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# Grain size

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### 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.

## Krumbein Phi Scale-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[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 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 4–62.5 µm 0.00015–0.0025 in Silt Mud
10 to 8 1-4 µm < 0.00015 in Clay Mud
20 to 10 1-1000 nm < 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 scale

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.[2]
ISO 14688-1[3]
Name Size range (mm)
Very coarse soil Large boulder LBo >630
Boulder Bo 200 – 630
Cobble Co 63 – 200
Coarse soil Gravel Coarse gravel CGr 20 – 63
Medium gravel MGr 6.3 – 20
Fine gravel FGr 2.0 - 6.3
Sand Coarse sand CSa 0.63 - 2.0
Medium sand MSa 0.2 - 0.63
Fine sand FSa 0.063 - 0.2
Fine soil Silt Coarse silt CSi 0.02 - 0.063
Medium silt MSi 0.0063 - 0.02
Fine silt FSi 0.002 - 0.0063
Clay Cl ≤0.002

## Sorting

An accumulation of sediment can also be characterized by the grain size distribution, called sorting. According to a formula[4] 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

## 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.org
3. ^ ISO 14688-1:2002 at scribd.com
4. ^ Folk, Robert L.; Ward, William C. (1957). "Brazos River bar: a study in the significance of grain-size parameters". Journal of Sedimentary Petrology 27 (1): 3–26.
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