3.2: Particle Morphology
- Page ID
- 25393
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Form
The overall form or shape of a particle can be important to note because it influences the hydrodynamic properties of the particle and because it might be influenced by mineralogy which in turn can impact how it responds to diagenesis and/or fluid flow. We commonly use the term "equant" to describe a particle that has broadly comparable dimensions in all directions (broadly spherical) and "platy" to describe a particle that is elongate in one direction (think elongate rectangle). Keep in mind that this refers to the overall shape not to the number or nature of the corners.
Roundness and Angularity
Roundness and angularity are end members that described the nature of the edges/corners of a particle. Keep in mind that this is different from the description of grain morphology.
Figure \(\PageIndex{1}\): Visual reference for describing rounding of equant (above) and platy (below) particles (Oceanography 101 (Miracosta) via source content; Public Domain).
Surface texture
One can also characterize the surficial texture of a particle. Common descriptors include things like bright, frosted, polished, pitted, scratched, etc.
Figure \(\PageIndex{2}\): Surface textures of sand grains. A) Frosted and pitted sand grains typical of aeolian environments (Mark Wilson via Wikimedia Commons; CC0 1.0) and B) bright, glassy sand grains more typical of fluvial transport (Krzem Anonim via Wikimedia Commons; CC BY-SA 4.0).
Packing
Packing refers to the arrangement of grains/particles in a rock. Although its not something that we can meaningfully describe in our work, its worth being aware of because of the potential to influence porosity and compaction of sediment. Have a look at the diagram below which shows various arrangements of spherical grains of the same size. By moving the grains around, you can go from a theoretical maximum of 47.64% porosity to a minimum of 25.95%.
Figure \(\PageIndex{3}\): Three arrangements of spherical, identically size particles showing how the packing of grains influences porosity. Diagram is after Graton, L.C. and Fraser, H.J, 1935, Systematic Packing of Spheres: With Particular Relation to Porosity and Permeability, Journal of Geology, v. 43, no. 8, pt.1, p. 785-909. (Michael Rygel via Wikimedia Commons; CC BY-SA 4.0).