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
where is the Krumbein phi scale, is the diameter of the particle or grain in millimeters (Krumbein and Monk's equation)PetroWiki: Estimating permeability based on grain size and is a reference diameter, equal to 1 mm (to make the equation dimensionally consistent). This equation can be rearranged to find diameter using φ:
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 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. + ISO 14688-1:2017Name Size range (mm) Size range (approx. in)Very coarse soil Large boulder lBo >630 >24.8031 Boulder Bo 200–630 7.8740–24.803 Cobble Co 63–200 2.4803–7.8740 Coarse soilGravel Coarse gravel cGr 20–63 0.78740–2.4803 Medium gravel mGr 6.3–20 0.24803–0.78740 Fine gravel fGr 2.0–6.3 0.078740–0.24803 Sand Coarse sand cSa 0.63–2.0 0.024803–0.078740 Medium sand mSa 0.2–0.63 0.0078740–0.024803 Fine sand fSa 0.063–0.2 0.0024803–0.0078740 Fine soilSilt Coarse silt cSi 0.02–0.063 0.00078740–0.0024803 Medium silt mSi 0.0063–0.02 0.00024803–0.00078740 Fine silt fSi 0.002–0.0063 0.000078740–0.00024803 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:
where
is the Inclusive Graphic Standard Deviation in phi units 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 < 0.35 very well sorted 0.35 < < 0.50 well sorted 0.50 < < 1.00 moderately sorted 1.00 < < 2.00 poorly sorted 2.00 < < 4.00 very poorly sorted 4.00 < extremely poorly sorted
See also Deposition (geology) Feret diameter Martin diameter Orders of magnitude (volume) Soil texture Substrate (biology) Unified Soil Classification System (USCS)
References
External links 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).