Staurolite, (Fe,Mg)4Al17(Si,Al)8O45(OH)3, occurs in medium-grade metamorphic rocks like this mica schist in brown crystals. (more below)
Well-formed staurolite crystals are commonly twinned, crossing at at 60° or 90° angles, that are called fairy stones or fairy crosses. I found this pair of staurolite twins in a streambed, where the sugary muscovite
, feldspar and occasional small garnets
around it were being eroded away. Large, clean staurolite specimens are found near Taos, New Mexico.
Staurolite is fairly hard, measuring 7 to 7.5 on the Mohs scale, and is used as an abrasive mineral in sandblasting.
Other Metamorphic Minerals