sedimentary particles

Sieves. A more sophisticated method makes the size of an irregular particle the diameter of a spherical particle which sinks in a viscous liquid at the same speed as the irregular particle.

American Geophysical Union system

Name Diameter
in millimeters
small boulders more than 256
large cobbles 128–256
small cobbles 128–64
very coarse gravel 64–32
coarse grave; 32–16
medium gravel 16–8
fine gravel 8–2
very fine gravel 4–2
very coarse sand 2–1
coarse sand 1–0.5
medium sand 0.5–0.25
fine sand 0.25–0.125
very fine sand 0.125–0.062
coarse silt 0.062–0.031
medium silt 0.031–0.062
fine silt 0.016–0.008
very fine silt 0.008–0.004
coarse clay 0.004–0.002
medium clay 0.002–0.001
fine clay 0.001–0.0005
very fine clay less than 0.0005

Emory Wilson Lane.
Report of the subcommittee on sediment terminology. Transactions of the American Geophysical Union, volume 28? number 6, 1947.

Wentworth system

One well-known classification of "detrital particles" was published by C. K. Wentworth in 1922.

Name Diameter
in millimeters
boulder over 256
cobble 64 to 256
pebble 2 to 64
sand 1/16 to 2
silt 1/256 to 1/16
clay-sized particle 1/256

C. K. Wentworth.
Journal of Geology, volume 30, pages 377-392, 1922.

Udden system

A revision and extension of the Wentworth system.

Name Diameter
in millimeters
boulder over 256
cobble 64–256
pebble 4–64
granule 2–4
very coarse sand 1–2
medium sand 0.5–1
fine sand 0.125–0.5
very fine sand 0.0625–0.125
silt 0.0039–0.0625
clay 0.00024–0.0039
colloid less than 0.0024

Phi value

Particles are sized by the negative logarithm to the base 2 of the particle's diameter in millimeters. Users often classify particles into ½Φ classes. Some examples:

Phi scale
value
Diameter,
millimeters
-12Φ 4096
-8Φ 256
-6Φ 64
-3Φ 8
1
1/16
1/64
1/256

U. S. Dept. of Agriculture

Name Diameter
in millimeters
stones over 250
cobble 250–76
gravel 76–2
very coarse sand 2–1
coarse sand 1–0.5
medium sand 0.5–0.25
fine sand 0.25–0.10
very fine sand 0.10–0.005
coarse silt 0.005–0.002
fine silt  
clay less than 0.002

USDA. 1951 Soil Survey Manual, page 208.

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