Grain Size Measurement

The size of grains in a sediment or sedimentary rock can be measured by a range of techniques, and choosing among them requires balancing the purpose of the study, the size range of the grains present, and whether the material is loose or cemented into rock. ^[1] No single technique works across the entire grain-size spectrum - from boulders down to clay - so a sedimentologist routinely draws on several methods depending on what size fraction is being studied.

Manual Measurement

For the coarsest particles - pebbles, cobbles, and boulders - whether in an unconsolidated deposit or a lithified rock, the simplest approach is direct manual measurement with a calliper or tape. ^[1] Grain size is expressed in terms of either the long dimension or the intermediate dimension of the particle. ^[1]

Sieving

Granule- to silt-size particles in unconsolidated sediments or in sedimentary rocks that can be disaggregated are most commonly sized by sieving through a set of nested, wire-mesh screens arranged from coarse to fine, mounted in a mechanical shaker. ^[1] An important subtlety of sieving is that it measures the intermediate (shortest horizontal) dimension of a particle, because it is the intermediate dimension that determines whether a grain can pass through a particular mesh opening - an elongated grain can slip through a mesh opening that its long axis would never fit, but only if its intermediate width is smaller than the aperture. ^[1]

Settling-Tube Analysis

Fine sand, silt, and similar grain sizes can be measured by settling techniques that relate the time a grain takes to fall through a water column to its equivalent size. ^[1] For coarser particles in this range, the settling time is compared empirically against a standard size-distribution calibration curve to obtain an equivalent millimetre or phi diameter. ^[1]

An important limitation of settling analysis is the effect of grain shape. Spherical particles settle faster than non-spherical ones of the same mass, so measuring the sizes of naturally angular or platy grains by settling will yield different values than sieving the same material. ^[1] This means that size data from settling tubes and sieving are not directly interchangeable without correction, and comparisons between datasets that used different methods must be made with caution.

Pipette Analysis and Stokes’ Law

For fine silt and clay, settling velocities become very low and cannot be measured by simple timing in ordinary settling tubes. The appropriate physical law for these fine sizes is Stokes’ Law, which relates the terminal settling velocity of a small sphere to its diameter and the viscosity and density of the surrounding fluid. ^[1] The standard laboratory implementation of this is pipette analysis, in which subsamples of suspension are withdrawn from a settling column at specific times and depths, and the concentration of sediment in each subsample reveals how much material of each size class has settled by that point. ^[1]

Automated Particle-Size Analysers

To reduce the laborious manual operations of pipette analysis, a range of automated instruments has been developed, each operating on a different physical principle.

A photohydrometer is a settling tube in which the intensity of a light beam passed through the suspension is recorded continuously: as particles settle and the suspension clears, the transmitted light intensity changes in a way that can be related empirically to the settling velocities and thus to particle sizes. ^[1]

A sedigraph works differently: it measures how the attenuation of a narrow, finely collimated X-ray beam changes as a function of both time and height within a settling suspension, using the progressive clearing of the beam path at each level to calculate the size distribution. ^[1]

A laser diffractometer exploits the fact that particles of a given size diffract laser light through a characteristic angle, with the angle increasing as grain size decreases - each size class leaves a distinctive diffraction signature that can be used to reconstruct the size distribution. ^[1]

Electroresistance analysers - such as the Coulter counter - measure grain size by detecting the change in an electrical field caused by a particle passing through it. Each particle displaces its own volume of electrolyte as it passes through the sensing zone, producing a voltage pulse whose magnitude is proportional to the particle volume; these pulses are scaled and counted to yield a size distribution. ^[1]

Image analysis systems use cameras to capture and digitise images of grains, and then calculate size diameters from the digitised grain outlines using appropriate computer software. ^[1]

Measuring Grains in Consolidated Rocks

When sedimentary rock cannot be disaggregated, none of the above techniques apply. Size and sorting of sand- and silt-size particles can be estimated using a reflected-light binocular microscope with a set of standard grain-size comparison cards as a visual reference. ^[1] More precise measurements can be made in thin sections under a petrographic microscope fitted with an ocular micrometer, or by image analysis applied to thin-section photographs. ^[1]

Both microscopic and image analysis methods tend to underestimate the true maximum diameter of grains because a thin section cuts through each grain at an arbitrary plane that rarely passes exactly through the grain’s centre - the result is that most measured diameters are smaller than the equatorial diameter of the grain. ^[1] Mathematically based corrections are therefore commonly applied to bring thin-section measurements into closer agreement with sieve data. ^[1] For the very finest material - clay-size grains in consolidated rock - only an electron microscope provides adequate resolving power, though it is rarely used for routine grain-size measurement. ^[1]

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