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Grain size
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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
Wentworth grain size chart from United States Geological Survey Open-File Report 2006-1195: Note size typos; 33.1mm is 38.1 & .545mm is .594
Beach cobbles at Nash Point, 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

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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[1] in 1934, is a logarithmic scale computed by the equation

where

is the Krumbein phi scale,
is the diameter of the particle or grain in millimeters (Krumbein and Monk's equation)[2] and
is a reference diameter, equal to 1 mm (to make the equation dimensionally consistent).

This equation can be rearranged to find diameter using φ:

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

[edit]

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.[3]

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

[edit]

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:[4]

where

is the Inclusive Graphic Standard Deviation in phi units
is the 84th percentile of the grain size distribution in phi units, etc.

The result of this can be described using the following terms:[5]

Diameter (phi units) Description
< 0.35 very well sorted
0.35 < < 0.50 well sorted
0.50 < < 1.00 moderately sorted
1.00 < < 2.00 poorly sorted
2.00 < < 4.00 very poorly sorted
4.00 < extremely poorly sorted

See also

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Grain size

View on Grokipedia
from Grokipedia
Grain size refers to the of individual particles, or clasts, in clastic sediments and sedimentary rocks, serving as a primary characteristic for classifying these materials and interpreting their depositional history. In geological contexts, it is typically measured as the average size of grains within a sample, influencing properties such as , permeability, and transportability by , , or ice. The classification of grain size follows standardized scales, with the widely used Wentworth scale dividing sediments into categories from clay (<0.004 mm) to boulders (>256 mm), based on phi units (φ = -log₂(d), where d is in mm) for precise logarithmic grading. This scale, established in , facilitates consistent description across studies and highlights how finer grains like (0.004–0.0625 mm) form in low-energy environments, while coarser ones like (>2 mm) indicate high-energy deposition. Grain size analysis is crucial for reconstructing paleoenvironments, as it correlates with the of transporting media—finer distributions suggest calm conditions, such as lakes, whereas coarser, poorly sorted grains point to rapid deposition in rivers or glaciers. techniques and statistical parameters are detailed in subsequent sections. Beyond , grain size governs diagenetic processes, affecting rock strength and hydrocarbon reservoir quality in .

Fundamentals

Definition

Grain size refers to the average diameter of individual particles, or clasts, in unconsolidated sediments or lithified clastic rocks. This parameter characterizes the texture of sedimentary materials derived from the mechanical breakdown and of pre-existing rocks or minerals. It is distinct from crystallite size, which denotes the dimensions of individual crystalline domains within a single grain, often relevant in or of polycrystalline structures. In sedimentary , grain size focuses on the overall particle dimensions rather than internal crystalline substructures. The term applies to a range of granular materials in sedimentary environments, including clay, silt, sand, gravel, cobbles, and boulders, which form through physical and deposition. It does not extend to crystalline rocks, such as igneous or metamorphic types, where grain size conventionally describes the size of constituent formed during solidification or recrystallization. For macroscopic grains, measurements are typically expressed in millimeters (mm).

Importance

Grain size analysis plays a central role in by providing insights into the transport distance of sediments, the energy levels of depositional environments, and the characteristics of source materials. Coarser grains typically indicate short transport distances and high-energy settings, such as riverbeds or beaches, while finer grains suggest longer transport and lower-energy environments like deep basins. This fundamental property influences sediment entrainment, , and deposition processes, allowing geologists to reconstruct the dynamics of ancient sedimentary systems. In and , grain size significantly impacts and properties, including permeability, , and potential. Larger grain sizes enhance permeability by creating wider pore spaces, facilitating fluid flow in aquifers and influencing movement. remains relatively independent of individual grain size but is affected by grain size distribution, while finer grains increase susceptibility under hydraulic stress, affecting stability and embankment design. These factors are critical for managing and preventing geotechnical failures. Environmentally, grain size classification aids in analyzing beach sands, riverbeds, and wind-blown deposits to reconstruct paleoenvironments. Variations in grain size distributions reveal past changes in energy and depositional processes, such as shifts from fluvial to aeolian conditions in deposits. This approach helps interpret histories and evolution over geological timescales. Economically, grain size influences oil reservoir quality by determining and permeability; coarser sands typically yield higher recovery rates due to better fluid connectivity. In construction, aggregate grain size selection for affects cohesion and strength, with finer grains enhancing mixture cohesion but potentially reducing permeability in specialized applications like permeable pavements. Grain size also relates to sorting, which together assesses overall maturity in depositional sequences.

