Density (ρ) is the mass (m) of a material divided by its volume (V), given in units of mass per unit
volume (kg/m3): ρ = m/V
The density of a material is a function of the density of the individual grains or crystals, the porosity of the material1, and the fluid filling the pore space. Even so, the variation in density of natural geologic materials is quite small, with only a 16% density increase from limestone to basalt.

Measurements of dry and wet samples are taken to calculate the dry density, wet density, and the grain density of a material. The dry density is the density of the material when the pore space is full of air, and is closely linked to porosity. The wet density is the density of the material when the pore space is filled with fluid (water or oil), and is related to the porosity and the fluid density. The grain density is the density of the solid material excluding porosity, and is closely linked to the mineralogy.

The density of a particular material will remain relatively stable under different circumstances, with little variation. Variation depends on saturation, the degree of compaction, cementation, or fracturing, and degree of metamorphism. Water has a higher density than air, so as water infills pore space, saturated materials are more dense than unsaturated materials. Materials that are more compact will be more dense as pore space is reduced, and intact materials will be more dense than those riddled by fractures or voids. Cementation can also infill void space, as can recrystallization, so more cemented or metamorphosed rocks are more dense than even compacted grains.

The critical density of a soil is the point at which a soil begins to demonstrate dilative behaviour when under high strain. This density will change with respect to the confining stress, the fabric of the material, the stress history, and the type and duration of loading, so is not constant for a particular geologic material under different conditions.

Specific gravity is the density of a soil normalized to the density of water, calculated as the material mass divided by the mass of an equal volume of water. It is a unitless measurement:
s.g. = ρsample/ ρwater

Because water density fluctuates with temperature, density and specific gravity measurements are made with respect to a reference temperature (typically 20◦C).

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