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sphericity An expression of how closely the shape of a grain resembles the shape of a sphere. Sphericity can be determined by examining the relation between the long (L), intermediate (I), and short (S) axes of the particle, the maximum projection sphericity, Ψ, being given by the expression Ψ = 3√(S2/LI). For a perfect sphere, Ψ = 1. Values less than one relate to increasingly less spherical shapes. See also PARTICLE SHAPE; and ROUNDNESS INDEX.