Grain size

Granulometry
Granulometry
Basic concepts
	Particle size, Grain size, Size distribution, Morphology
Methods and techniques
	Mesh scale, Optical granulometry, Sieve analysis, Soil gradation
Related concepts
	Granulation, Granular material, Mineral dust, Pattern recognition, Dynamic light scattering

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.

Krumbein phi scale

Size ranges define limits of classes that are given names in the Wentworth scale (or Udden–Wentworth scale named after geologists Chester K. Wentworth and Johan A. Udden) used in the United States. The Krumbein phi (φ) scale, a modification of the Wentworth scale created by W. C. Krumbein in 1934, is a logarithmic scale computed by the equation

φ = - \log_{2} \frac{D}{D_{0}},

where

φ

is the Krumbein phi scale,

D

is the diameter of the particle or grain in millimeters (Krumbein and Monk's equation) and

D_{0}

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 = D_{0} \cdot 2^{- φ}

φ Scale			Size Range						Terminology
φ Scale			Metric			Inches (approx.)			Wentworth
≤ −8			≥ 256 mm			≥ 10.1 in			Boulder
−8	to	−7	256 mm	to	128 mm	10.1 in	to	5.04 in	Cobble
−7	to	−6	128 mm	to	64 mm	5.04 in	to	2.52 in	Cobble
−6	to	−5	64 mm	to	32 mm	2.52 in	to	1.26 in	Pebble
−5	to	−4	32 mm	to	16 mm	1.26 in	to	0.63 in
−4	to	−3	16 mm	to	8 mm	0.63 in	to	0.31 in
−3	to	−2	8 mm	to	4 mm	0.31 in	to	0.16 in
−2	to	−1	4 mm	to	2 mm	0.16 in	to	0.08 in	Granule
−1	to	0	2 mm	to	1 mm	0.08 in	to	0.04 in	Sand
0	to	1	1 mm	to	0.5 mm	0.04 in	to	0.02 in
1	to	2	0.5 mm	to	0.25 mm	0.02 in	to	0.01 in
2	to	3	250 μm	to	125 μm	0.01 in	to	0.005 in
3	to	4	125 μm	to	62.5 μm	0.005 in	to	0.0025 in
4	to	5	62.5 μm	to	31.3 μm	0.0025 in	to	0.0012 in	Silt
5	to	6	31.3 μm	to	15.6 μm	0.0012 in	to	0.0006 in
6	to	7	15.6 μm	to	7.81 μm	0.0006 in	to	0.0003 in
7	to	8	7.81 μm	to	3.91 μm	0.0003 in	to	0.00015 in
8	to	9	3.91 μm	to	1.95 μm	0.00015 in	to	7.7×10^-5 in	Clay
9	to	10	1.95 μm	to	0.98 μm	7.7×10^-5 in	to	3.8×10^-5 in	Clay
10	to	20	977 nm	to	0.95 nm	3.8×10^-5 in	to	3.8×10^-8 in	Colloid

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

International scale

ISO 14688-1:2017 establishes the basic principles for identifying and classifying soils based on those material and mass characteristics most commonly used for soils for engineering purposes. ISO 14688-1 applies to natural soils in situ, similar man-made materials in situ and soils redeposited by people.

ISO 14688-1:2017
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

Sorting

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:

σ_{I} = \frac{ϕ 84 - ϕ 16}{4} + \frac{ϕ 95 - ϕ 5}{6.6}

where

σ_{I}

is the Inclusive Graphic Standard Deviation in phi units

ϕ 84

is the 84th percentile of the grain size distribution in phi units, etc.

The result of this can be described using the following terms:

Diameter (phi units)	Description
$σ_{I}$ < 0.35	very well sorted
0.35 < $σ_{I}$ < 0.50	well sorted
0.50 < $σ_{I}$ < 1.00	moderately sorted
1.00 < $σ_{I}$ < 2.00	poorly sorted
2.00 < $σ_{I}$ < 4.00	very poorly sorted
4.00 < $σ_{I}$	extremely poorly sorted

Coastal geography
Landforms	Anchialine pool Archipelago Atoll Avulsion Ayre Barrier island Bay Bight Bodden Brackish marsh Cape Channel Cliff Coast Coastal plain Coastal waterfall Continental margin Continental shelf Coral reef Cove Dune cliff-top Estuary Firth Fjard Fjord Freshwater marsh Fundus Gat Geo Gulf Gut Hapua Headland Inlet Intertidal wetland Island Islet Isthmus Liman Lagoon Machair Mudflat Natural arch Peninsula Reef Ria Salt marsh Shoal Skerry Sound Spit Stack Strait Strand plain Submarine canyon Tidal island Tidal marsh Tide pool Tied island Tombolo Waituna Windwatt
Beaches	Beach cusps Beach evolution Beach ridge Beach wrack Beaches in estuaries and bays Beachrock Coastal morphodynamics Pocket beach Raised beach Recession Shell beach Shingle beach Storm beach Wash margin
River mouths	Debouch Mouth bar River delta mega regressive
Processes	Blowhole Cliffed coast Coastal biogeomorphology Coastal erosion Concordant coastline Current Cuspate foreland Discordant coastline Emergent coastline Feeder bluff Flat coast Graded shoreline Ingression coast Large-scale coastal behaviour Longshore drift Marine regression Marine transgression Raised shoreline Rip current Rocky shore Sea cave Sea foam Shoal peresyp Steep coast Submergent coastline Surf break Surf zone Surge channel Swash Undertow Volcanic arc Wave-cut platform Wave shoaling Wind fetch Wind wave
Management	Accretion Coastal management Integrated coastal zone management Submersion
Related	Bulkhead line Coastal engineering Grain size boulder clay cobble granule gravel pebble sand shingle silt Intertidal zone Littoral zone Physical oceanography Region of freshwater influence River plume
Category

Grain size

Krumbein phi scale

International scale

Sorting

See also