Particle size (grain size)

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

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

United States

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, (Krumbein & Sloss 1963) is a logarithmic scale computed by the equation

ϕ = − log ₂D / Do,

where

ϕ is the Krumbein phi scale,

D is the diameter of the particle, and

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

This equation can be rearranged to find diameter using φ:

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

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	½–1 mm	0.020–0.039 in	Coarse sand
2 to 1	¼–½ 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	3.90625–62.5 µm	0.00015–0.0025 in	Silt	Mud
< 8	< 3.90625 µm	< 0.00015 in	Clay	Mud
< 10	< 1 µm	< 0.000039 in	Colloid	Mud

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

International

ISO 14688-1, 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 man.

ISO 14688-1

name			size range
Very coarse soil		Large boulder, LBo	>630 mm
		Boulder, Bo	>200 - 630 mm
		Cobble, Co	>63 - 200 mm
Coarse soil	Gravel	Coarse gravel, CGr	>20 - 63 mm
		Medium gravel, MGr	>6.3 - 20 mm
		Fine gravel, FGr	>2.0 - 6.3 mm
	Sand	Coarse sand, CSa	>0.63 - 2.0 mm
		Medium sand, MSa	>0.2 - 0.63 mm
		Fine sand, FSa	>0.063 - 0.2 mm
Fine soil	Silt	Coarse silt, CSi	>0.02 - 0.063 mm
		Medium silt, MSi	>0.0063 - 0.02 mm
		Fine silt, FSi	>0.002 - 0.0063 mm
	Clay, Cl		≤0.002 mm

See also

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

References

• 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).