size–density relationship that governs the deposition of mineral particles from flowing water. Two particles of different sizes and densities are said to be hydraulically equivalent if they are deposited at the same time under a given set of conditions; the smaller particle will have the higher density. Thus, it is not uncommon to find sedimentary deposits containing coarse quartz particles together with fine particles of heavy minerals.

Heavy minerals have a size distribution similar to that of quartz; they occur in silt, sand, and even larger sizes. Because of this fact, it was possible for the U.S. geologist Gordon Rittenhouse to determine (1943) the hydraulic equivalence for various heavy minerals in terms of the Udden grade scale, which is based on the size of quartz grains (see grain size scale); these equivalents express the number of Udden grades by which the heavy mineral particle is smaller than the equivalent quartz particle. In general order of increasing density, and thus hydraulic equivalence, some examples include tourmaline (0.2), amphibole (hornblende, 0.2), pyroxene (0.3), apatite (0.4), titanite (0.5), garnet (0.6), zircon (0.9), ilmenite (1.0), and magnetite (1.0).