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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 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) used in the United States. The Krumbein phi (φ) scale, a modification of the Wentworth scale created by W. C. Krumbein in 1937, is a logarithmic scale computed by the equation
- is the Krumbein phi scale,
- is the diameter of the particle,[clarification needed] 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
|<−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).
ISO 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.
|Name||Size range (mm)||Size range (approx. in)|
|Very coarse soil||Large boulder||LBo||>630||>24.8031|
|Coarse soil||Gravel||Coarse gravel||CGr||20–63||0.78740–2.4803|
|Fine soil||Silt||Coarse silt||CSi||0.02–0.063||0.00078740–0.0024803|
An accumulation of sediment can also be characterized by the grain size distribution, called sorting. According to a formula 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|
- Orders of magnitude (volume)
- Soil texture
- Substrate (biology)
- Unified Soil Classification System (USCS)
- Martin diameter
- Feret diameter
- Krumbein, W. C.; Aberdeen, Esther (April 1937). "The Sediments of Barataria Bay". Journal of Sedimentary Petrology. 7 (1). Retrieved 11 May 2014. (Subscription required (. ))
- "ISO 14688-1:2002 – Geotechnical investigation and testing – Identification and classification of soil – Part 1: Identification and description". International Organization for Standardization (ISO).
- Folk, Robert L.; Ward, William C. (1957). "Brazos River bar: a study in the significance of grain-size parameters" (PDF). Journal of Sedimentary Petrology. 27 (1): 3–26. doi:10.1306/74d70646-2b21-11d7-8648000102c1865d. Retrieved 11 May 2014.
- 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).
- J A Udden, "Mechanical composition of clastic sediments", Bull. Geol. Soc. Am. 25, 655-744 (1914).
- C K Wentworth, "A scale of grade and class terms for clastic sediments", J. Geology V. 30, 377-392 (1922).