iD59113_2 Click to enlarge image
Four small, yellow, water-worn sapphire grain from Neogene Nsanaragati gem deposit, Mamfe Basin, Cameroon. Image: Dayna McGeeney
© Australian Museum


Porosity is the volume of voids (spaces) within a rock which can contain liquids. The porosity of a sedimentary rock made up of perfectly closely-packed spherical particles is 27%, while in one with more openly-packed particles it rises to 47%. Because sedimentary rocks are never perfectly packed the porosity generally varies from 1 to 50%. In sandstones, porosity varies from 5 to 15%, while in loose sands and gravel it may reach 45%. Clays are exceedingly porous, up to 50%. The degree of porosity largely depends upon the geometrical arrangement of particles in the sediment.


Permeability is the ability of water or other liquids (e.g. oil) to pass freely through a rock. Importantly, some rocks (e.g. sandstones) may have a high porosity but may still be impermeable. A rock is pervious if it is permeable via mechanical discontinuities (cracks) such as joints and bedding planes.


Roundness is a measure of the roughness of the surface of the sedimentary grain. In general, grains become more rounded the further they are from their source rock. This is an important tool in mineral exploration, particularly for diamonds and other gemstones. Sphericity refers to the shape of the grain and is largely inherited from the host rock.


Sorting relates to the range of particle sizes in a sediment or sedimentary rock. In general, sediments which have travelled relatively long distances from their source are well sorted while those that haven't travelled far are poorly sorted.


The matrix is the fine-grained material (usually clays or silt) that is deposited originally with the coarser-grained material (e.g. sands and gravels) in a sediment. It is also derived from the weathering and erosion of the source rocks. This fine grained matrix can also be referred to as cement.