Long prisms of Staurolite in a mica schist. Locality - Russia
A penetration twin of Staurolite called a Fairy Cross twin. This crystal
grew in a mica schist associated with garnets.
Another penetration twin of
Staurolite. This is the common 60o twin. Note the long,
slender single crystal on the left side.
Staurolite
from Minas Gerais, Brazil. The crystal on the left is about 1" tall.
Top view of the same crystal on the right.
CHEMISTRY (Fe,Mg)2Al9(Si,Al)4O20(O,OH)4
CRYSTALLOGRAPHY Monoclinic (Pseudo Orthorhombic)
CRYSTAL GROWTH AND HABITS Very commonly found as prismatic crystals, often twinned at 60o, also found twinned at 90o to produce cruciform twins (also known as fairy crosses).
COLOR AND OTHER OPTICAL PROPERTIES Transparent to opaque; reddish brown to blackish or yellowish brown.
HARDNESS 7 - 7.5
SPECIFIC GRAVITY 3.7 - 3.8
LUSTER Sub vitreous to sub resinous
STREAK White to grayish
BREAKABILITY One distinct cleavage; sub conchoidal fracture; brittle
OCCURRENCE Normally found in mica schists or in gneisses.
ASSOCIATED MINERALS Almandine, Sillimanite, Kyanite, Tourmaline, Muscovite, Quartz
NAME From the Greek word for cross, in allusion to the cruciform twins.