orthorhombic Nesosilicates
Staurolite (90 Degree Cross)
Formula Fe2Al9O6(SiO4)4(O,OH)2
Crystal Structure
orthorhombic[mmm]:{110}@1.0 + {010}@0.7 + {001}@0.2 | twin(staurolite_90) orthorhombic
mmm
{110} {010} {001}
Quick Facts
Hardness
7.0
Specific Gravity
3.65
Refractive Index
1.736
Optical Character
Biaxial +
Physical Properties
- Crystal System
- orthorhombic
- Hardness (Mohs)
- 7.0
- Specific Gravity
- 3.65
- Cleavage
- Poor {010}
- Fracture
- Subconchoidal to uneven
- Lustre
- Vitreous to resinous
- Twin Law
- 90 degree cross (staurolite law)
Optical Properties
- Refractive Index
- 1.736
- Birefringence
- 0.009-0.015
- Optical Character
- Biaxial +
- Dispersion
- 0.021
- Pleochroism
- strong: Yellow / Reddish-brown / Dark red (Trichroic; best seen in translucent crystals)
Characteristics
Colours
Reddish-brownYellowish-brownBrownDark brownBlack
Localities
Georgia (USA)Virginia (USA)Russia (Kola Peninsula)France (Brittany)Switzerland
Crystal Forms
prismpinacoidtwin
Diagnostic Features
Distinctive cross-shaped penetration twins (60° and 90°); brown colour; high hardness (7-7.5)
Notes
More rare than 60-degree twins; highly sought by collectors
Related Minerals
Other minerals in the orthorhombic system
