What is specific gravity in mineralogy ?

Specific gravity : definition

Specific gravity (usual) is the quotient of the mass of a body by the mass of the same volume of pure water at 4 °C. Specific gravity is therefore a dimensionless number.

In mineralogy, the specific gravity can be appreciated "by hand". With some practice, we can distinguish light minerals (specific gravity less than 2.5), heavy minerals (specific gravity greater than 4), and very heavy minerals (specific gravity greater than 6). The density of the earth's crust being 2.6, minerals with a density close to this figure appear to us to be of "normal" weight.

Specific gravity is a characteristic and important value of minerals which can be easily measured with a pycnometer. We can also calculate the density (ms) expressed in g/cm3, equivalent to the specific gravity, and improperly called calculated density.

It responds to the formula: ms = Z x M / N x V
M : Atomic mass of the mineral
V : Volume of the elementary cell
Z : Number of formulas - units in the mesh
N : Number of Avogadro

Deviations between measured specific gravity and calculated density are frequent in minerals. They result from the fact that the measured specific gravity is frequently distorted by the presence of trace elements, inclusions of other mineral phases, vacancies and imperfections of the crystal lattice, while the calculated density considers an ideal and perfect mineral, this which is far from being the case in Nature.