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Grain size

Diameter of individual grains of sediment, or of lithified particles in clastic rocks

Wentworth grain size chart from United States Geological Survey Open-File Report 2006-1195: Note size typos; 33.1mm is 38.1 & .545mm is .594

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 in 1934, is a logarithmic scale computed by the equation :\varphi=-\log_2{\frac{D}{D_0}}, where :\varphi is the Krumbein phi scale, :D is the diameter of the particle or grain in millimeters (Krumbein and Monk's equation) 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 \cdot 2^{-\varphi},

φ scale	Size range
(metric)	Size range
(approx. inches)	Aggregate name
(Wentworth class)	Other names
	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 14688-1:2017 establishes the basic principles for identifying and classifying soils based on those material and mass characteristics most commonly used for soils for engineering purposes. ISO 14688-1 applies to natural soils in situ, similar man-made materials in situ and soils redeposited by people.

Name	Size range (mm)	Size range (approx. in)
Very coarse soil	Large boulder	lBo
Boulder	Bo	200–630
Cobble	Co	63–200
Coarse soil	Gravel	Coarse gravel
Medium gravel	mGr	6.3–20
Fine gravel	fGr	2.0–6.3
Sand	Coarse sand	cSa
Medium sand	mSa	0.2–0.63
Fine sand	fSa	0.063–0.2
Fine soil	Silt	Coarse silt
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. 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: :\sigma_I=\frac{\phi 84 - \phi 16}{4} + \frac{\phi 95 - \phi 5}{6.6}

where

:\sigma_I is the Inclusive Graphic Standard Deviation in phi units :\phi 84 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
\sigma_I	very well sorted
0.35 \sigma_I	well sorted
0.50 \sigma_I	moderately sorted
1.00 \sigma_I	poorly sorted
2.00 \sigma_I	very poorly sorted
4.00 \sigma_I	extremely poorly sorted

References

Krumbein, W. C.. (1934). "Size frequency distributions of sediments". Journal of Sedimentary Petrology.
[https://petrowiki.org/Estimating_permeability_based_on_grain_size PetroWiki: Estimating permeability based on grain size]
"ISO 14688-1:2017 – Geotechnical investigation and testing – Identification and classification of soil – Part 1: Identification and description". International Organization for Standardization (ISO).
(1957). "Brazos River bar: a study in the significance of grain-size parameters". Journal of Sedimentary Petrology.
Folk, Robert L.. (1980). "Petrology of sedimentary rocks". Hemphill Pub. Co.

Info: Wikipedia Source

This article was imported from Wikipedia and is available under the Creative Commons Attribution-ShareAlike 4.0 License. Content has been adapted to SurfDoc format. Original contributors can be found on the article history page.

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