stauroliteFe

StauroliteFe is the iron-rich variant of staurolite, a monoclinic silicate mineral with the general formula (Fe2+,Mg)2Al9Si4O22(OH)2. In this endmember, iron (Fe2+) substitutes for magnesium (Mg) in the crystal structure, producing a spectrum of Fe–Mg compositions that petrographic literature sometimes refers to as Fe-rich staurolite or staurolite-Fe.

Staurolite crystals are best known for their distinctive cruciform twinning, producing cross-shaped intergrowths that resemble a cross or “fairy cross.” Iron content can influence crystal color and transparency, but the cross-twin remains a diagnostic feature of the mineral.

Formation occurs during regional metamorphism of aluminous rocks, typically at greenschist to upper amphibolite facies. Staurolite commonly occurs in schists and gneisses and is often found alongside garnet, kyanite or sillimanite, quartz, and feldspar.

Physical properties include a brown to reddish-brown color, a vitreous lustre, and a hardness around 7 to 7.5 on the Mohs scale. The specific gravity typically ranges from about 3.7 to 3.9. Crystals are usually prismatic and may appear translucent to subopaque, with the cross twinning visible in higher-quality specimens.

Etymology derives from Greek stauros, meaning cross, and lithos, meaning stone, in reference to the characteristic twin morphology. Staurolite is primarily of interest to mineral collectors and as a metamorphic indicator mineral; it is not a major ore mineral.