Metamorphic Minerals

• Actinolite
• Alurgite
• Andalusite
• Andalusite (chiastolite)
• Antigorite
• Anthophyllite-Gedrite
• Biotite
• Brucite
• Calcite
• Cassiterite
• Charoite
• Chlorite
• Chloritoid
• Chrysocolla
• Cordierite
• Corundum
• Corundum (ruby)
• Kyanite
• Kornerupine
• Kyanite (reticite)
• Deerite
• Dumortierite
• Epidotes
Allanite
Pistacite
Piemontite
Zoisite

• Fassaite
• Fluorite
• Glaucophane
• Garnet
• Hercynite
• Hedenbergite
• Hydrogrossular
• Howieite
• Humites
• Jadeite
• Lawsonite
• Lazulite
• Lazurite
• Malachite
• Minnesotaite
• Muscovite
• Olivine
• Omphacite
• Omphacite Cr
• Pargasite
• Piemontite (Praborna)
• Pyrite
• Prehnite
• Pumpellyite
• Quartz
• Rhodonite
• Rutile
• Sapphirine
• Scapolite
• Serpentine
• Sillimanite
• Sillimanite (fibrolite)
• Staurolite
• Stilpnomelane
• Sugilite
• Talc
• Taramellite
• Thulite
• Tinaksite
• Tinzenite
• Titanite
• Tremolite
• Tourmaline
• Vesuvianite
• Violan
• Wollastonite
• Yoderite
• Zussmanite

Texture and Microstructure

• Augen
• Book shelf sliding
• Boudinage
• Corona
• Decussate
• Deformation lamellae
• Deformation twins
• Crenulation cleavage
• Fringes
• Granoblastic
• Kelyphite
• Kink Bands
• Lepidoblastic
• Mesh
• Mica fish
• Mylonitic
• Nematoblastic
• Oblique foliations
• Polygonal
• Porphyroblastic
• Porphyroclastic
• Pseudomorph
• Ribbon
• S-C fabric
• Skeletal garnet
• Symplectite
• Veins

Metamorphic Rocks

• Albite greenshist
• Amphibolite (felsic)
• Amphibolite (mafic)
• Amphibolite (mylonitic)
• Amphibolite with epidote
• Andalusite schist
• Anyolite
• Augen gneiss
• Blueschist
• Blueschist (Ile de Groix)
• Blueschist (retrograde)
• Buchite
• Calc-schists
• Carbonated peridotite
• Cataclasite
• Charoitite
• Chlorite schist
• Chloritoid Schist (Ile de Groix)
• Cordierite slate
• Corundum-fuchsite rock
• Deformed oolite
• Dunite (mylonitic)
• Eclogite
• Eclogite Porto Ottiolu
• Epidosite
• Gabbro (mylonitic)
• Garnet micaschist
• Glaucophane granofels
• Granite (mylonitic)
• Granulite (felsic)
• Granulite (mafic)
• Granulite (retrograde)
• Granulite (sapphirine-bearing)
• Greenschist
• Hornfels
• Hornfels (albite-epidote)
• Hornfels (cordierite, plagioclase)
• Hornfels (corundum, plagioclase)
• Hornfels (hydrogarnet)
• Jadeite metagranite
• Jadeitite
• Kinzigite
• Lapis lazuli
• Marble
• Marble (calc-silicate)
• Marble (diopside)
• Marble (forsterite)
• Marble (mylonitic)
• Marble (tremolite)
• Mylonite
• Ocean jasper
• Ophicalcite
• Peridotite (mylonitic)
• Phyllite
• Prasinite
• Pseudotachylyte
• Pseudotachylyte (Ivrea-Verbano)
• Quartzite
• Quartzite (tourmaline)
• Rhyolite (mylonitic)
• Rodingite
• Schist
• Seafloor basalt
• Serpentinite
• Serpentinized lherzolite
• Skarn
• Skiddaw Metamorphic Aureole
Skiddaw
1) Spotted slate
2) Andalusite slate
3) Cordierite slate

• Slate
• Slate (ottrelite)
• Talc schist
• Tourmalinite
• Ultramylonite (granitic)
• Ultramylonite (mafic)

Staurolite - (Fe²⁺,Mg,Zn)₂(Al,Fe³⁺,Ti)₉O₆[(Si,Al)O₄]₄(O,OH)₂

The name "staurolite" is from the Greek word "stauros" which means "cross". The mineral commonly occurs as twinned, six-sided crystals that sometimes intersect at 90 degrees to form a cross (an intersection angle of 60 degrees is more common). In some localities these twinned crystals are collected, made into jewelry, and sold under the name "fairy crosses". Staurolite structure consist basically of layers Al₂SiO₅ (kyanite composition) and layers of AlFe₂O₂(OH)₂ alternating parallel to (010). kyanite and staurolite are often found in parallel intergrowth.

Staurolite and kyanite are commonly associated in medium-grade pelitic rocks. Both minerals often occur as epitaxial, macroscopic intergrowths, as in the sample from the Alpe Sponda in the Swiss Alps described by Kenngot (1866). The close relationship between the structures of both minerals, which contain similar Al₂SiO₅ layers, should facilitate submicroscopic intergrowth of kyanite and staurolite. Although rare in collector grade crystals, such intergrowths are not uncommon, in particular in rocks of multiple metamorphic events with staurolite replacing kyanite or vice versa.

Staurolite is a regional metamorphic mineral of intermediate to high grade. It occurs with almandine garnet, micas, kyanite; as well as albite, biotite, and sillimanite in gneiss and schist of regional metamorphic rocks. Staurolite is quite resistant to weathering but may alter to sericite, chlorite and limonite. With increasing metamorphic grade, staurolite becomes almandine and kyanite or sillimanite:

Staurolite + muscovite + quartz = almandine + Al₂SiO₅ + biotite + H₂O.

Optical properties:

• Form: Equant prism.
• Color: Yellow, pale-yellow.
• Pleochroism: Yellow to colorless.
• Relief: High.
• Cleavage: Perfect octahedral; Four direction are distinctive and commonly produce fragments as trigonal pyramids or triangular shapes.
• Interference colors: Low, I order grey-yellow.
• Twinning: Staurolite is famous for its twinned crystals that form into the shape of a cross; staurolite forms two twin types; one that is nearly 90 degrees and one that is nearly 60 degrees. The 60 degree type is more common but the 90 degree type is the most sought after.

Staurolite 60 degree type twin. From ThoughtCo.

Staurolite 90 degree type twin ("fairy crosses"). Taos, New Mexico. From Stan Celestian.

Kyanite and staurolite (in epitaxy). Pizzo forno, Swiss. From David Darricau.

microscopic intergrowths of kyanite and staurolite (yellow), PPL and XPL image, 10x (Field of view = 2mm).

Panoramic view of staurolite schif from Sardinia, Italy. Image by Andy Tindle (Virtual Microscope). PPL image, field of view = 2.5cm.

Panoramic view of staurolite schif from Sardinia, Italy. Image by Andy Tindle (Virtual Microscope). XPL image, field of view = 2.5cm.

Bibliography

• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Cesare, B., & Grobety, B. (1995). Epitaxial replacement of kyanite by staurolite: A TEM study of the microstructures. American Mineralogist, 80(1-2), 78-86.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.
• Vernon, R. H. (2018). A practical guide to rock microstructure. Cambridge university press.

