Grain size

{{short description|Diameter of individual grains of sediment, or of lithified particles in clastic rocks}}

{{distinguish|text=crystallite size, which is referred to as "grain sizes" by metallurgists}}

File:Cobbles Nash Point.jpg, South Wales]]

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 <span class="anchor" id="Udden–Wentworth scale"></span>

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{{Cite journal |title = Size frequency distributions of sediments|last = Krumbein|first = W. C.|date = 1934|journal = Journal of Sedimentary Petrology |issue = 4|volume = 2|doi=10.1306/D4268EB9-2B26-11D7-8648000102C1865D}} in 1934, is a logarithmic scale computed by the equation

: $\varphi=-\log_2{\frac{D}{D_0}},$

where

: $\varphi$ is the Krumbein phi scale,

: $D$ is the diameter of the particle or grain in millimeters (Krumbein and Monk's equation)[https://petrowiki.org/Estimating_permeability_based_on_grain_size PetroWiki: Estimating permeability based on grain size] 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:

: $D=D_0 \cdot 2^{-\varphi}\,$

class="wikitable"
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 \| 0.5–1 mm \| 0.020–0.039 in \| Coarse sand \|
2 to 1 \| 0.25–0.5 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 \| 3.9–62.5 μm \| 0.00015–0.0025 in \| Silt \| Mud
10 to 8 \| 0.98–3.9 μm \| 3.8×10⁻⁵–0.00015 in \| Clay \| Mud
20 to 10 \| 0.95–977 nm \| 3.8×10⁻⁸–3.8×10⁻⁵ in \| Colloid \| Mud

class="wikitable"

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

| 0.5–1 mm

| 0.020–0.039 in

| Coarse sand

2 to 1

| 0.25–0.5 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

| 3.9–62.5 μm

| 0.00015–0.0025 in

| Silt

| Mud

10 to 8

| 0.98–3.9 μm

| 3.8×10⁻⁵–0.00015 in

| Clay

| Mud

20 to 10

| 0.95–977 nm

| 3.8×10⁻⁸–3.8×10⁻⁵ in

| Colloid

| Mud

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.{{cite web |url=https://www.iso.org/standard/66345.html |title=ISO 14688-1:2017 – Geotechnical investigation and testing – Identification and classification of soil – Part 1: Identification and description |publisher=International Organization for Standardization (ISO) }}

class="wikitable" \|+ ISO 14688-1:2017
colspan=4 \|Name ! Size range (mm) ! Size range (approx. in)
colspan=2 rowspan=3\|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
rowspan=6\|Coarse soil \| rowspan=3\|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
rowspan=3\|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
rowspan=4\|Fine soil \| rowspan=3\|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
colspan=2\|Clay \| Cl \| ≤0.002 \| ≤0.000078740

class="wikitable"

|+ ISO 14688-1:2017

colspan=4 |Name

! Size range (mm)

! Size range (approx. in)

colspan=2 rowspan=3|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

rowspan=6|Coarse soil

| rowspan=3|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

rowspan=3|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

rowspan=4|Fine soil

| rowspan=3|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

colspan=2|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:{{Cite journal|url = http://www.er.uqam.ca/nobel/aqqua1/articles/Folk_Ward_27%281%29-3.pdf|title = Brazos River bar: a study in the significance of grain-size parameters|last1 = Folk|first1 = Robert L.|date = 1957|journal = Journal of Sedimentary Petrology|access-date = 11 May 2014|doi = 10.1306/74d70646-2b21-11d7-8648000102c1865d|pages = 3–26|last2 = Ward|first2 = William C.|volume = 27|issue = 1|bibcode = 1957JSedR..27....3F|archive-url = https://web.archive.org/web/20140512213708/http://www.er.uqam.ca/nobel/aqqua1/articles/Folk_Ward_27%281%29-3.pdf|archive-date = 12 May 2014|url-status = dead}}

: $\sigma_I=\frac{\phi 84 - \phi 16}{4} + \frac{\phi 95 - \phi 5}{6.6}$

where

: $\sigma_I$ is the Inclusive Graphic Standard Deviation in phi units

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

The result of this can be described using the following terms:{{Cite book |last=Folk |first=Robert L. |title=Petrology of sedimentary rocks |date=1980 |publisher=Hemphill Pub. Co |isbn=978-0-914696-14-8 |location=Austin, Tex}}

class="wikitable"
Diameter (phi units) ! Description
style="text-align:center" \| $\sigma_I$ < 0.35 \| very well sorted
style="text-align:center" \| 0.35 < $\sigma_I$ < 0.50 \| well sorted
style="text-align:center" \| 0.50 < $\sigma_I$ < 1.00 \| moderately sorted
style="text-align:center" \| 1.00 < $\sigma_I$ < 2.00 \| poorly sorted
style="text-align:center" \| 2.00 < $\sigma_I$ < 4.00 \| very poorly sorted
style="text-align:center" \| 4.00 < $\sigma_I$ \| extremely poorly sorted

class="wikitable"

Diameter (phi units)

! Description

style="text-align:center" |

\sigma_I

< 0.35

| very well sorted

style="text-align:center" | 0.35 <

\sigma_I

< 0.50

| well sorted

style="text-align:center" | 0.50 <

\sigma_I

< 1.00

| moderately sorted

style="text-align:center" | 1.00 <

\sigma_I

< 2.00

| poorly sorted

style="text-align:center" | 2.00 <

\sigma_I

< 4.00

| very poorly sorted

style="text-align:center" | 4.00 <

\sigma_I

| extremely poorly sorted

References

External links

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).
{{cite journal|last1=Udden|first1=J. A.|title=Mechanical composition of clastic sediments|journal=Geological Society of America Bulletin|date=1914|volume=25|issue=1|pages=655–744|doi=10.1130/GSAB-25-655|bibcode=1914GSAB...25..655U}}
{{cite journal|last1=Wentworth|first1=C. K.|title=A Scale of Grade and Class Terms for Clastic Sediments|journal=The Journal of Geology|date= 1922|volume=30|issue=5|pages=377–392|jstor=30063207|doi=10.1086/622910|bibcode=1922JG.....30..377W|s2cid=128682870|url=https://zenodo.org/record/1431467}}

Category:Sedimentary rocks

Category:Pedology

Grain size

Krumbein phi scale <span class="anchor" id="Udden–Wentworth scale"></span>

International scale

Sorting

See also

References

External links