This measure is called a uniformity coefficient . Clay particles tend to electrostatically repell one-another along the surface of the particles. This results in a relatively large proportion of voidspace. Primary vs secondary porosity Primary porosity is the porosity that exists between individual grains in the rock. Secondary porosity is the porosity that results from fracturing, dissolution, and separation of the rock after its formation.
Water is sticky .
We see the sticky nature of water as surface tension and as the tendency for water to coat surfaces with a thin film. Specific retention , contrariwise, is the amount of water that the rock retains as a surface film on the sediments. The sum of specific yield and specific retention is the total porosity. Grain surface area - as grainsize decreases, total surface area increases, leading to smaller specific yields at smaller grain sizes. The highest specific yields come from coarse sands and fine gravels.