Heavy minerals are minerals with a density greater than 2.8…2.9 g/cm³. Why these numbers and what for is this range needed? Why not just one number? The most common minerals in most sand samples are quartz and feldspar. Calcite and dolomite are common cementing minerals in sandstones. All of these minerals have densities below the range shown above. Heavy minerals are usually volumetrically insignificant. However, there are a large number of heavy mineral species, each of them having their own story to tell. Therefore geologists often need heavy minerals to get as much information out of the studied rock as possible.

Heavy minerals in beach sand from Sri Lanka. Most intensely colored grains are spinel crystals. Width of view 20 mm.

Heavy minerals including gold on a gold pan. This concentrate is panned from river sediments in Lapland, Finland.

It is very uncomfortable to do if only one grain out of one hundred or less is what we are looking for. We therefore seek methods to somehow separate heavy minerals from the bulk of the sand. Obvious way to do that is to use some heavy liquids which have a density greater than that of quartz (2.65 g/cm³) but lighter than that of most minerals.

Several liquids with slightly different densities have been used. That’s the reason why there is range instead of fixed value. The liquid used to separate heavy minerals from the rest is usually bromoform (its density is 2.89 g/cm³). It is liquid at room temperature and feels abnormally heavy as one is usually not used to liquids as dense as bromoform. Its high density is a result of three bromine atoms in its chemical formula (CHBr₃). This liquid has a nasty downside. It is poisonous and has a disgusting odor. I guess it is actually a good thing because you don’t want to be exposed to the vapors of it. There are some more recently developed alternatives though which do not have such an adverse health effects, most notably polytungstate liquids.

Heavy minerals are useful to study the provenance of a sand or sandstone. “Provenance” is a fancy term geologists love to use when they talk about the place where the sand grain broke out of its parent rock and began its journey as a sediment particle. How do we study provenance? We take a look at the heavy mineral fraction and make sure what is its composition. Let’s say it contains garnet, staurolite, and kyanite. What can we say about that? I think we can reasonably safely assume that this sand is a weathering product of a metamorphic terrane because this mineral assemblage is very typical to metamorphic rocks. If there are lots of augite, magnetite, and olivine, then it probably comes from an igneous source.

Heavy minerals are sorted out by a running water near the coastline at Pfeiffer Beach, California.

A closer look at the Pfeiffer Beach sand. Main heavy mineral species are garnet, epidote, zircon, magnetite, spinel, staurolite, etc. Width of view 8 mm.

It is a very broad approach. Unfortunately more detailed studies into the provenance often give debatable results because there are so many factors that can alter the composition of the heavy mineral suite. These are mostly weathering, burial diagenesis, hydrodynamic sorting, and mechanical abrasion during the transport.

Heavy minerals in beach sand from Sri Lanka. Most intensely colored grains are spinel crystals. Width of view 20 mm.

Many diamond-bearing kimberlite pipes are discovered by studying heavy mineral fraction of a sand. We need to look for pyrope (which is heavy mineral) for example. This is a rare Mg-bearing garnet that is associated with diamonds in kimberlite pipes. Its presence in the river sand may give us a hint that a kimberlite pipe may be nearby. To hunt down its location, we have to go upstream and take many samples until pyrope and some other index minerals found in abundance in kimberlite pipes suddenly disappear. Heavy minerals have other applications in forensics, oil and gas industry, etc.

Heavy minerals sometimes get naturally concentrated as a heavy mineral sand and there were, of course, no bromoform involved. It was moving water either in a stream or beach that did the job. Sometimes the sand is so concentrated in heavy minerals that it has a real economic value as an ore. Sand collectors also love these black sand deposits. Such heavy mineral concentrates are called placers.

Gold panning is an activity used to separate gold flakes and nuggets from these placers. However, gold is not the only mineral that is mined from placers. These minerals are also cassiterite (tin ore), ilmenite (titanium), magnetite (iron), rutile (titanium), monazite (rare earths), chromite (chromium), zircon (zirconium), etc. Australia is particularly well-known heavy mineral source, but heavy mineral deposits occur in many places.

Some common and not so common heavy minerals in sand and some of their properties:

Basalt is full of heavy minerals. Here are basalt pebbles going through the weathering processes on the coast of La Palma, The Canary Islands.

Further reading

Deer, W. A., Howie, R. A. & Zussman, J. (1996). An Introduction to the Rock-Forming Minerals, 2nd Edition. Prentice Hall.