Specific Gravity of Minerals

The specific gravity of a mineral is a number which expresses the ratio existing between its weight and the weight of an equivalent volume of water. If a mineral has a specific gravity of 2, it means that a given specimen of that mineral weights twice as much as the same volume of water. The specific gravity of a mineral which does not vary in its composition is a constant factor, the identification.

After a little experience one can frequently judge quite accurately the specific gravity of a mineral by weighing it in the hand. Minerals containing the heavy metals like lead, copper, iron, etc., can be at once differentiated from those containing lighter elements by this means. And by practice one can become expert enough to be able to distinguish from each other minerals that have comparatively small differences in specific gravity; for instance, topaz (sp.gr = 3.52) from orthoclase (sp.gr=2.57), and fluorite (sp.gr=3.18) from quartz (sp.gr=2.6)

In order to accurately determine the specific gravity of a mineral, the following condition must be observed: The mineral must be pure. It must also be solid, with no cracks or cavities within which bubbles or films of air could be imprisoned. The fragment used should be reasonably large, about one cubic inch being a convenient size. If these conditions cannot be met, it is of little use to attempt a specific gravity determination by any rapid and simple method.

The necessary steps in making an ordinary specific gravity determination are briefly as follows: The mineral is first weighed in air. Let this weight be represented by x. it is them immersed in water and weighed again. Under these conditions it weight in water lent to the weight of the water displaced. Let the weight caused by immersion in water, or the weight of an equal volume of water.

The expression x / x-y will therefore yield a number which in various ways, those most commonly used being described below.

By Means of a Chemical Balance
The most accurate method of determining the specific gravity of a mineral is by the use of a chemical balance. To one beam of the balance is suspended a wire basket which is so arranged that it can be immersed in a beaker of water (see Fig. 194). The basket is hung in the water and then counterbalanced by weights on the opposite pan of the balance. The mineral specimen to be tested, having been first weighed on the balance in the ordinary fashion, is now placed in the basket under the water and weighed again. These two weights are the necessary data for calculation the specific gravity as explained above.