Staurolite

In hand sample, staurolite frequently occurs as twinned prismatic crystals known as “fairy stones” or “fairy crosses.” In thin section, it displays a poikiloblastic texture which people often liken to vomit spatter (gross, but effective).

Physical Properties
Chemical formula Fe2Al9Si4O23(OH)
Class Nesosilicate
Isolated tetrahedra
Crystal system Monoclinic
Habit Prismatic
Color Brown to black
Hardness 7 to 7.5
Specific gravity 3.75
Cleavage Poor {010}
Fracture Subconchoidal
Luster Vitreous
Resinous
Transparency Translucent
Streak White or grey
Optical Properties
PPL Poikiloblastic texture
Clear to yellow or light brown
XPL Max 1st order yellows
Frequent quartz inclusions
δ 0.013
Twinning Penetration
Cruciform
after Perkins, 346

Staurolite in Hand Sample

Staurolite hand sample

Staurolite in muscovite-staurolite schist

Staurolite in Thin Section

Thin Section GigaPans

Staurolite in  plane polars

Staurolite in crossed polars

Staurolite, PPL
Staurolite, XPL

Further Reading

Staurolite at webmineral.com

Staurolite at mindat.org