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Grain size

From Academic Kids

This article deals with grain size in the context of geology, see crystallite for grain size in materials science.

Grain size refers to the physical dimensions of particles of rock or other solid. This is different from the crystallite size, which is the size of a single crystal inside the solid (a grain can be made of several single crystals).

Grain sizes can range from very small colloidal particles, through clay, silt, sand, and gravel, to boulders.

Size ranges define limits of classes that are given names in the Wentworth scale used in the United States. The Krumbein phi (φ) scale, a modification of the Wentworth scale created by W. C. Krumbein, is a logarithmic scale computed by the equation:

<math>\phi=-\log_2(\hbox{grain size in mm})<math>.
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 –1 mm 0.020–0.039 in Coarse sand
2 to 1 – mm 0.010–0.020 in Medium sand
3 to 2 125–250 m 0.0049–0.010 in Fine sand
4 to 3 62.5–125 m 0.0025–0.0049 in Very fine sand
8 to 4 3.90625–62.5 m 0.00015–0.0025 in Silt
> 8 < 3.90625 m < 0.00015 in Clay
 
<10 < 1 m < 0.000039 in Colloid

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).da:Jordpartikler ja:粒径 de:Korngre

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