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Geology is the Way

Staurolite

Monoclinic (pseudo-orthorhombic)

(Fe2+,Mg)2(Al,Fe3+)9[(Si,Al)O4]4(O,OH)2

Staurolite is a ferromagnesian and Al-bearing nesosilicate that occurs mainly in schistose rocks metamorphosed during regional metamorphism. The name ‘staurolite’, derived from the Greek stauros ‘cross’, and lithos, ‘stone’, is a reference to its tendency to form cross-twins, intersecting at angles of 90° or 60°, between 2 or 3 crystals.

Structure and chemistry
The structure of staurolite consists of packed (Si,Al)O4 tetrahedrons, (Fe,Mg)O4 tetrahedrons, and octahedral sites containing Al, Fe3+, and Ti cations. Its structure can be visualized as a series of Al2SiO5 and Fe-rich Al0.7Fe2O2(OH2) layers, with Al-octahedrons forming chains along the c-axis. The OHcontent varies within the structure, because the occupation of octahedral sites is variable (some of them are vacant).
The chemical variability of staurolite is governed by several substitutions within the crystal lattice, the main ones being Mg2+ for Fe2+, Si4+ for Al3+, Al3+ for Mg2+, Fe3+, or Ti4+, O2- for OH. Other elements, like Li, Zn, Co, and Cr, are also present in many staurolites. Staurolites are generally iron-rich (Mg/Fe < 0.3), but magnesian staurolites are known to occur in eclogites.

Staurolite is technically monoclinic but practically orthorhombic (pseudo-orthorhombic), since the angle between the a- and c-axes is nearly 90°.

Staurolite crystal habit

Sketch of the crystal habit of staurolite. Modified after Optical Mineralogy: Principles and Practice.

twinned staurolite

Twinned staurolite “fairy cross”. Size: 4.1 x 3.9 x 3.1 cm. Rubelita, Minas Gerais, Brazil. Photo © Robert M. Lavinsky.

Properties
Habit: prismatic
Hardness: 7
Cleavage: {010} moderate (parallel to long axis)
Twinning: {023}, {232} interpenetrated cross-twins at angles of about 60° and 90° between the c-axes of the twins
Color: dark brown to reddish brown and yellow-brown
Luster: subvitreous, resinous
Streak: white to light grey
Alteration: sericite, chlorite, clay minerals, iron oxides
In thin section…
α(//b): 1.736-1.747
β(//a): 1.742-1.753
γ(//c): 1.748-1.761
2Vγ
: 80-90°
Color: pale yellow
Pleochroismα colorless, β pale yellow, γ golden yellow
Birefringence (δ): 0.011-0.014 (first order grey to yellow/orange)
Relief: high
Optic sign: +
[Mindat]

Field features
Staurolite occurs as dark-colored (brown, reddish, yellowish) prismatic crystals in metamorphic rocks, commonly with kyanite, garnet, plagioclase, and micas. It is a hard mineral (7 on the Mohs scale) but can cleave along its long axes. Staurolite crystals can be twinned and produce cross-shaped associations of crystals oriented at 60° or 90° with respect to one another.

Cross-twinned staurolite

Cross-twinned staurolite crystal. Lech Darski Collection. Source: wikimedia.org.

staurolite garnet kyanite schist

Micaschist with prismatic crystals of staurolite (dark brown), kyanite (blue), and garnet (red), surrounded by micaceous muscovite. Width of sample 7 cm. Photo © Siim Sepp.

Kyanite staurolite schist

Association of staurolite (dark brown) and kyanite (blue) in a metasedimentary rock from the Engandine Window, Switzerland. Size: 7.7 x 4.1 x 2.2 cm. Photo © Robert M. Lavinsky.

Staurolite schist

Staurolite crystals in a mica schist. Michigamme, Michigan, USA. Photo © James St. John.

Staurolite in thin section
Staurolite is easy to recognize in thin section because of its characteristic colorless to golden yellow pleochroism, the typical pale yellow color, and prismatic habit with 6-sided basal sections. At parallel polars, staurolite shows high relief and a moderately-developed cleavage along {010}, a single set of cleavage planes parallel to the long axis visible both on prismatic and basal sections. At crossed polars, staurolite is characterized by straight extinction and shows first-order grey to yellow, maximum orange interference colors. Staurolite can alter to chlorite or sericite and to a mixture of clay minerals and iron oxides during retrograde metamorphism and weathering.

Staurolite and kyanite can be found associated as epitaxial intergrowths, thanks to their closely related crystal structure.

CPL
CPL
CPL
PPL
PPL

Above: basal (6-sided) section of staurolite, surrounded by quartz and muscovite. Note the right-dipping cleavage planes. Width: 5 mm. Posada Valley, Sardinia, Italy.

CPL
CPL
CPL
PPL
PPL

Above: prismatic staurolite crystals in a micaschist, with oriented inclusions of oxides. Width: 5 mm. Posada Valley, Sardinia, Italy.

CPL
CPL
CPL
PPL
PPL

Above: staurolite including an euhedral garnet and surrounded by muscovite in a micaschist. Width: 5 mm. Posada Valley, Sardinia, Italy.

CPL
CPL
CPL
PPL
PPL

Above: staurolite crystals surrounded by muscovite and biotite. The crystal in the center shows interpenetrated twinning. Width: 5 mm. Posada Valley, Sardinia, Italy.

Occurrence
Staurolite is a distinctive mineral of medium-grade regional metamorphic terranes, occurring in schistose metasediments rich in Al and Fe. Staurolite forms from the breakdown of chlorite and/or chloritoid, stable in the low grade, and commonly coexist with garnet and kyanite in Barrovian metamorphic terranes or cordierite and andalusite/sillimanite in contact metamorphic rocks or Buchan terranes. At high grade, staurolite is replaced by garnet and kyanite/sillimanite. Staurolite more rarely occurs in eclogitic rocks with unusual Al-rich composition, where it shows magnesian composition. Staurolite is relatively resistant to weathering and can be found as a detrital mineral in siliciclastic sedimentary rocks, especially in placers of heavy minerals.

References

        

Petrography websites:
Alexstrekeisen.it
Virtual Microscope

it_IT Italiano
Mineral Properties
Minerals

 

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