Staurolite

Staurolite is a common constituent of a heavy mineral (its density is 3.7…3.8 g/cm³) fraction of a sand. Despite this, it is surprisingly obscure sand mineral. Even geologists often don’t know more than that it is one of the metamorphic minerals and often forms crossed twins (more about that later). I believe it is mostly because staurolite often gets misidentified as a garnet.

I know that sand collectors sometimes talk about “pink and orange garnets”. I did that too before I understood that these “orange garnets” are often different minerals. This confusion has several roots. First, garnet is far better known mineral. Second reason is an optical illusion. To the naked eye, garnet is dark red while staurolite tends to be almost opaque. But if you look smaller grains with stereomicroscope, red garnets become light pink and “opaque” staurolite often gets beautiful and deep brownish red color. The fact that garnet is not a single mineral only adds confusion as one is easily tricked to believe that there are many different garnets in his or her sand sample — hence the difference in color.

The story gets even more confusing because staurolite and garnet seem to enjoy each others company. Both are metamorphic minerals (garnet could be igneous as well). They both contain lots of aluminium in their crystal structures and inhabit the same medium- to high-grade metamorphic rocks. If these rocks are disintegrated by the weathering processes, garnet and staurolite crystals become sand grains and are therefore very often together in the sand samples too. Due to the crystal structure, staurolite is often elongated but garnet is not.

However, it is not very good idea to rely on color only. Garnet is a group of several minerals. Pink garnet is probably almandine (Fe-bearing garnet). If the grain is orange, then it could be spessartine (Mn-bearing garnet) as well as staurolite. If you want to know for sure then I recommend to use polarising microscope to distinguish one from another. Simply take a look at the grains using crossed polars. If the mineral is garnet (no matter whether it is almandine or spessartine), it will be dark (extinct). If it is staurolite, it has to have interference colors. You will get better results if you choose as small grains as you possibly can because light needs to penetrate the grains.

Staurolite has one interesting habit — it forms crossed twins. However, you are extremely unlikely to find beautiful staurolite crosses (penetration twins) in your sand sample. You have to seek patiently for subtle hints of it. Check your sand grain by grain and if you are lucky, you will find a grain that seems to be like a blend of two crystals that have grown together at an angle of 60 (mostly) or 90 degrees.

Blue circles mark garnet and green circles staurolite grains. Sand sample is from the Mediterranean Coast of France (Rayol-Canadel-sur-Mer). Other grains are quartz, K-feldspar, and plagioclase. The width of the view is 10 mm.