Particle size or grain size refers to the diameter of a grain of granular material, such as sediment or the lithified particles in clastic rock. Granular material can range from very small colloidal particles, through clay, silt, sand, and gravel, to boulders.
In contrast, crystallite size is the size of a single crystal inside the grain. A single grain can be composed of several crystals.
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Wentworth
The Wentworth scale or Udden-Wentworth scale is used in the United States. It names size ranges in inches.
Krumbein φ
A modification of the Wentworth scale created by W. C. Krumbein, the Krumbein phi scale, is a logarithmic scale defined by:
- D = D02 − φ
Such that:
- D is the diameter of the particle
- D0 is a reference diameter, equal to 1 mm
- φ is the phi scale
Chart
| φ scale | size range | Wentworth range | name | other names |
| -8 to -∞ | 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 |
| ∞ to 8 | 1/∞-3.9 µm | 1/∞ –0.00015 in | clay | mud |
| ∞ to 10 | 1/∞-1 µm | 1/∞–0.000039 in | colloid | mud |
In some schemes[which?] gravel is anything larger than sand and includes granule, pebble, cobble, and boulder in the above table. In the chart above, pebble covers the size range 4 to 64 mm.
See also
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Wikimedia Commons has media related to: Sediments by particle size |
- Unified Soil Classification System
- Soil texture
- Photoanalysis
- Optical granulometry
- Sandpaper uses an older system of grit size.
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 30:377–392 (1922).
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