Staurolite is a common mineral in medium-grade aluminous metamorphic rocks. The composition is very rich in aluminum: Fe2Al9O6[(Si,Al)O4]4(OH)2. This indicates that the protolith had to be a clay-rich sedimentary rock. It is a hydrous mineral which makes it unstable at very high pressures.
Staurolite is an orthosilicate: ratio of (Si,Al):O is 1:4 and the silica tetrahedra are isolated from each other. Structurally similar silicate minerals are kyanite and garnet which tend to occur often together with staurolite as they are all Al-rich minerals formed during metamorphism. Other common metamorphic minerals that often occur with them are kyanite, muscovite, biotite, chloritoid, and cordierite.
Crystals are elongated and dark brown. Edges of crystals may be lighter and even orange in color if the light is able to penetrate the crystal. Twinning is very common (crystals crossed with 90 or 60 degree angles between them) which are very characteristic and therefore useful in identification.
This mineral is resistant to weathering and is therefore very common constituent in sand. It is part of the heavy mineral fraction, its density is 3.7…3.8 g/cm3. It may be easily mistaken for garnet because small grains are often not elongated and they are signifiantly lighter in color than large crystals in rocks. Staurolite as a sand grain is orange, garnet is usually pink (spessartine (Mn-rich garnet) may resemble staurolite in color, though). Polarizing microscope may be helpful to tell them apart. Garnet is an isometric mineral which will be dark in crossed polars while staurolite has interference colors.
Blue circles mark garnet and green circles staur. grains. Sand sample is from the Mediterranean Coast of France (Rayol-Canadel-sur-Mer). Other grains are quartz, K-feldspar, and plagioclase. Width of view 10 mm.
Staurolite is mostly used as an abrasive (hardness 7-7.5) in sandblasting applications. So in this regard it is also very similar to garnet. Twinned crystals have a commercial value as they resemble crosses which can be used as earrings or pendants.