Photo

Cross twinned staurolite crystal. Posada Asinara shear zone, Sardinia (Italy). PPL image, 2x (Field of view = 7mm)	Cross twinned staurolite crystal. Posada Asinara shear zone, Sardinia (Italy). XPL image, 2x (Field of view = 7mm)	Cross twinned staurolite crystal. Posada Asinara shear zone, Sardinia (Italy). XPL image, 10x (Field of view = 2mm)
Staurolite crystal altered by sericite. Posada Asinara shear zone, Sardinia (Italy). XPL image, 10x (Field of view = 2mm)	Staurolite crystal altered by sericite. Posada Asinara shear zone, Sardinia (Italy). XPL image, 10x (Field of view = 2mm)	Staurolite crystal altered by sericite. Posada Asinara shear zone, Sardinia (Italy). XPL image, 10x (Field of view = 2mm)
Staurolite crystal altered by Sericite. Posada Asinara shear zone, Sardinia (Italy). XPL image, 10x (Field of view = 2mm)	Staurolite crystal (yellow), ilmenite (balck), tourmaline (green), musvovite and quartz colorless. Posada Asinara shear zone, Sardinia (Italy). PPL image, 10x (Field of view = 2mm)	Staurolite crystal (yellow), ilmenite (balck), tourmaline (green), musvovite and quartz colorless. Posada Asinara shear zone, Sardinia (Italy). PPL image, 10x (Field of view = 2mm)
Staurolite crystal (low interference colors), musvovite (high interference coloros) and quartz. Posada Asinara shear zone, Sardinia (Italy). XPL image, 10x (Field of view = 2mm)	Staurolite crystal (yellow), ilmenite (balck), tourmaline (green), musvovite and quartz colorless. Posada Asinara shear zone, Sardinia (Italy). PPL image, 10x (Field of view = 2mm)	Staurolite crystal (low interference colors), musvovite (high interference coloros) and quartz. Posada Asinara shear zone, Sardinia (Italy). XPL image, 10x (Field of view = 2mm)
Staurolite crystal (low interference colors), musvovite (high interference coloros) and quartz. Posada Asinara shear zone, Sardinia (Italy). XPL image, 10x (Field of view = 2mm)	Staurolite crystal (yellow), ilmenite (balck), tourmaline (green), musvovite and quartz colorless. Posada Asinara shear zone, Sardinia (Italy). PPL image, 10x (Field of view = 2mm)	Staurolite crystal (low interference colors), musvovite (high interference coloros) and quartz. Posada Asinara shear zone, Sardinia (Italy). XPL image, 10x (Field of view = 2mm)
Staurolite crystal (low interference colors), musvovite (high interference coloros) and quartz. Posada Asinara shear zone, Sardinia (Italy). XPL image, 10x (Field of view = 2mm)	Staurolite crystal (yellow), ilmenite (balck), tourmaline (green), musvovite and quartz colorless. Posada Asinara shear zone, Sardinia (Italy). PPL image, 10x (Field of view = 2mm)	Staurolite crystal (yellow), ilmenite (balck), tourmaline (green), musvovite and quartz colorless. Posada Asinara shear zone, Sardinia (Italy). PPL image, 10x (Field of view = 2mm)
Staurolite crystal (yellow), ilmenite (balck), tourmaline (green), musvovite and quartz colorless. Posada Asinara shear zone, Sardinia (Italy). PPL image, 10x (Field of view = 2mm)	Staurolite crystal (low interference colors), musvovite (high interference coloros) and quartz. Posada Asinara shear zone, Sardinia (Italy). XPL image, 10x (Field of view = 2mm)	Staurolite crystal (yellow), ilmenite (balck), tourmaline (green), musvovite and quartz colorless. Posada Asinara shear zone, Sardinia (Italy). PPL image, 10x (Field of view = 2mm)
Staurolite crystal (low interference colors), musvovite (high interference coloros) and quartz. Posada Asinara shear zone, Sardinia (Italy). XPL image, 10x (Field of view = 2mm)	Staurolite crystal (yellow), ilmenite (balck), tourmaline (green), musvovite and quartz colorless. Posada Asinara shear zone, Sardinia (Italy). PPL image, 10x (Field of view = 2mm)	Staurolite with internal foliation. PPL image, 2x (Field of view = 7mm)
Staurolite with internal foliation. PPL image, 2x (Field of view = 7mm)	Staurolite with internal foliation. PPL image, 2x (Field of view = 7mm)	Staurolite with internal foliation. PPL image, 2x (Field of view = 7mm)
Staurolite with internal foliation and garnet inclusions. PPL image, 2x (Field of view = 7mm)	Staurolite with internal foliation and garnet inclusions. XPL image, 2x (Field of view = 7mm)	Staurolite with internal foliation and garnet inclusions. PPL image, 2x (Field of view = 7mm)
Staurolite with internal foliation. PPL image, 2x (Field of view = 7mm)	Staurolite with internal foliation. XPL image, 2x (Field of view = 7mm)	Staurolite with internal foliation and garnet inclusions. PPL image, 2x (Field of view = 7mm)
Staurolite with internal foliation and garnet inclusions. PPL image, 2x (Field of view = 7mm)	Staurolite with internal foliation and garnet inclusions. PPL image, 2x (Field of view = 7mm)	Staurolite with internal foliation and garnet inclusions. PPL image, 2x (Field of view = 7mm)
Staurolite with internal foliation and garnet inclusions. PPL image, 2x (Field of view = 7mm)	Staurolite with internal foliation and garnet inclusions. XPL image, 2x (Field of view = 7mm)	Staurolite with internal foliation and garnet inclusions. PPL image, 2x (Field of view = 7mm)
Staurolite with internal foliation and garnet inclusions. PPL image, 2x (Field of view = 7mm)	Staurolite with internal foliation and garnet inclusions. PPL image, 2x (Field of view = 7mm)	Staurolite with internal foliation and garnet inclusions. XPL image, 2x (Field of view = 7mm)
Staurolite crystals in a sialic eclogite. PPL image, 1x (Field of view = 9mm)	Staurolite crystals in a sialic eclogite. PPL image, 1x (Field of view = 9mm)	Staurolite crystals in a sialic eclogite. XPL image, 1x (Field of view = 9mm)
Staurolite crystals in a sialic eclogite. PPL image, 1x (Field of view = 9mm)	Staurolite crystals in a sialic eclogite. PPL image, 1x (Field of view = 9mm)	Staurolite crystals in a sialic eclogite. PPL image, 1x (Field of view = 9mm)
Staurolite crystals in a sialic eclogite. PPL image, 1x (Field of view = 9mm)	Staurolite crystals in a sialic eclogite. PPL image, 1x (Field of view = 9mm)	Staurolite crystals in a sialic eclogite. PPL image, 1x (Field of view = 9mm)
Staurolite crystals in a sialic eclogite. XPL image, 1x (Field of view = 9mm)	Staurolite crystals in a sialic eclogite. PPL image, 1x (Field of view = 9mm)	Staurolite crystals in a sialic eclogite. PPL image, 1x (Field of view = 9mm)
Staurolite crystals in a sialic eclogite. XPL image, 1x (Field of view = 9mm)	Staurolite crystals in a sialic eclogite. PPL image, 1x (Field of view = 9mm)	Staurolite crystals in a sialic eclogite. PPL image, 1x (Field of view = 9mm)
Staurolite crystals in a sialic eclogite. XPL image, 1x (Field of view = 9mm)	Staurolite crystals in a sialic eclogite. PPL image, 1x (Field of view = 9mm)	Staurolite crystals in a sialic eclogite. PPL image, 1x (Field of view = 9mm)
Staurolite crystals in a sialic eclogite. PPL image, 1x (Field of view = 9mm)	Staurolite crystals in a sialic eclogite. PPL image, 1x (Field of view = 9mm)	Staurolite crystals in a sialic eclogite. PPL image, 1x (Field of view = 9mm)