Sillimanite - Al2SiO5

Sillimanite was named by George Thomas Bowen in 1824 in honor of Benjamin Silliman (Professor of Chemistry and Geology, Yale University, New Haven, Connecticut, USA). The minerals kyanite, adalusite and sillimanite are polymorphs of Al2SiO5, each occurring under different temperature-pressure regimes and are therefore rarely found together in the same rock. All three polymorphs can be used as index minerals in metamorphic rocks. They all develop in alumina-rich pelites under different conditions of temperature and pressure.

A common variety of sillimanite is known as fibrolite, so named because the mineral appears like a bunch of fibres twisted together when viewed in thin section or even by the naked eye. Both the fibrous and traditional forms of sillimanite are common in metamorphosed sedimentary rocks. It is an index mineral indicating high temperature but variable pressure.

Optical properties:

Form: Usually found as elongate prisms or in fibrous crystals.
Color: Colourless.
Relief: high.
Cleavage: (010) perfect: thus a basal section of sillimanite, wich is diamond shaped, has cleavage parallel to the long axis.
Interference colors: Low (pale yallow).

al2sio5.png

Al2SiO5 phase diagram



Bibliography



• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• 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
sillimanitegranulite(2).jpg

Colorless, high Relief, sillimanite crystals in a sialic granulite, Calabria, Italy. PPL image, 10x (Field of view = 2mm)
sillimanitegranulite(3).jpg

Colorless, high Relief, sillimanite crystals in a sialic granulite, Calabria, Italy. PPL image, 10x (Field of view = 2mm)
sillimanitegranulite(4).jpg

sillimanite crystals (yellow-orange) in a sialic granulite, Calabria, Italy. XPL image, 10x (Field of view = 2mm)
sillimanitegranulite(5).jpg

Colorless, high Relief, sillimanite crystals in a sialic granulite, Calabria, Italy. PPL image, 10x (Field of view = 2mm)
sillimanitegranulite(6).jpg

sillimanite crystals (yellow-orange) in a sialic granulite, Calabria, Italy. XPL image, 10x (Field of view = 2mm)
sillimanitegranulite(7).jpg

sillimanite crystals (yellow-orange) in a sialic granulite, Calabria, Italy. XPL image, 10x (Field of view = 2mm)
sillimanitegranulite(8).jpg

Colorless, high Relief, sillimanite crystals in a sialic granulite, Calabria, Italy. PPL image, 10x (Field of view = 2mm)
sillimanitegranulite(9).jpg

sillimanite crystals (yellow-orange) in a sialic granulite, Calabria, Italy. XPL image, 10x (Field of view = 2mm)
sillimanitegranulite(10).jpg

sillimanite crystals (yellow-orange) in a sialic granulite, Calabria, Italy. XPL image, 10x (Field of view = 2mm)
sillimanitegranulite(11).jpg

sillimanite crystals (yellow-orange) in a sialic granulite, Calabria, Italy. XPL image, 10x (Field of view = 2mm)
sillimanitegranulite(12).jpg

Colorless, high Relief, Sillimanite (basal section) crystals in a sialic granulite, Calabria, Italy. PPL image, 10x (Field of view = 2mm)
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Colorless, high Relief, sillimanite crystals in a sialic granulite, Calabria, Italy. PPL image, 2x (Field of view = 7mm)
sillimanitegranulite(14).jpg

sillimanite crystals (yellow-orange) in a sialic granulite, Calabria, Italy. XPL image, 2x (Field of view = 7mm)
sillimanitegranulite(15).jpg

sillimanite crystals (yellow-orange) in a sialic granulite, Calabria, Italy. XPL image, 2x (Field of view = 7mm)
sillimanitegranulite(16).jpg

sillimanite crystals (yellow-orange) in a sialic granulite, Calabria, Italy. XPL image, 2x (Field of view = 7mm)
sillimanitegranulite(17).jpg

Colorless, high Relief, sillimanite crystals around a garnet in a sialic granulite, Calabria, Italy. PPL image, 2x (Field of view = 7mm)
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sillimanite crystals (yellow-orange) around a garnet in a sialic granulite, Calabria, Italy. XPL image, 2x (Field of view = 7mm)
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Basal sections of sillimanite, Calabria, Italy. PPL image, 10x (Field of view = 2mm)
sillimanitegranulite(20).jpg

Basal sections of sillimanite, Calabria, Italy. XPL image, 10x (Field of view = 2mm)
sillimanitegranulite(21).jpg

sillimanite crystals (yellow-orange) in a sialic granulite, Calabria, Italy. XPL image, 2x (Field of view = 7mm)
sillimanitegranulite(22).jpg

Basal sections of sillimanite, Calabria, Italy. XPL image, 10x (Field of view = 2mm)
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Colorless, high Relief, sillimanite crystals in association with biotite in a sialic granulite, Calabria, Italy. PPL image, 2x (Field of view = 7mm)
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sillimanite crystals (gray) in association with biotite in a sialic granulite, Calabria, Italy. PPL image, 2x (Field of view = 7mm)
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Colorless, high Relief, sillimanite crystals in association with biotite in a sialic granulite, Calabria, Italy. PPL image, 10x (Field of view = 2mm)
sillimanite(7).jpg

sillimanite crystals (gray) in association with biotite in a sialic granulite, Calabria, Italy. PPL image, 10x (Field of view = 2mm)
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sillimanite crystals (yellow) in association with biotite in a sialic granulite, Calabria, Italy. PPL image, 10x (Field of view = 2mm)
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sillimanite crystals (gray) in association with biotite in a sialic granulite, Calabria, Italy. PPL image, 10x (Field of view = 2mm)
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Colorless, high Relief, sillimanite crystals in association with biotite in a sialic granulite, Calabria, Italy. PPL image, 10x (Field of view = 2mm)
granuliteasill(9).jpg

sillimanite crystals in association with biotite in a sialic granulite, Calabria, Italy. XPL image, 10x (Field of view = 2mm)
granuliteasill(13).jpg

sillimanite crystals in association with biotite in a sialic granulite, Calabria, Italy. XPL image, 10x (Field of view = 2mm)
granuliteasill(21).jpg

Colorless, high Relief, sillimanite crystals in association with biotite in a sialic granulite, Calabria, Italy. PPL image, 10x (Field of view = 2mm)
granuliteasill(19).jpg

sillimanite crystals in association with biotite in a sialic granulite, Calabria, Italy. XPL image, 10x (Field of view = 2mm)
granuliteasill(20).jpg

Colorless, high Relief, sillimanite crystals in association with biotite in a sialic granulite, Calabria, Italy. PPL image, 10x (Field of view = 2mm)
granuliteasill(10).jpg

Basal sections of sillimanite, Calabria, Italy. PPL image, 10x (Field of view = 2mm)
granuliteasill(11).jpg

Basal sections of sillimanite, Calabria, Italy. PPL image, 10x (Field of view = 2mm)
granuliteasill(12).jpg

Basal sections of sillimanite, Calabria, