Grain size explained

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

\varphi=-log
2{D
D0
},where

\varphi

is the Krumbein phi scale,

D

is the diameter of the particle or grain in millimeters (Krumbein and Monk's equation)[2] and

D0

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=D02-\varphi

φ scaleSize range
(metric)
Size range
(approx. inches)
Aggregate name
(Wentworth class)
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 (comprising granule, pebble, cobble, and boulder in the table above).

International scale

ISO 14688-1:2017, 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.[3]

ISO 14688-1:2017
NameSize range (mm)Size range (approx. in)
Very coarse soilLarge boulder lBo>630 >24.8031
Boulder Bo200–630 7.8740–24.803
Cobble Co63–200 2.4803–7.8740
Coarse soilGravelCoarse gravel cGr20–63 0.78740–2.4803
Medium gravel mGr6.3–20 0.24803–0.78740
Fine gravel fGr2.0–6.3 0.078740–0.24803
SandCoarse sand cSa0.63–2.0 0.024803–0.078740
Medium sand mSa0.2–0.63 0.0078740–0.024803
Fine sand fSa0.063–0.2 0.0024803–0.0078740
Fine soilSiltCoarse silt cSi0.02–0.063 0.00078740–0.0024803
Medium silt mSi0.0063–0.02 0.00024803–0.00078740
Fine silt fSi0.002–0.00630.000078740–0.00024803
Clay Cl≤0.002 ≤0.000078740

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]

\sigma
I=\phi84-\phi16
4

+

\phi95-\phi5
6.6

where

\sigmaI

is the Inclusive Graphic Standard Deviation in phi units

\phi84

is the 84th percentile of the grain size distribution in phi units, etc.

The result of this can be described using the following terms:

Diameter (phi units)Description

\sigmaI

< 0.35
very well sorted
0.35 <

\sigmaI

< 0.50
well sorted
0.50 <

\sigmaI

< 1.00
moderately sorted
1.00 <

\sigmaI

< 2.00
poorly sorted
2.00 <

\sigmaI

< 4.00
very poorly sorted
4.00 <

\sigmaI

extremely poorly sorted

See also

External links

Notes and References

  1. Size frequency distributions of sediments. Krumbein. W. C.. 1934. Journal of Sedimentary Petrology . 4. 2. 10.1306/D4268EB9-2B26-11D7-8648000102C1865D.
  2. https://petrowiki.org/Estimating_permeability_based_on_grain_size PetroWiki: Estimating permeability based on grain size
  3. Web site: ISO 14688-1:2017 – Geotechnical investigation and testing – Identification and classification of soil – Part 1: Identification and description . International Organization for Standardization (ISO) .
  4. Brazos River bar: a study in the significance of grain-size parameters. Folk. Robert L.. 1957. Journal of Sedimentary Petrology. 11 May 2014. 10.1306/74d70646-2b21-11d7-8648000102c1865d. 3–26. Ward. William C.. 27. 1. 1957JSedR..27....3F. https://web.archive.org/web/20140512213708/http://www.er.uqam.ca/nobel/aqqua1/articles/Folk_Ward_27%281%29-3.pdf. 12 May 2014. dead.