diameter of individual grains of sediment, or of lithified particles in clastic rocks
(Redirected from Particle size (grain size))
|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||½–1 mm||0.020–0.039 in||Coarse sand|
|2 to 1||¼–½ 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||3.90625–62.5 µm||0.00015–0.0025 in||Silt||Mud|
|> 8||< 3.90625 µm||< 0.00015 in||Clay||Mud|
|>10||< 1 µm||< 0.000039 in||Colloid||Mud|
In some schemes "gravel" is anything larger than sand (>2.0 mm), and includes "granule", "pebble", "cobble", and "boulder" in the above table. In this scheme, "pebble" covers the size range 4 to 64 mm (−2 to −6 φ).
- Size ranges define limits of classes that are given names in the Wentworth scale (or Udden-Wentworth) used in the United States. The Krumbein phi (φ) scale, a modification of the Wentworth scale created by W. C. Krumbein, (Krumbein & Sloss 1963) is a logarithmic scale computed by the equation:
- is the Krumbein phi scale, and
- is the diameter of the particle
- is a reference diameter, equal to 1 mm (to make the equation dimensionally consistent.)
- 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).