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Variation in sizes of sediment particles deposited together under the same hydraulic regime is related to normal fluctuations in fluid properties and to differences between particle shape and specific gravity. Differences in size-frequency distributions between mineral species can be ascribed to these fluid and sediment properties. A useful way of comparing the differences is by modal sizes.
In heavy-mineral analysis the differences in modal sizes (^dgr^phgr) between each heavy mineral and the light minerals can be used to express the hydraulic equivalence of heavy and light minerals. Modal separation (^dgr^phgr) is a function of shape of the heavy mineral (ZH) and the light mineral (ZL), and of the relative specific gravities of the heavy mineral (^rgrH^prime) and light minerals (^rgrL^prime) in the fluid stream:
A test of this function was made in the heavy minerals in Tertiary sediments of Huerfano Park, Colorado, whose age, mode of deposition, direction of transport, and local source areas had already been established by independent field observations. The mean computed (<pl>^<above>^dgr</pl>^phgr) and measured (^dgr^phgrm) modal separations and their statistical deviations are as follows.
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