Grain size

Wentworth grain size chart from United States Geological Survey Open-File Report 2006-1195

Beach cobbles at Nash Point, South Wales.

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 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) used in the United States. The Krumbein phi (φ) scale, a modification of the Wentworth scale created by W. C. Krumbein^[1] in 1937, is a logarithmic scale computed by the equation

$\phi=-\log_2{D/D_0},$

where

$\phi$ is the Krumbein phi scale,

$D$ is the diameter of the particle, and

$D_0$ is a reference diameter, equal to 1 mm (to make the equation dimensionally consistent).

This equation can be rearranged to find diameter using φ:

$D=D_0 \times 2^{-\phi}\,$

φ scale	Size range (metric)	Size range (approx. inches)	Aggregate name (Wentworth class)	Other names
<−8	>256 mm	>10.1 in	Boulder
−6 to −8	64–256 mm	2.5–10.1 in	Cobble
−5 to −6	32–64 mm	1.26–2.5 in	Very coarse gravel	Pebble
−4 to −5	16–32 mm	0.63–1.26 in	Coarse gravel	Pebble
−3 to −4	8–16 mm	0.31–0.63 in	Medium gravel	Pebble
−2 to −3	4–8 mm	0.157–0.31 in	Fine gravel	Pebble
−1 to −2	2–4 mm	0.079–0.157 in	Very fine gravel	Granule
0 to −1	1–2 mm	0.039–0.079 in	Very coarse sand
1 to 0	0.5–1 mm	0.020–0.039 in	Coarse sand
2 to 1	0.25–0.5 mm	0.010–0.020 in	Medium sand
3 to 2	125–250 µm	0.0049–0.010 in	Fine sand
4 to 3	62.5–125 µm	0.0025–0.0049 in	Very fine sand
8 to 4	3.9–62.5 µm	0.00015–0.0025 in	Silt	Mud
10 to 8	0.98–3.9 µm	3.8×10^−5–0.00015 in	Clay	Mud
20 to 10	0.95–977 nm	3.8×10^−8–3.8×10^−5 in	Colloid	Mud

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

International scale

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.^[2]

ISO 14688-1:2002

Name				Size range (mm)	Size range (approx. in)
Very coarse soil		Large boulder	LBo	>630	>24.8031
		Boulder	Bo	200–630	7.8740–24.803
		Cobble	Co	63–200	2.4803–7.8740
Coarse soil	Gravel	Coarse gravel	CGr	20–63	0.78740–2.4803
		Medium gravel	MGr	6.3–20	0.24803–0.78740
		Fine gravel	FGr	2.0–6.3	0.078740–0.24803
	Sand	Coarse sand	CSa	0.63–2.0	0.024803–0.078740
		Medium sand	MSa	0.2–0.63	0.0078740–0.024803
		Fine sand	FSa	0.063–0.2	0.0024803–0.0078740
Fine soil	Silt	Coarse silt	CSi	0.02–0.063	0.00078740–0.0024803
		Medium silt	MSi	0.0063–0.02	0.00024803–0.00078740
		Fine silt	FSi	0.002–0.0063	0.000078740–0.00024803
	Clay		Cl	≤0.002	≤0.000078740

Sorting

An accumulation of sediment can also be characterized by the grain size distribution, called sorting. According to a formula^[3] the sorting can be quantified as

φ < 0.35	0.35 < φ < 0.50	0.50 < φ < 0.71	0.71 < φ < 1.00	1.00 < φ < 2.00	2.00 < φ
very well sorted	well sorted	moderately well sorted	moderately sorted	poorly sorted	very poorly sorted

See also

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

References

1. Krumbein, W. C.; Aberdeen, Esther (April 1937). "The Sediments of Barataria Bay". Journal of Sedimentary Petrology 7 (1). Retrieved 11 May 2014. (subscription required (help)).
2. "ISO 14688-1:2002 – Geotechnical investigation and testing – Identification and classification of soil – Part 1: Identification and description". International Organization for Standardization (ISO).
3. 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. doi:10.1306/74d70646-2b21-11d7-8648000102c1865d. 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).
• 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 V. 30, 377-392 (1922).