An average grain volume for carbonate sands composed of various shaped particles has remained undetermined owing to the unpredictable effect of different fossil particle shapes on grain volume distribution. In this study, grain volume was measured for 250 single, dual and multicomponent packs of varied shaped, particulate, natural and artificial sands and gravels. Single-shape packs of simple, regular shaped particles have average grain volumes for moderately well sorted sediments as follows: 1) disks = 65%, 2) spheres = 57%, and 3) rods = 54%. Grain volumes of single-shape packs of various radical shaped particles, eg. bivalve shells, range from 35% ( Transenella ) to 17% ( Anomia ). Packing of two or more shapes tends to block and cancel the effects of radical shape which in single-shape packs causes a wide grain volume range. Multiple-shape packs have grain volumes proportional to the percent of the dominant component of the mix, making the grain volume of multiple-component mixtures essentially predictable. The ecologic and hydraulic factors involved in carbonate sedimentation produce a bimodal shape distribution. Grain volume values at the two modes are 59% and 28% for multiple-component shape packs. Lacking any evidence that the biologic and physical forces which molded the final composition and fabric of a carbonate sand tended to preserve a deposit of mainly radical-shape mixtures (i.e. low grain volume mixtures), it is reasonable to presume that an otherwise undefined carbonate sand can be expected to have an average grain volume of 59%, subject to correction based on subsequent analysis of shape-mixture limiting factors.

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