Measurement Methods

Field Techniques

Field techniques for assessing grain size in natural sediment settings emphasize non-destructive, on-site methods suitable for immediate evaluation without sample extraction. These approaches are particularly valuable in reconnaissance surveys of rivers, beaches, and exposed deposits, where rapid assessment informs broader geological or environmental studies. Visual estimation relies on comparison to standardized charts or direct observation with hand lenses to categorize grains roughly, such as distinguishing sand (0.0625–2 mm) from gravel (>2 mm). Geologists use portable grain size charts, often based on the Wentworth scale, to match sediment appearance against visual references for quick classification during fieldwork. Hand lenses (typically 10x magnification) aid in inspecting finer fractions like sand on site, though precision is limited to broad categories. This method, described in early field geology manuals, allows for subjective but efficient categorization in diverse terrains. For coarser sediments like and gravel, the Wolman pebble count method involves systematically selecting and measuring the intermediate (b-) axis of 100 or more particles along transects using or a gravelometer template. The gravelometer, a portable tool with apertures corresponding to classes (e.g., from 2 mm to 256 mm), enables direct by passing or fitting particles through slots, commonly applied in riverbeds and beaches. This technique, introduced in , provides a representative distribution for coarse fractions without full sieving. In situ sieving uses portable sieve sets to process loose surface sediments directly in the field, particularly for sand and coarse sand fractions on beaches or riverbars. Operators collect small volumes of unconsolidated material (e.g., via scoops or syringes) and pass it through stacked sieves of decreasing sizes (e.g., 0.063 mm to 4 mm), weighing retained fractions on portable balances to derive distributions. This approach suits accessible, dry-to-moist environments and is often combined with pebble counts for mixed beds. Photographic analysis captures images of exposed sediment surfaces using rulers or grids for scale, followed by digital processing to measure grain dimensions. Field photos of gravel or sand patches are analyzed with software like ImageJ, where grains are outlined or segmented to compute sizes via pixel calibration, offering a non-contact alternative for larger areas. Recent advancements as of 2025 include machine learning-based object detection for automated grain sizing from images, unmanned aerial vehicle (UAV) photogrammetry for mapping gravel-bed rivers, and hand-held devices like Instagrain, which provide instant (~2 seconds) grain size measurements using on-device AI. These methods, refined in studies of streambeds and coastal sediments, automate sizing for hundreds of particles but require clear visibility and post-processing. These techniques face limitations in accuracy due to environmental factors, such as wet or compacted sediments obscuring grain boundaries, and challenges with heterogeneous mixtures where fractions (<0.063 mm) evade detection. Visual and photographic methods are prone to operator bias and reduced precision in turbid or vegetated settings, making them best for preliminary surveys rather than definitive analysis; validation often complements laboratory methods for finer resolutions.

