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ZACK'S ROCKS & MINERALS - Geology

Particle Size (Grain Size) Chart

Particle size, also called grain size, refers to 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 is the size of a single crystal inside the particles or grains. A single grain can be composed of several crystals. Granular material 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 (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

φ is the Krumbein phi scale,
D is the diameter of the particle,
D0 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 Size range Aggregate class Other names
(metric) (inches) (Wentworth)
< -8 > 256 mm > 10.1 in Boulder  
-6 to -8 64256 mm 2.510.1 in Cobble  
-5 to -6 3264 mm 1.262.5 in Very coarse gravel Pebble
-4 to -5 1632 mm 0.631.26 in Coarse gravel Pebble
-3 to -4 816 mm 0.310.63 in Medium gravel Pebble
-2 to -3 48 mm 0.1570.31 in Fine gravel Pebble
-1 to -2 24 mm 0.0790.157 in Very fine gravel Granule
0 to -1 12 mm 0.0390.079 in Very coarse sand  
1 to 0 0.51 mm 0.0200.039 in Coarse sand  
2 to 1 0.250.5 mm 0.0100.020 in Medium sand  
3 to 2 125250 m 0.00490.010 in Fine sand  
4 to 3 62.5125 m 0.00250.0049 in Very fine sand  
8 to 4 3.962.5 m 0.000150.0025 in Silt Mud
> 8 < 3.9 m < 0.00015 in Clay Mud
>10 < 1 m < 0.000039 in Colloid Mud

Source: Wikipedia

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