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Grain shape

Clasts in sedimentary rocks have different shapes, ranging from round (equant) to elongate and flat. Equant grains, whether they are cubes or spheres, can be approximated to spheres; they hence have high sphericity. Elongate and flat grains can be considered respectively as ellipsoids with a long axis (like a cigar) and flat ellipsoids with a short axis and two long axes. In other words, they have low sphericity. Sphericity describes how much a grain can be approximated to a sphere and differs from roundness, the textural parameters defining how well the outline of a grain is rounded.

Some processes tend to produce grains with spherical shape, like rivers, where sediment is transported on the bed, rolling and becoming equant in the process. Others processes, like waves on a gravel beach, tend to act always on the same side of a clast favoring flatter (low sphericity) shapes, which are yet well rounded. Composition of clasts is also important: lithic clasts of foliated rocks tend to break along the foliation during, facilitating the development of flatter, low-sphericity shapes. In a similar fashion, a rock with a strong lineation can produce elongate grains during erosion and subsequent transport.

Relationships between sphericity and roundness. Modified after wikimedia.org.

References
Compton, R. R. (1962). Manual of field geology. Soil Science93(4), 295.
Dott, R. H. (1964). Wacke, graywacke and matrix; what approach to immature sandstone classification?. Journal of Sedimentary Research34(3), 625-632.
Folk, R. L. (1980). Petrology of sedimentary rocks. Hemphill publishing company.
Lewis, D. W., & McConchie, D. (2012). Analytical sedimentology. Springer Science & Business Media.
Powers, M. C. (1953). A new roundness scale for sedimentary particles. Journal of Sedimentary Research23(2), 117-119.
Wentworth, C. K. (1922). A scale of grade and class terms for clastic sediments. The journal of geology30(5), 377-392.
        

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Detrital and Authigenic Minerals
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Sedimentary Structures
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Sedimentary Rocks

 

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