Laboratory Techniques

Laboratory techniques for grain size analysis provide high-precision quantification of particle size distributions in sediment samples, typically involving controlled separation and measurement processes under standardized conditions. These methods are essential for obtaining detailed histograms or cumulative curves that describe the proportions of different size fractions, often achieving resolutions down to micrometers. Sample preparation is a critical initial step to ensure accurate representation of individual grains without alteration. This includes disaggregation using mechanical or chemical means to break apart aggregates, oven-drying at low temperatures (e.g., 60–105°C) to remove moisture while preserving grain integrity, and organic removal via hydrogen peroxide treatment or sieving to eliminate biogenic material that could bias size measurements. Dry sieving is widely used for coarser fractions, such as sands ranging from 0.063 to 2 mm. It involves stacking a series of mesh screens with progressively smaller apertures (e.g., ASTM or ISO standard sizes) atop a receiver pan, placing the dried sample on the top sieve, and mechanically shaking the stack for a set duration (typically 10–15 minutes) using a sieve shaker. Retained material on each sieve is then weighed to determine the weight percentage of each size fraction, yielding a discrete distribution. For finer, cohesive materials like silts that may form aggregates, wet sieving employs water or dispersants to suspend particles during separation. The prepared sample is soaked and gently agitated in water, then passed through a stack of sieves (often a single 63 μm mesh to isolate sands from fines), with continuous rinsing to ensure complete disaggregation. The coarse fraction is dried and weighed, while the suspended fines proceed to further analysis; this method minimizes clogging and improves accuracy for particles below 63 μm. Sedimentation analysis targets particles smaller than 0.063 mm, relying on gravitational settling in a liquid medium. The pipette method withdraws subsamples at specific depths and times from a settling column containing dispersed fines, with particle concentration measured gravimetrically to infer sizes via settling velocities. The hydrometer method similarly monitors density changes over time using a hydrometer immersed in the suspension. Both approaches are based on , which relates settling velocity to particle diameter, fluid viscosity, and density differences, enabling calculation of size distributions for silts and clays. Modern instrumental techniques extend analysis across broader ranges with greater efficiency. Laser diffraction measures the full spectrum from 0.001 to 2000 μm by dispersing particles in a fluid and analyzing the angular distribution of scattered laser light, which correlates to size via Mie or Fraunhofer theory approximations; this non-destructive method provides rapid, continuous distributions suitable for both coarse and fine fractions. For very fine grains (e.g., clays <2 μm), scanning electron microscopy (SEM) offers direct imaging of individual particles, where high-resolution scans allow manual or automated measurement of dimensions, often combined with energy-dispersive X-ray spectroscopy for compositional context. As of 2025, X-ray microtomography (micro-CT) enables non-destructive, high-resolution (micrometer-scale) 3D mapping of grain sizes in sediment cores, facilitating upscale predictions without sample alteration.

Classification Scales

Wentworth Scale

The Wentworth scale is a linear classification system for describing the grain sizes of clastic sediments, developed as a refinement of earlier work by J.A. Udden. Udden introduced the foundational geometric progression for sediment grading in his 1898 publication, The Mechanical Composition of Wind Deposits, which emphasized measurable particle diameters to categorize wind-blown materials. In 1922, Chester K. Wentworth expanded and standardized this approach in his seminal paper, A Scale of Grade and Class Terms for Clastic Sediments, tailoring it specifically for geological applications in describing unconsolidated deposits. Wentworth's version adopted a consistent geometric ratio (primarily powers of 2) to define boundaries in millimeters, ensuring precise and reproducible categorization across sediment types. The scale delineates sediment classes based on the longest axis or nominal diameter of particles, providing descriptive terms that directly correspond to metric measurements. Key categories include:
CategorySubcategorySize Range (mm)
-< 0.004
-0.004–0.0625
Very coarse1–2
Coarse0.5–1
Medium0.25–0.5
Fine0.125–0.25
Very fine0.0625–0.125
Granules2–4
Pebbles4–64
Cobbles64–256
Boulders-> 256
These boundaries facilitate straightforward field and lab identification without requiring complex computations. A primary advantage of the Wentworth scale is its intuitiveness for non-specialists, as the terms (e.g., "fine sand" or "pebbles") evoke visual and tactile familiarity while being anchored to explicit millimeter limits, promoting consistent communication in geological reporting. This direct linkage to metric units avoids ambiguity in measurements, making it particularly suitable for practical sediment descriptions. It has become the standard reference in sedimentology for classifying unconsolidated deposits, such as those in fluvial, beach, or glacial environments, and is routinely applied in studies of sediment transport and deposition. This linear scale can be adapted into logarithmic scales for statistical analysis of grain size distributions.

