Specific Gravity of Minerals
Another tool that is available for identifying unknown mineral specimens is specific gravity. A person who studies minerals can measure the specific gravity of a specimen by comparing its weight with the weight of an equal volume of water. In actual practice a mineralogist may use the following procedure:
- First, weigh the unknown specimen while it is dry.
- 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 equal to the volume of the mineral specimen.
- 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 barite as follows:
Dry weight: 2.00 oz.
Weight when suspended in water: 1.56 oz.
Loss of weight: 0.44 oz.
Dry weight divided by loss equals specific gravity: 4.5
In fact, the specific gravity of barite varies between 4.3 and 4.6. This results in part from impurities that may be present in varying quantities in different specimens of this mineral. Most minerals have variable specific gravities. In some the variation may be as much as 25%. Therefore, it is necessary for a person to consider as many of the physical and chemical characteristics as possible when identifying a mineral specimen.