Grain size (or particle size) is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which refers to the size of a single crystal inside a particle or grain. A single grain can be composed of several crystals. Granular material can range from very small colloidal particles, through clay, silt, sand, gravel, and cobbles, to boulders.
| \varphi=-log | ||||
|
\varphi
D
D0
D=D0 ⋅ 2-\varphi
| 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 | 3.9–62.5 μm | 0.00015–0.0025 in | Silt | Mud | |
| 10 to 8 | 0.98–3.9 μm | 3.8×10−5–0.00015 in | Clay | Mud | |
| 20 to 10 | 0.95–977 nm | 3.8×10−8–3.8×10−5 in | Colloid | Mud |
In some schemes, gravel is anything larger than sand (comprising granule, pebble, cobble, and boulder in the table above).
ISO 14688-1:2017, establishes the basic principles for the identification and classification of soils on the basis of those material and mass characteristics most commonly used for soils for engineering purposes. ISO 14688-1 is applicable to natural soils in situ, similar man-made materials in situ and soils redeposited by people.[3]
| Name | Size range (mm) | Size range (approx. in) | |||
|---|---|---|---|---|---|
| Very coarse soil | Large boulder | lBo | >630 | >24.8031 | |
| Boulder | Bo | 200–630 | 7.8740–24.803 | ||
| Cobble | Co | 63–200 | 2.4803–7.8740 | ||
| Coarse soil | Gravel | Coarse gravel | cGr | 20–63 | 0.78740–2.4803 |
| Medium gravel | mGr | 6.3–20 | 0.24803–0.78740 | ||
| Fine gravel | fGr | 2.0–6.3 | 0.078740–0.24803 | ||
| Sand | Coarse sand | cSa | 0.63–2.0 | 0.024803–0.078740 | |
| Medium sand | mSa | 0.2–0.63 | 0.0078740–0.024803 | ||
| Fine sand | fSa | 0.063–0.2 | 0.0024803–0.0078740 | ||
| Fine soil | Silt | Coarse silt | cSi | 0.02–0.063 | 0.00078740–0.0024803 |
| Medium silt | mSi | 0.0063–0.02 | 0.00024803–0.00078740 | ||
| Fine silt | fSi | 0.002–0.0063 | 0.000078740–0.00024803 | ||
| Clay | Cl | ≤0.002 | ≤0.000078740 | ||
An accumulation of sediment can also be characterized by the grain size distribution. A sediment deposit can undergo sorting when a particle size range is removed by an agency such as a river or the wind. The sorting can be quantified using the Inclusive Graphic Standard Deviation:[4]
| \sigma | ||||
|
+
| \phi95-\phi5 | |
| 6.6 |
where
\sigmaI
\phi84
The result of this can be described using the following terms:
| Diameter (phi units) | Description | |
|---|---|---|
\sigmaI | very well sorted | |
| 0.35 < \sigmaI | well sorted | |
| 0.50 < \sigmaI | moderately sorted | |
| 1.00 < \sigmaI | poorly sorted | |
| 2.00 < \sigmaI | very poorly sorted | |
| 4.00 < \sigmaI | extremely poorly sorted |