Gems – Density

Among the physical properties of minerals, there are scalar and vector properties. The first are direction independent, in which they are tested in the crystal, the other, however, change depending on the direction. Examples of scalar properties are density and specific heat, vector properties - hardness, cleavage, some thermal properties, electric, optical, etc..

Density is of practical importance for the recognition of minerals, hardness, cleavage and optical properties.

Density (specific mass) the mineral ρ is the ratio of its mass m do jego objętości V:

ρ = m / V

The openness of the density depends on the temperature. Almost all minerals decrease in density with increasing temperature. The temperature should therefore be indicated for precise markings, in which the measurement was made. The normal temperature for determining the density is + 20 ° C.

Density is an important characteristic of a given mineral. If the considered mineral is constant, invariable chemical composition, e.g.. kwarc SiO2 or CaCO calcite3, its density is constant: quartz - 2,65 g/cm3, kalcyt — 2,71 g/cm3. The deviation from the normal density value indicates the presence of foreign bodies in the mineral. However, the chemical composition of a large number of minerals varies within certain limits, what they show e.g.. chemical analyzes of pyroxenes, amfiboli, grenades. The density value of these minerals varies depending on the chemical composition: the pyroxenes have a density of 3.2-3.7, amfibole 3,0—3.5, granaty 3.5-4.5 g / cm3.

Minerals differ significantly in the density value. Common minerals, being the main components of rocks, have densities ranging from 2 to 3.5 g / cm3.

The density of most gemstones is clearly higher than that of common rock components, mainly from feldspar and quartz, which has already been pointed out in the description of the occurrence of precious stones in nature. Because thanks to the higher density, precious stones washed from primary rocks and carried by the water of streams and rivers can settle among sands and river gravel in these places, where there is a decrease in the speed of the water. In this way, secondary river and coastal concentrations of precious stones may arise.

Density determination. Do oznaczania gęstości minerałów służą: 1) pycnometer, 2) hydrostatic balance, 3) Jolly's spring scale, 4) the weight of Kratschmar and 5) heavy liquids.

Pycnometer jest naczyniem szklanym o pojemności 5—100 cm3, closed and ground, elongated, glass stopper, having a narrow channel allowing excess liquid to drain. In order to determine the density of the mineral, it is ground into a powder, and then accurately weighs an approximate amount of powder 1/21/4 objętości piknometru. Po oznaczeniu ciężaru piknometru napełnionego wodą Qpw opróżnia się go, wsypuje odważoną ilość sproszkowanej substancji Qs, dopełnia piknometr przegotowaną wodą destylowaną i zamyka korkiem; excess water flowing out of the channel is removed by drying with tissue paper, then weighed again. The density of the test substance can be calculated from the formula:

ρ = (Qpw+Qs+Qpws) / Qs

where: Qs — masa próbki, Qpw — masa piknometru z wodą, Qpws - mass of the pycnometer with water and substance.

Density determination with a pycnometer is an accurate method, but it can only be used when testing powdered material, fine crumbs or small ground stones.

Hydrostatic balance różni się od zwyczajnej wagi laboratoryjnej tym, that one of the dishes is hanging higher. Under this pan there is a hook for hanging the sample on a thin thread, so that it can be weighed first in the air, and then in water placed under the pan in a suitable vessel.

Różnica masy minerału w powietrzu Q i jego masy po zanurzeniu w wodzie Qw indicates an amount of water equal to the volume of the weighed mineral sample. Density is obtained according to Archimedes' law, from the formula:

ρ = Q / (Q Qw)

You can also use an ordinary laboratory balance as follows: A small table is placed under one of the dishes so, so that it does not interfere with changing the position of the pan. There is a water cylinder on the table, in which a test sample is immersed, suspended by a thread with a hook or a wire ending in a spiral. The mineral density is calculated as above.

The Jolly Scale is a spring scale allowing it to be less accurate, but faster determination of the density of gemstones.