Densities of Common Rocks and Minerals

Density is a measure of the mass of a substance per unit measure. For example, the density of a one-inch cube of iron is much greater than the density of a one-inch cube of cotton. In most cases, denser objects are also heavier.

The densities of rocks and minerals are normally expressed as specific gravity, which is the density of the rock relative to the density of water. This isn't as complex as you may think because water's density is 1 gram per cubic centimeter or 1 g/cm³. Therefore, these numbers translate directly to g/cm³, or tonnes per cubic meter (t/m³).

Rock densities are useful to engineers, of course. They're also essential for geophysicists who must model the rocks of the Earth's crust for calculations of local gravity.

Mineral Densities

As a general rule, non-metallic minerals have low densities while metallic minerals have high densities. Most of the major rock-forming minerals in the Earth's crust, like quartz, feldspar, and calcite, have very similar densities (around 2.6 to 3.0 g/cm³). Some of the heaviest metallic minerals, like iridium and platinum, can have densities as high as 20. 

Rock Densities

Rock density is very sensitive to the minerals that compose a particular rock type. Sedimentary rocks (and granite), which are rich in quartz and feldspar, tend to be less dense than volcanic rocks. And if you know your igneous petrology, you will see that the more mafic (rich in magnesium and iron) a rock is, the greater its density.

As you can see, rocks of the same type can have a range of densities. This is partly due to different rocks of the same type containing different proportions of minerals. Granite, for example, can have a quartz content anywhere between 20% and 60%. 

Porosity and Density

This range of densities can also be attributed to a rock's porosity (the amount of open space between mineral grains). This is measured either as a decimal between 0 and 1 or as a percentage. In crystalline rocks like granite, which have tight, interlocking mineral grains, porosity is normally quite low (less than 1 percent). On the other end of the spectrum is sandstone, with its large, individual sand grains. Its porosity can reach 10 percent to 35 percent.

Sandstone porosity is of particular importance in petroleum geology. Many people think of oil reservoirs as pools or lakes of oil under the ground, similar to a confined aquifer holding water, but this is incorrect. The reservoirs are instead located in porous and permeable sandstone, where the rock behaves like a sponge, holding oil between its pore spaces.