Specific Gravity

Specific Gravity (SG) is the weight of substance divided by the weight of an equal volume of water at 23oC. SG is a valuable, non-destructive statistic that is available for identifying unknown mineral specimens. Specific gravity is given in most descriptions of gems and minerals. The specific gravity is obtained by comparing a mineral's weight with the weight of a volume of water equal to that of the volume of the mineral.  The easiest method to use is to displace the volume of the mineral by dipping it into water.  The increase in the water volume when the mineral is dipped is equal to the mineral's volume.  By convention, 1 gram of water has a volume of one cubic centimeter (1 cm3 = 1 ml).  In actual practice the following method may be used :

SG =(weight of mineral in air)/(the weight of an equal volume of water)

 1) Weigh the unknown specimen while it is dry.

 2) Then using a special scale designed for this purpose, weigh the specimen while it is suspended in water. This is done because the amount of weight that is lost when a mineral is weighed in water is the same as the weight of the amount of water displaced by the mineral (volume of the mineral specimen). Typically it is hard to know the exact volume of a mineral, so weighing in water is the easiest method.

3) Finally, the mineralogist divides the loss of weight into the dry weight. This gives the specific gravity of the unknown mineral specimen.

For an example, we can use a sample of gold as follows:
Dry weight: 50 gms.
Weight when suspended in water: 47.368 gm.
Loss of weight: 2.632 gm.
Dry weight divided by loss equals specific gravity ,about: 19

 In fact, the specific gravity of native gold varies between 19 and 20. This results in part from impurities or even hollow spaces (bubbles) that may be present in varying quantities in different specimens of the mineral. Most minerals have a range of potential specific gravities. In some the variation may be as much as 25%. Therefore, it is wise to choose as pure a mineral specimen as possible when testing the physical properties of an unknown mineral specimen. Different varieties of the same mineral may have different SGs, for example garnet group minerals or tourmaline group minerals depending on the major element composition of the varieties.

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