Kenneth Pye Associates Ltd.
 Scientific Research, Consultancy and Investigations

              
.    ..    

Particle Shape and Surface Texture Analysis
...

Particle shape has many aspects, but four are particularly important: form (reflected by the degree of particle elongation or flatness), roundness (reflected by the degree of sharpness of corners and edges), sphericity (reflected by the degree to which the external envelope of the particle approximates that of a true sphere); and irregularity (reflected by the number and size of projections and indentations). The form of large gravel particles can most easily be measured using calipers of various sizes. Small gravel and sand particles can be photographed digitally under an optical microscope in three dimensions and image processing software used to calculate a number of parameters. Form can be measured from the axial dimensions. Sphericity can be calculated by measuring the area and perimeter. Roundness can be estimated by measuring the radii of inscribed and circumscribed circles. Irregularity can be estimated by measuring the relative size of bumps and hollows on the particle outline.

Where large numbers of grains are to be measured, or if only a qualitative measure is required, a number of visual comparators have been developed. These are available in PDF format can be downloaded using the link below. Often the best method of comparing a number of measurements of particle form is to plot the values graphically, and two different diagrams are commonly  used: that suggested by Zingg (1935), and that developed by Sneed and Folk (1958). Blank versions of these diagrams can also be downloaded below. In addition, particle form can be classified on the basis of the degree of elongation and flatness using the method suggested by Blott and Pye (2007). Again, diagrams illustrating the classification can be downloaded below. 

• Visual comparators of particle elongation and flatness, roundness, circularity, sphericity and irregularity (PDF document, 1.1 MB)

• Classification of particle form in terms of elongation and flatness (PDF document, 1.1 MB)

• Blank Sneed and Folk triangular diagram, for plotting measurements of particle elongation and flatness (PDF document, 211 KB)

Surface texture refers to the nature, density and distribution of small-scale topographic features present on the surface of particles and which are independent of overall size and shape. For general descriptive purposes, sand grain can be classified using a binocular microscope the terms glassy (very smooth), waxy (smooth), frosted (rough) and pitted (very rough). If a quantitative measure is required , the number of projections and indentations along a unit length of the particle can be measured from digital images. More detailed examination can be conducted using the scanning electron microscope, with the presence/absence or relative abundance of particular features being recorded. In addition, different combinations of features can be used to define specific particle 'types'.

    
Rounded and sub-rounded chert, chalk and quartzite            Angular assemblage of predominantly crushed dolerite
pebbles from Aldeburgh beach, Suffolk, UK                        used as an ornamental garden dressing

    
Rounded and sub-rounded chert, with some angular             Highly irregular, branching coral from a tropical beach 
edges and corners caused by fracturing, glacial drift              in North Queensland, Australia
deposits, Berkshire, UK

    
Honeycomb-weathered banded sandstone from an              Orange brick (with adhering grey mortar) which has been
arid desert environment                                                       rounded by abrasion in a marine beach environment

Examples of sand particles imaged using a scanning electron microscope, allowing
topographical and compositional information to be measured.

Example comparison of Fourier analysis shape data for two groups of dune sand
samples from North Queensland, Australia (Group A are older than Group B).
The histograms compare the frequency distributions of mean amplitude for harmonics
2, 4 and 19. Harmonics 2 and 4 provide information about the gross two dimensional
form , whereas harmonic 19 provides information about particle irregularity, angularity
and surface texture. Differences between the means are significant at the 99% level,
reflecting differences in the degree of weathering experienced by the two groups of
sands.

Backscattered SEM micrographs of polished grain mounts of dune sands from
North Queensland, Australia, showing (a) a mixture of rounded to subangular quartz
grains and heavy minerals from a young, parabolic dune; (b) angular to subangular
quartz grains from an older parabolic dune; (c) angular quartz showing evidence of
extensive surface disintegration and silica overgrowth from an older parabolic dune;
(d) a single grain showing evidence of surface disintegration and dissolution along
lines of intracrystalline weakness. 

Publications

Blott, S.J. and Pye, K. (in press) Particle shape: a review and new methods of characterization and classification. Sedimentology.

Pye, K. and Mazzullo, J.M. (1994) Effects of tropical weathering on quartz grain shape: an example from northeastern Australia. Journal of Sedimentary Research A 64, 500-507.

Kenneth Pye Associates Ltd. - Environmental Consultants
Page last modified: July 11, 2007