Siltstone and sandstone are both granular materials but they are distinguished by grain size. The Wentworth scale places silt particles between 0.0039 to 0.0625 mm. This is smaller than very fine grained sand.
Siltstone and Sandstone may have identical mineralogy. They are generally comprised of quartz, feldspar and rock fragments. Because siltstone is comprised of smaller grains, it has smaller pore throats, resulting in lower permeability and higher capillary pressures than in an equivalent porosity sandstone.
