Grain size

Particle size, also called grain size, means the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be used for other granular materials.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: 

where
 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.)
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 3.90625–62.5 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).

 References 

 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).

Category:Geology