ABSTRACT

The shapes of sand particles that affect their hydraulic properties in a moving fluid during entrainement, transport and deposition are closely approximated by their dynamic shape factor (DSF), the squared ratio of fall velocity of the particle to the fall velocity of its nominal sphere. We determined the DSF values for 126 specimens of 15 common heavy minerals (magnetite, tourmaline, garnet, etc.) by weighing each grain and by measuring its fall velocity in water. The results are compared with geometric shape measures obtained from the triaxial dimensions of each particle. The geometric shape measures all show a high correlation with the DSF.

Lines of equal DSF on a dimensionless (C_D-R) plot are shown to be logarithmically distributed according to the function DSF=Z=(8F/v²)/(C_DR²), of gradient, Z=Z(1/C_D²+4/R²)^1/2.

Among particles of the same mineral, shape can influence their hydraulic properties (drag coefficient) to about the same magnitude as variation in specific gravity among the heavy mineral species. Thus, variation in shapes of heavy minerals can produce about the same differences in size frequency distribution as the expected variations in specific gravity. The effects can be great. If minerals are to be used to discriminate sedimentary environments, or to establish consanguinity of heavy mineral suites, their hydraulic shapes should be accurately measured, and the effect of shape on the mineral distribution evaluated.