Measuring specific gravitySpecific gravity is one of the most diagnostic properties of minerals. It is also easy to measure. More than any other property, therefore, this is the one to prioritise.
Specific gravity (aka specific density or just density) is simply defines as the mass per unit volume of the mineral. As density, this is usually measured in terms of grammes per cubic centimetre (g/cc). As specific gravity it is a simple ratio, relative to water (which can be assumed to have a density of 1.0). Density is traditionally measured by weighing the mineral in a clean dry state, then immersing it in water in a finely graduated container to calculate the volume by displacement. Specific gravity is measured by a simple trick that avoids the need to calculate the volume. This is done as follows: The procedure is therefore as follows:
Alternatively, if you cannot tare the scales, weigh the empty container and record this as Wc. The specific gravity is then calculated as (Ws-Wc)/(Ww-Wc)
For a listing of specific gravities of common minerals click here. |
The specific gravity of mineralsSpecific gravity is a non-dimensional measure of the density of a mineral, and like other properties of minerals depends primarily on the chemistry and structure of the crystal. Dense minerals are those that are made of inherently heavy (i.e. dense) elements, or ones that are built from densely packed atoms and molecules. Light (non-dense) minerals are generally composed of lighter elements, and ones that are arranged more loosely.
The density of atoms is described by its atomic weight. The heaviest are elem ents you've probably never heard of, such as Organneson, Tennessine, Livermorium and Moscovium - the names of which perhaps tell of where they were discovered or first made (or which football team the researchers supported) - all with atomic weights estimated to be around 294, In the mid and lower 200s are more familiar radioactive elements such as uranium and radium. Lead comes in ad 207 and gold at 196. The more common mineral-forming metals rarely exceed an atomic weight of a third of that: some are listed in the table below. Atomic weight of some common mineral-forming elements
In the case of raw metals, such as as gold or calcium, that's more-or less the end of the story. But most minerals have a more complex chemistry that includes other elements, especially the non-metals such a oxygen and hydrogen which act to bind the elements into molecules. Others consist of a mix of metals and metaloids (especially silicon). The relative proportions of these elements obviously affect the density of the minerals. So, too, does the closeness of their bonds as we saw when discussing mineral hardness. Minerals built with covalent bonds, which are strong and tight, tend to be denser; those with metallic bonds that are weaker ted to be less dense. Putting all these things together makes specific gravity difficult to predict, though in a series of minerals with identical structures, those composed of elements with higher atomic weight will have higher specific gravities. Likewise, if two minerals have the same chemical composition, differences in specific gravity will reflect variations in their internal packing. What's crucial, though, in identifying minerals is that specific gravity will usually be consistent for any mineral, so it is a highly diagnostic property. Learning how to measure it is a worthwhile skill, therefore, for the amateur geologist. |