Discussion
Sphericity of a grain is defined as
Answer: A
Correct answer: The equal-volume sphere's surface area over the grain's own surface area
Sphericity measures how nearly a particle approaches a sphere. Formally it is the surface area of a sphere having the same volume as the grain, divided by the grain's actual surface area:
\[ \Phi = \frac{A_{\text{sphere, equal volume}}}{A_{\text{particle}}} \]
Because a sphere has the least surface area of any body enclosing a given volume, the numerator can never exceed the denominator, so \( \Phi \le 1 \) always, reaching 1 only for a true sphere. In practice it is computed from the three principal dimensions as \( \Phi = (LWT)^{1/3} / L \), the geometric mean diameter over the longest dimension. The other options describe a packing fraction, an aspect ratio and porosity respectively - none of which is bounded at one in the same way.
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