Grain Size Problems
Updated 31-Dez-09 18:51h
Microscopic (optical) sizing
Microscopic (optical) sizing has accuracy limitations. We do not see grain size correctly. In order to measure the sand grain size and magnify its picture by microscope, the angle of the rays’ convergence grows with the magnification. This is why we do not focus onto the maximum, but to about up to 27% smaller projection. More complicated is the apparent size reduction in thin sections, the 0.03 mm thin slices of rocks. The random cuts of large grains are about 27% smaller on average. However, the thin sections cut finer grains (as fine as the slice thickness or smaller) seldom or not at all. We are viewing the fine grains in projections close to the “correct” grain size. However, projections are 2-dimensional pictures of the grain – a 3-dimensional body. We are loosing one dimension!
Sieving has almost no value because:
1) Sedimentation velocity can not be correctly calculated from the sieve grain size;
2) too wide intervals (0,25 PHI instead of 0.02 PHI), too few fractions in sand range (25 instead of >400);
3) grain shape affects the sieve grain size inconsistently with sedimentation velocity: sieves measure non-spherical grains coarser but they sediment slower and have greater specific surface;
4) sieving is time consuming and inaccurate.
Sieving can hardly meet the accuracy requirements, woven sieves not at all. They can easily become defective by an almost invisible shift of a wire, which may enlarge some openings. We should prefer micro precision sieves with circular holes (e.g. by VECO, produced by galvanic deposition on thin nickel plates, accuracy ±1 micron), support the sieving by a liquid (alcohol), use vibration in random 3D-directions and monitor it up to constant weight to check whether there are still some grains to pass. On screens with holes finer than about 0.2 mm, the load should not exceed 0.3 g/cm2 for each screen, because each grain must get a chance to be measured, i.e., it must “wait” in maximum 10 layers. A 20-gram sand sample may be sieved on 8 screens for 8 hours; ultrasound cleaning and microscopic checking of sieves may take more than one hour.
A grain size definition includes grain shape, which is irregular and randomly different in natural sands. However, one may eliminate the shape influence on the size by using the diameter of a volume equivalent (nominal) sphere. Because the volume measurement of sand particles is difficult and inaccurate, we should define the grain size as the size of a sedimentation velocity equivalent geometric body. Usually, a sphere has been chosen as such a body.
Because a sphere sediments much faster than a naturally shaped volume equivalent sand particle, Jiri Brezina (1979) suggested the hydraulic equivalence to a rotational ellipsoid with the typical Corey’s Shape Factor value, SF = 0.65. He also developed the converting equations and the most precise Sand Sedimentation Analyzer™.
We usually know exactly all variables needed to solve the converting equations: the grain material and thus its mass per volume, density, and the laboratory gravity acceleration. The grain mass is perfectly sensed by acceleration as a buoyant force (weight) resisted by a fluid frictional force in an equilibrium at the terminal sedimentation velocity. The Analyzer measures the sedimentation time in milliseconds over the sedimentation length of 180 cm and calculates the fluid density and viscosity from the water temperature measured to ± 0.1°C. Sand grains in water accelerate sufficiently by gravity to sediment separately.
For genetic interpretations in sedimentology, the most precise distribution data are needed to decompose them into up to 5 Gaussian components. These components help reconstruct recent and fossil streams, which deposited them, trace the sand transport etc., because they control the location of each grain during its deposition we found it = the sediment origin.
Sedimentation velocity describes the grain size most accurately.
The sedimentation velocity of sand grains is more related to their deposition (origin) than their size.