Grain size


Grain 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 used in the United States. The Krumbein phi scale, a modification of the Wentworth scale created by W. C. Krumbein in 1934, is a logarithmic scale computed by the equation
where
This equation can be rearranged to find diameter using φ:
φ scaleSize range
Size range
Aggregate name
Other names
<−8>256 mm>10.1 inBoulder
−6 to −864–256 mm2.5–10.1 inCobble
−5 to −632–64 mm1.26–2.5 inVery coarse gravelPebble
−4 to −516–32 mm0.63–1.26 inCoarse gravelPebble
−3 to −48–16 mm0.31–0.63 inMedium gravelPebble
−2 to −34–8 mm0.157–0.31 inFine gravelPebble
−1 to −22–4 mm0.079–0.157 inVery fine gravelGranule
0 to −11–2 mm0.039–0.079 inVery coarse sand
1 to 00.5–1 mm0.020–0.039 inCoarse sand
2 to 10.25–0.5 mm0.010–0.020 inMedium sand
3 to 2125–250 μm0.0049–0.010 inFine sand
4 to 362.5–125 μm0.0025–0.0049 inVery fine sand
8 to 43.9–62.5 μm0.00015–0.0025 inSiltMud
10 to 80.98–3.9 μm3.8×10−5–0.00015 inClayMud
20 to 100.95–977 nm3.8×10−8–3.8×10−5 inColloidMud

In some schemes, gravel is anything larger than sand.

International scale

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.

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:
where
The result of this can be described using the following terms:
Diameter Description
< 0.35very well sorted
0.35 < < 0.50well sorted
0.50 < < 1.00moderately sorted
1.00 < < 2.00poorly sorted
2.00 < < 4.00very poorly sorted
4.00 <extremely poorly sorted