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Grain size (or particle size) is the
diameter In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid fo ...
of individual grains of
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...
, or the
lithified Lithification (from the Ancient Greek word ''lithos'' meaning 'rock' and the Latin-derived suffix ''-ific'') is the process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock. Essentially, lithificatio ...
particles in
clastic rock Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rock ...
s. The term may also be applied to other
granular material A granular material is a conglomeration of discrete solid, macroscopic particles characterized by a loss of energy whenever the particles interact (the most common example would be friction when grains collide). The constituents that compose ...
s. This is different from the
crystallite A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. Crystallites are also referred to as grains. Bacillite is a type of crystallite. It is rodlike with parallel longulites. Stru ...
size, which refers to the size of a single
crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macr ...
inside a particle or grain. A single grain can be composed of several
crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macr ...
s. Granular material can range from very small colloidal particles, through
clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay part ...
,
silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel wh ...
,
sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a soil texture, textur ...
,
gravel Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone. Gravel is classif ...
, and cobbles, to
boulder In geology, a boulder (or rarely bowlder) is a rock fragment with size greater than in diameter. Smaller pieces are called cobbles and pebbles. While a boulder may be small enough to move or roll manually, others are extremely massive. In ...
s.


Krumbein phi scale

Size ranges define limits of classes that are given names in the Wentworth scale (or Udden–Wentworth scale) used in the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., federal district, five ma ...
. The Krumbein ''phi'' (φ) scale, a modification of the Wentworth scale created by W. C. Krumbein in 1934, is a
logarithmic scale A logarithmic scale (or log scale) is a way of displaying numerical data over a very wide range of values in a compact way—typically the largest numbers in the data are hundreds or even thousands of times larger than the smallest numbers. Such a ...
computed by the equation :\varphi=-\log_2, where :\varphi is the Krumbein phi scale, :D is the
diameter In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid fo ...
of the particle or grain in millimeters (Krumbein and Monk's equation)PetroWiki: Estimating permeability based on grain size
/ref> 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^\, In some schemes, gravel is anything larger than sand (comprising granule, pebble, cobble, and boulder in the table above).


International scale

ISO 14688-1:2002, establishes the basic principles for the identification and classification of soils on the basis of those material and mass characteristics most commonly used for soils for engineering purposes. ISO 14688-1 is applicable to natural soils ''in situ'', similar man-made materials ''in situ'' and soils redeposited by people.


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: :\sigma_I=\frac + \frac 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:


See also

* Feret diameter * Martin diameter * Orders of magnitude (volume) *
Soil texture Soil texture is a classification instrument used both in the field and laboratory to determine soil classes based on their physical texture. Soil texture can be determined using qualitative methods such as texture by feel, and quantitative methods ...
*
Substrate (biology) In biology, a substrate is the surface on which an organism (such as a plant, fungus, or animal) lives. A substrate can include biotic or abiotic materials and animals. For example, encrusting algae that lives on a rock (its substrate) can be its ...
*
Unified Soil Classification System The Unified Soil Classification System (USCS) is a soil classification system used in engineering and geology to describe the texture and grain size of a soil. The classification system can be applied to most unconsolidated materials, and is rep ...
(USCS)


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). * * {{coastal geography Sedimentary rocks Pedology