# Grain size

Grain sizes from 0.016 mm to 2.0 mm

The term grain size describes the size of individual particles (also called grains ) in a mixture . The grain or particle size distribution has a significant influence on the material properties in many technical and scientific areas such as construction , sedimentology and soil science as well as in metallurgy (see grain growth and grain refinement ). A large number of grain or particle mixtures are used there. These grain mixtures include all types of bulk material such as sand and gravel , flour , cocoa , plastic granulate and pigments . In metallurgy, the microstructures within metallic materials are also referred to as grain .

Due to the multitude of methods for determining, describing and interpreting the grain size as well as other grain properties according to EN ISO 14688 (shape, rounding and surface), granulometry has developed as an independent discipline.

## Equivalent diameter

Assuming that grains or particles exist as perfect spheres, one could simply use the sphere diameter as a measure of the grain size. However, this assumption is inadequate in practice, since naturally formed or technically produced particles exist in a wide variety of forms. The equivalent diameter is therefore used to describe their size . This means that you determine another measurable property and relate the measured values ​​to balls of the same size (equivalent).

A simple example of an equivalent diameter is the sieve diameter. A ball with a diameter of 1 mm and an elongated grain in the form of a pencil with a diameter of 1 mm fit through the square hole of a sieve with, for example, an edge length of 1 mm. Over the diagonal of the sieve hole, this also applies to a flat grain in the form of a coin with a diameter significantly more than 1 mm. All three grains have the same equivalent diameter of 1 mm.

Other examples of equivalent diameters are hydrodynamic diameter (same speed of fall in a column of water as a ball) or aerodynamic diameter (same speed of fall in air as a ball).

## Grain size analysis

Sieves with different fineness

To determine the composition of a mixture with regard to the particle sizes, one can choose from a large number of methods in which an equivalent diameter is ultimately always determined. The suitable method depends on the grain size range, the question or regulations (e.g. DIN standards ).

Very large particles (approximately 63 mm or more) are measured individually by hand or the size is determined from photos.

In the case of particles in the range from 10 µm to button size, the size can be determined by sieving . Here, a set with downwardly finer sieves is placed on top of one another. The sample to be analyzed is poured into the uppermost sieve and the sieve set is then clamped into a sieve machine . The machine then shakes or vibrates the sieve set for a certain period of time. If there is a high proportion of fine grains, sieving is carried out with running water (wet sieving). The grain sizes determined in this way are usually given in millimeters. The unit of measurement mesh is often used in Anglophone countries .

For very fine particles (<10 µm), methods are used in which the particles are allowed to settle in a column of water (coarse particles fall faster than fine ones) and the density of the suspension is regularly determined (with the help of an areometer ) or the mass of the settled Particles determined (sediment scales). Modern methods work with the scattering of laser light on the particles, which varies depending on the particle size, or with digital image processing. In soil science , the sludge analysis is used from a grain size of 0.063 mm = 63 µm (and smaller) , in building materials science it is determined in a washout test .

For the determination of the nutrient content classes, the grain size can be determined by experienced specialists with a finger test. Standard samples are available for self-control

In the agricultural soil investigation, the finger test is used in routine operation for the division of the analysis values ​​into nutrient content classes to determine the soil type and grain size. The amount of lime required is determined based on the proportions of sand, silt and clay and a proposal is made for an environmentally friendly and needs-based fertilization .

## Grain size distribution

Grain size distribution of different soils in line representation. The cumulative curves in the diagram are called

The result of a grain size analysis is the grain size distribution, i.e. a frequency distribution in the form of a bar or line diagram. The percentage (weight percent) of the grains is plotted against the classified equivalent diameter (abscissa). The usual statistical parameters, such as mean value , median , percentile values , scatter or skew of the distribution, as well as the number of irregularities , can be calculated and thus the sample can be characterized in terms of its grain size.

In production processes in which defined grain sizes are important for raw materials or products, grain size analysis is an essential part of quality control . In sedimentology and soil science, the grain size distribution is a very important feature for characterizing soils and sediments. It is used to classify them and determine properties, for example in terms of water balance, compaction potential or slope stability.

## Grain size in sedimentology and soil science

In sedimentology and soil science, grain size distributions serve the classification and nomenclature of soils, sediments and sedimentary rocks and allow conclusions to be drawn about the formation and certain properties of these natural materials. In soil science, the proportions of the various grain sizes are used to define the type of soil that is addressed in the course of soil mapping in the area using the finger test .

In principle, the wide range of grain sizes occurring in the geosphere, from well below a micrometer to several meters, is logarithmically divided into classes. In detail, the classification within the various geoscientific disciplines varies from author to author or between different countries. The classification according to DIN 4022 is most widespread in German-speaking countries .

### Grain type

When looking at soils, a distinction must be made between sieve grain and sludge grain . The sieve grain can be seen with the naked eye and has a grain size of more than 0.063 mm. In contrast, the slurry can only be made visible under a microscope . The grain size range is between 0.0002 mm and 0.063 mm.