Krumbein Phi Scale

The Krumbein phi scale, introduced by W. C. Krumbein in , transforms grain diameters into a to normalize the typically skewed distributions of sizes, facilitating their statistical analysis in . This approach addresses the limitations of linear measurements by converting them into dimensionless units that approximate normal distributions for arithmetic operations. The scale is defined by the formula
ϕ=−log⁡2D,\phi = -\log_2 D,
where DD is the grain diameter in millimeters and the reference diameter D0=1D_0 = 1 mm corresponds to ϕ=0\phi = 0. The inverse calculation yields the diameter as
D=2−ϕ.D = 2^{-\phi}.
This logarithmic base-2 transformation ensures that each unit increase in ϕ\phi represents a halving of the grain size, providing a uniform progression across orders of magnitude. For instance, a 2 mm grain corresponds to ϕ=−1\phi = -1, while a 0.5 mm grain is ϕ=1\phi = 1. A key benefit of the phi scale is its compatibility with standard statistical techniques; sediment size data in phi units can be treated as normally distributed, allowing straightforward computation of parameters like the grain size and standard deviation to quantify and sorting. This enables quantitative comparisons of sediment populations, such as assessing transport history or depositional environments, without the distortions inherent in linear scales. The phi scale categorizes grains from coarse boulders to fine colloids, with negative values for larger particles and positive values for smaller ones. It builds on earlier descriptive categories by assigning precise phi intervals. The table below outlines the primary size classes, their phi ranges, corresponding diameters, and names, extending from boulders (less than -8 ϕ\phi) to colloids (greater than 10 ϕ\phi):
ϕ\phi RangeDiameter (mm)Class Name
< -8> 256
-8 to -664 to 256Cobble
-6 to -416 to 64
-4 to -24 to 16Coarse gravel
-2 to -12 to 4Granule
-1 to 01 to 2Very coarse sand
0 to 10.5 to 1Coarse sand
1 to 20.25 to 0.5Medium sand
2 to 30.125 to 0.25Fine sand
3 to 40.0625 to 0.125Very fine sand
4 to 80.0039 to 0.0625
8 to 100.00098 to 0.0039Clay
> 10< 0.00098Colloid
These classes provide a framework for classifying sediments, with very coarse sand spanning 0 to 1 ϕ\phi (1 to 0.5 mm) and clay encompassing sizes finer than 8 ϕ\phi (less than 0.004 mm).

International Scale

The International Scale refers to the standardized classification system for soil particle sizes outlined in ISO 14688-1:2017, which provides rules for the identification and description of soils in geotechnical investigations. This standard establishes a flexible framework applicable to natural soils, man-made materials in situ, and redeposited soils, prioritizing characteristics relevant to engineering behavior such as particle size distribution, composition, and structure. It is designed for use by experienced practitioners in field and laboratory settings to ensure consistent terminology across international projects. Soils are broadly divided into very fine soil, fine soil, and coarse soil based on particle diameters, with further subdivisions for precision in engineering assessments. The categories and their size ranges are as follows:
CategorySubcategorySize Range (mm)
Very fine soilClay≤ 0.002
Silt> 0.002 to ≤ 0.063
Fine soil> 0.063 to ≤ 2
Coarse soil> 2 to ≤ 63
Cobbles> 63 to ≤ 200
Boulders> 200
This classification overlaps with the Wentworth scale for sands and gravels but differs in boundaries, such as the gravel upper limit at 63 mm versus 64 mm in Wentworth, reflecting an engineering focus on practical soil behavior over geological sediment transport dynamics. In , the scale supports critical applications including foundation design, where grain size influences and settlement predictions, and contamination assessment, aiding in the evaluation of pollutant migration through pores. The 2017 revision, superseding the 2002 edition and incorporating a amendment, introduces finer distinctions between organic and inorganic fractions to better account for their impacts on plasticity and strength.

