Particle size (grain size)

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

$\phi=-\log_2{D/Do},$

where

$\phi$ is the Krumbein phi scale,

$D$ is the diameter of the particle, and

$Do$ is a reference diameter, equal to 1 mm (to make the equation dimensionally consistent).

This equation can be rearranged to find diameter using φ:

$D=Do \times 2^{-\phi}\,$

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	Mud
< 8	< 3.90625 µm	< 0.00015 in	Clay	Mud
< 10	< 1 µm	< 0.000039 in	Colloid	Mud

In some schemes, gravel is anything larger than sand (comprising granule, pebble, cobble, and boulder in the table above).

References

R D Dean & R A Dalrymple, Coastal Processes with Engineering Applications (Cambridge University Press, 2002)
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 V. 30, 377-392 (1922).

Coastal geography

Landforms	Anchialine pool Avulsion Archipelago Atoll Ayre Barrier bar Barrier island Bay Baymouth bar Bight Brackish marsh Cape Channel Cliff Cliffed coast Cliff-top dune Coast Coastal plain Coastal waterfall Continental margin Continental shelf Coral reef Cove Dune Estuary Firth Fjard Fjord Flat coast Freshwater marsh Fundus Geo Graded shoreline Gulf Headland Inlet Intertidal wetland Island Islet Isthmus Lagoon Marine terrace Mouth bar Raised shorelines Machair Mega delta Mudflat Natural arch Peninsula Reef Ria River delta Rocky shore Salt marsh Shore Sound Stack Steep coast Strait Strand plain Submarine canyon Tied island Tidal island Tidal marsh Tide pool Tombolo More...

Beaches	Beach cusps Beach evolution Beach ridge Beachrock Modern recession of beaches Pocket beach Raised beach Storm beach Shingle beach

Processes	Blowhole Coastal erosion Concordant coastline Current Cuspate foreland Discordant coastline Emergent coastline Feeder bluff Fetch Headlands and bays Large scale coastal behaviour Longshore drift Marine regression Marine transgression Rip current Sea cave Sea foam Shoal Spit Submergent coastline Surf break Surf zone Surge channel Swash Volcanic arc Wave-cut platform Wave shoaling Wind wave Wrack zone

Management	Coastal management Accretion Integrated coastal zone management Submersion

Related	Bulkhead line Coastline length Intertidal zone Littoral zone Particle size boulder shingle pebble sand silt clay Physical oceanography Region of freshwater influence Scree More...

Particle size (grain size)

See also

References