### Grain size classification

The classification as given by DIN 4022 ( naming and describing soil and rock ). The DIN 18196 ( soil classification for structural purposes ) is thus largely compliant, but marks something different and sets different framework conditions. Depending on the author and especially in the USA, the class boundaries are slightly to significantly different, although only the names of the large groups are internationally uniform.

designation Equivalent
diameter
in mm
clear comparison symbol Soil type Grain type
Large group Small group
rounded angular-edged fineness Cohesion
Stones 8 Blocks 1 > 200 bigger than chicken eggs Y Coarse soil ( soil skeleton ) non-cohesive soils Sieve grain
Boulders , debris Rough stones ( rubble ) 63-200 X
Gravel 2 Coarse gravel Middle stones (gravel) 7 20-63 smaller than hen's egg, larger than hazelnuts gG G
Medium gravel Fine stones (chippings) 6  7 6.3-20 smaller than hazelnuts, larger than peas mG
Fine gravel 3 Grus 6 2-6.3 smaller than peas, bigger than match heads fG
Sand 2 Coarse sand 3 0.63-2 smaller than match heads, larger than grain semolina gS S. Fine floor
Medium sand 10 0.2-0.63 like semolina mS
Fine sand 5  10 0.063-0.2 like flour (~ 150 μm) and smaller, but still visible to the naked eye fS
Silt 2
( Silt 4 )
Coarse silt 0.02-0.063 no longer visible to the naked eye PDO U cohesive soils Slurry grain
Medium silt 0.0063-0.02 mU
Fine silt 0.002-0.0063 fU
Tone 9
( fine grain )
Coarse tone 0.00063-0.002 gT T
Midrange 0.0002-0.00063 mT
Fine tone <0.0002 fT
1Depending on the genesis of debris , block scree , stone run
2Referring to von Engelhardt , the terms Pelit (<0.063 mm), Psammit (0.063-2 mm) and Psephit (> 2 mm) were introduced in 1953
3According to von Engelhardt, the border area between coarse sand and fine gravel is also called Grand
4thAccording to von Engelhardt Silt for the border area between coarse clay and fine sand (silt according to DIN)
5According to EN 12620 among other things in construction <0.063 mm rock powder , see Broken Minerals
6thEN 12620 among others: 2–32 mm grit
7thEN 12620 among others: 32–63 mm ballast
8th Schroppen in construction
9According to Robert L. Folk (1962), carbonate rocks are classified as micrite , lutite , siltite , arenite and rudite with increasing grain size
10It is also - not according to DIN - fine sand precipitated as 0.125-0.250 mm

## Grain sizes of crystalline stones and mineral aggregates

In the petrology of igneous rocks and metamorphic rocks as well as in mineralogy , a distinction is made between the absolute and the relative grain size in the structure of rocks and mineral aggregates . In contrast to sedimentology, the term “grain” does not stand for detritic particles or particles that have otherwise emerged from exogenous sedimentary processes, but for crystals that either primarily emerged from a melt or that grew secondarily in the course of the transformation of a rock.

### Absolute grain size

The absolute grain size can partly be estimated with the naked eye, but partly only under the microscope. The terms macrocrystalline (recognizable with the naked eye), microcrystalline (only recognizable under the light microscope) and cryptocrystalline (cannot be resolved under the light microscope) are used to differentiate .

Coarse to giant-grain crystal structures have an average grain size of 5–30 mm. Medium-grain crystal structures have an average grain size of 1–5 mm. A fine-grain structure is present with an average grain diameter of less than one millimeter. In the case of microliths or crystallites, the average grain size is only a few micrometers.

### Relative grain size

The relative grain size makes statements about the size ratio of the mineral grains in the overall structure of a rock. In the case of an evenly grained (homogeneous) structure, the grains show only small differences in size from one another.

In the case of a non-uniform grain (heterogeneous) structure, however, the size differences are larger and also more variable. If larger crystals, so-called sprinkles , are embedded in a macroscopically indissoluble homogeneous matrix , this is called a porphyry structure . Microscopically, a further distinction can be made between in vitrophyrical structures , in which the matrix is glass-like , and microlithic structures , in which the matrix is ​​microcrystalline. In the case of glomerophyric structures , the single fragments are present as crystal aggregates .

With regard to the statistical distribution of the grain sizes in crystalline rocks with uneven grain structure, a serial ( continuous ) distribution is distinguished from a hiatal ( discontinuous ) distribution, with at least two maxima and the complete lack of some grain size intervals . A hiatal distribution is characteristic of porphyry structures.

## Grain size in metallography

In metallography , the term grain size describes the mean diameter or the mean area of ​​the crystallites (grains) within a multicrystalline metal . The mean grain diameter is usually a few µm to a few mm, but with a nanocrystalline structure it can also be in the range of a few nanometers. Different grain sizes can be set through appropriate solidification conditions, mechanical and thermal processing. The grain size of the metals influences the mechanical properties of the materials, as well as their machinability and corrosion behavior; fine-grain structures are usually tougher and harder and form a thicker , passive layer that protects against corrosion .

## literature

• Robert L. Folk : Practical petrographic classification of limestones. In: Bulletin of the American Association of Petroleum Geologists. Vol. 43, 1959, , pp. 1-38, doi: 10.1306 / 0BDA5C36-16BD-11D7-8645000102C1865D .
• Robert L. Folk: Spectral subdivision of limestone types. In: William E. Ham (Ed.): Classification of Carbonate Rocks. A Symposium (= American Association of Petroleum Geologists. Memoir. Vol. 1, ). American Association of Petroleum Geologists, Tulsa OK 1962, pp. 62-84.
• Wolfhard Wimmenauer: Petrography of igneous and metamorphic rocks. Enke-Verlag, Stuttgart 1985, ISBN 3-432-94671-6 .