Sediment Characteristics

Sorting

Sorting refers to the degree of variation in sizes within a sample, which provides insights into the processes that deposited the . Well-sorted sediments exhibit a narrow range of sizes, typically resulting from prolonged transport and selective deposition by agents like or waves that separate particles based on size and . In contrast, poorly sorted sediments contain a wide range of sizes, often from rapid deposition with minimal sorting. Sorting is measured using cumulative frequency curves derived from data, where grain sizes are plotted in (φ) units on a probability scale. The key statistic is the inclusive graphic standard deviation (σᵢ), which quantifies the spread of the distribution by emphasizing the tails. The formula is: σi=ϕ84−ϕ164+ϕ95−ϕ56.6\sigma_i = \frac{\phi_{84} - \phi_{16}}{4} + \frac{\phi_{95} - \phi_5}{6.6} where φ₈₄, φ₁₆, φ₉₅, and φ₅ are the grain sizes at the 84th, 16th, 95th, and 5th percentiles, respectively, from the -scale distribution. This measure depends on the scale for logarithmic transformation of grain diameters. Sediments are categorized by σᵢ values as follows:
Categoryσᵢ (φ units)
Very well sorted< 0.35
Well sorted0.35–0.50
Moderately sorted0.50–0.71
Poorly sorted0.71–1.00
Very poorly sorted1.00–2.00
Extremely poorly sorted> 2.00
Good sorting arises from selective transport mechanisms, such as aeolian or wave action, which preferentially move and deposit grains of similar sizes. Poor sorting, conversely, results from processes like debris flows or deposition, where diverse grain sizes are moved and deposited together without significant segregation. Grain shape refers to the overall form of sedimentary particles, characterized by their angularity and three-dimensional geometry, such as spherical, equant, oblate (platy), prolate (elongated), or bladed forms. These shapes influence particle packing density and fluid permeability in sediments; for instance, more equant or spherical grains allow for looser arrangements with higher void spaces, while platy or bladed shapes can restrict flow paths. Zingg's classification system, based on ratios of the three principal axes (longest, intermediate, and shortest diameters), categorizes particles into these classes to assess their morphological variability. Advanced measurements employ Fourier analysis of particle outlines to quantify shape complexity beyond simple axial ratios. Roundness describes the degree of edge smoothing on grains, resulting from abrasion during transport and , with angular grains retaining sharp corners near their source and progressively rounding downstream. It is quantitatively assessed using Wadell's index, defined as the ratio of the average at particle corners to the radius of the largest inscribed , providing a precise measure of surface modification. Visual estimation often relies on Powers' scale, which divides roundness into six categories from very angular (sharp edges) to well-rounded (smooth, sub-spherical), facilitating rapid field assessments of maturity. Sphericity measures how closely a grain approximates a , independent of size or roundness, and affects hydraulic equivalence and behavior in fluids. It is calculated as ψ = \frac{\pi^{1/3} (6V)^{2/3}}{S}, where V is the particle and S is its surface area, yielding values approaching 1 for perfect spheres and lower for irregular forms; this index highlights deviations in surface-to- ratios that influence drag and transport dynamics. These properties interact synergistically with size: finer, rounded, and high- grains promote greater by reducing and allowing efficient packing with more interstitial space, as evidenced by empirical relations showing porosity increasing with sphericity. Conversely, angular shapes enhance particle , boosting and stability in applications like , though they may lower porosity in dense packs. For measurement, are used for coarse grains (>2 mm) to directly gauge axial dimensions and , while automated image analysis techniques process digital photographs of finer sediments to compute shape descriptors like roundness and sphericity with high throughput.

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