Editing Sediment (section)

== Classification ==
[[File:Sediment in the Gulf of Mexico.jpg|thumb|Sediment in the [[Gulf of Mexico]]]]
[[File:Sediment off the Yucatan Peninsula.jpg|thumb|Sediment off the [[Yucatán Peninsula]]]]

Sediment can be classified based on its [[Particle size (grain size)|grain size]], grain shape, and composition.

===Grain size===
{{main|Particle size (grain size)}}
{{see also|soil texture|Unified Soil Classification System}}

Sediment size is measured on a log base 2 scale, called the "Phi" scale, which classifies particles by size from "colloid" to "boulder".

{| class="wikitable"
|-
! φ scale !! Size range<br>(metric) !! Size range<br>(inches) !! Aggregate class<br>(Wentworth) !! Other names
|-
| < −8 || &gt; 256&nbsp;mm || > 10.1 in || [[Boulder]]
|-
| −6 to −8 || 64–256&nbsp;mm || 2.5–10.1 in || [[Cobble (geology)|Cobble]]
|-
| −5 to −6 || 32–64&nbsp;mm || 1.26–2.5 in || Very coarse [[gravel]] || [[Pebble]]
|-
| −4 to −5 || 16–32&nbsp;mm || 0.63–1.26 in || Coarse gravel || Pebble
|-
| −3 to −4 || 8–16&nbsp;mm || 0.31–0.63 in || Medium gravel || Pebble
|-
| −2 to −3 || 4–8&nbsp;mm || 0.157–0.31 in || Fine gravel || Pebble
|-
| −1 to −2 || 2–4&nbsp;mm || 0.079–0.157 in || Very fine gravel || [[Granule (geology)|Granule]]
|-
| 0 to −1 || 1–2&nbsp;mm || 0.039–0.079 in || Very coarse [[sand]]
|-
| 1 to 0 || 0.5–1&nbsp;mm || 0.020–0.039 in || Coarse sand
|-
| 2 to 1 || 0.25–0.5&nbsp;mm || 0.010–0.020 in || Medium sand
|-
| 3 to 2 || 125–250 [[micrometre|μm]] || 0.0049–0.010 in || Fine sand
|-
| 4 to 3 || 62.5–125&nbsp;μm || 0.0025–0.0049 in || Very fine sand
|-
| 8 to 4 || 3.9–62.5&nbsp;μm || 0.00015–0.0025 in || [[Silt]] || [[Mud]]
|-
| > 8 || < 3.9&nbsp;μm || < 0.00015 in || [[Clay]] || Mud
|-
| > 10 || < 1&nbsp;μm || < 0.000039 in || [[Colloid]] || Mud
|}

===Shape===
[[File:Rounding & sphericity EN.svg|thumb|300px|Schematic representation of difference in grain shape. Two parameters are shown: sphericity (vertical) and [[Roundness (geology)|rounding]] (horizontal).]]
The shape of particles can be defined in terms of three parameters. The ''form'' is the overall shape of the particle, with common descriptions being spherical, platy, or rodlike. The ''roundness'' is a measure of how sharp grain corners are. This varies from well-rounded grains with smooth corners and edges to poorly rounded grains with sharp corners and edges. Finally, ''surface texture'' describes small-scale features such as scratches, pits, or ridges on the surface of the grain.<ref>{{cite book |last1=Boggs |first1=Sam |title=Principles of sedimentology and stratigraphy |date=2006 |publisher=Pearson Prentice Hall |location=Upper Saddle River, N.J. |isbn=0131547283 |edition=4th |page=65}}</ref>

====Form====
{{See also|Sphericity}}
Form (also called ''sphericity'') is determined by measuring the size of the particle on its major axes. [[William C. Krumbein]] proposed formulas for converting these numbers to a single measure of form,<ref>{{cite journal |last1=Krumbein |first1=William C. |title=Measurement and Geological Significance of Shape and Roundness of Sedimentary Particles |journal=SEPM Journal of Sedimentary Research |date=1941 |volume=11 |pages=64–72 |doi=10.1306/D42690F3-2B26-11D7-8648000102C1865D}}</ref> such as

:<math>\psi_l = \sqrt[3]{\frac{D_S D_I}{D_L^2}}</math>

where <math>D_L</math>, <math>D_I</math>, and <math>D_S</math> are the long, intermediate, and short axis lengths of the particle.{{sfn|Boggs|2006|p=582}} The form <math>\psi_l</math> varies from 1 for a perfectly spherical particle to very small values for a platelike or rodlike particle.

An alternate measure was proposed by Sneed and Folk:<ref>{{cite journal |last1=Sneed |first1=Edmund D. |last2=Folk |first2=Robert L. |title=Pebbles in the Lower Colorado River, Texas a Study in Particle Morphogenesis |journal=The Journal of Geology |date=March 1958 |volume=66 |issue=2 |pages=114–150 |doi=10.1086/626490|bibcode=1958JG.....66..114S |s2cid=129658242 }}</ref>

:<math>\psi_p = \sqrt[3]{\frac{D_S^2}{D_L D_I}}</math>

which, again, varies from 0 to 1 with increasing sphericity.

====Roundness====
{{Main|Roundness (geology)}}
[[File:Rounding.gif|thumb|Comparison chart for evaluating roundness of sediment grains]]
Roundness describes how sharp the edges and corners of particle are. Complex mathematical formulas have been devised for its precise measurement, but these are difficult to apply, and most geologists estimate roundness from comparison charts. Common descriptive terms range from very angular to angular to subangular to subrounded to rounded to very rounded, with increasing degree of roundness.{{sfn|Boggs|2006|pp=66-67}}

====Surface texture====
Surface texture describes the small-scale features of a grain, such as pits, fractures, ridges, and scratches. These are most commonly evaluated on [[quartz]] grains, because these retain their surface markings for long periods of time. Surface texture varies from polished to frosted, and can reveal the history of transport of the grain; for example, frosted grains are particularly characteristic of [[Aeolian processes|aeolian]] sediments, transported by wind. Evaluation of these features often requires the use of a [[scanning electron microscope]].{{sfn|Boggs|2006|pp=68-70}}

===Composition===
Composition of sediment can be measured in terms of:
* Parent [[Rock (geology)|rock]] [[lithology]]
* [[Mineral]] composition
* [[Chemical]] make-up

This leads to an ambiguity in which [[clay]] can be used as both a size-range and a composition (see [[clay mineral]]s).