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phi scale The expression of grain sizes on a logarithmic scale. The phi value (φ) is related to grain diameter (d) by the expression φ = −log2d. Increasing positive phi values are for grain diameters progressively finer than 1000 μm, and increasing negative values for grain diameters progressively coarser than 1000 μm (e.g. 2φ = 250 μm; 1φ = 500 μm; 0φ = 1000 μm; −1φ = 2 mm; −2φ = 4 mm).