Grain Size Distribution Analysis

Grain Size Distribution Analysis is usually employed in sedimentology. It is employed to determine the size of the different grains that make up a sedimentary rock or soil unit under consideration.

From the analysis, the following can be deduced:

            The sortability of the grains;

            The environment of deposition (Paleoenvironmental reconstruction);

            The energy of deposition (Paleocurrent current reconstruction);

            Transportation agent.

Please note that this analysis does not in any way show / determine the mineralogical composition of the rock / soil unit.

Grain sizes range from gravel (conglomerates and coarser particles) to clay (fine particles). It is determined by its diameter and measured in millimeters (mm), micrometers (µm) or phi (ø).

Below are the soils ranges (in mm):

            Boulders                                              >200

            Cobbles                                               63-200

            Gravel

(Coarse)                                  20-63

                        (Medium)                                6.3-20

                         (Fine)                                      2 - 6.3

            Sand

                        (Coarse)                                  0.63-2

                        (Medium)                                0.2-0.63

                        (Fine)                                       0.063-0.02

            Silt

                        (Coarse)                                  0.02-0.063

                        (Medium)                                0.006-0.02

                        (Fine)                                       0.002-0.006

            Clay                                                     <0.002

It is worth noting that no particular grain size exists naturally. It always exists as mixtures / intercalations e.g silty-clay, silty-sand. However, a particular grain size usually dominates the other.

Whenever Grain size (or Particle size) distribution analysis is mentioned, what usually comes to the mind is sieve analysis. Sieve analysis is the rudimentary part of grain size distribution analysis. Other methods exist. Examples are:

            Laser Diffraction

            Dynamic Light Scattering

            Sedimentation

Sieve Analysis

Just as the name implies ‘sieve analysis’. It is the passage of a weighted amount of soil (usually dried) through a set of sieves with known diameters. The coarses settle on the sieves while the fines pass through to the next sieve till the finest which settles in the tray. It has its pros and cons. It is cheap and easy to operate manually. But it burns the energy of the operator as a result of the constant and vigorous shaking required.

Laser diffraction:

This measures the angular dependence of laser light scattered by particles. It does not require a sieve analysis first. It is done in situ and always accurate. It has the capacity of detecting particle sizes of <100nm to 3mm.

Dynamic Light Scattering:

This measures the intensity of scattered light variations due to Brownian motion of particles in suspension. It is usually accurate and suitable for very fine particles (<1nm – 1µm).

Sedimentation:

It measures the rate of sedimentation of soil particles in water. It is used for very fine soil particles.

Laser diffraction and dynamic light scattering techniques are newer methods and are more accurate.