Science Fair Project Encyclopedia
Grain size
- 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:
- φ = - log2(grain size in mm).
| 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 | 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 | |
| > 8 | < 3.90625 µm | < 0.00015 in | Clay | |
| <10 | < 1 µm | < 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).
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