957). "Brazos River bar: a study in the significance of grain-size parameters" (PDF). Journal of Sedimentary Petrology. 27 (1): 3–26. Bibcode:1957JSedR..27....3F. doi:10.1306/74d70646-2b21-11d7-8648000102c1865d. Archived from the original (PDF) on 12 May 2014. Retrieved 11 May 2014.

External links

• 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.