STAUROLITE
Staurolite is a moderately common metamorphic mineral,
usually found in schists & gneisses. It is an iron aluminum
hydroxysilicate mineral, ideally Fe2Al9O6(SiO4)4(O,OH)2.
Staurolite is famous for its tendency to form interpenetrating twins -
individual crystals tend to cross each other at acute angles, forming
"X"s or crosses. These are called cruciform twins.
Perfect cross-shaped staurolite twins are sometimes called "fairy
crosses" or "fairy stones". Many fairy crosses offered for
sale are fakes (carved & dyed rock). Staurolite is nonmetallic in
luster, typically dark brown in color, quite hard (H=7 to 7.5), and has no
cleavage. Staurolite tends to alter to muscovite mica if potassium (K) is
added. The iron of altered staurolite goes into forming hematite.
Staurolite (each is ~7 to 9 mm across)
Staurolite cruciform twin from Pilar, New Mexico, USA.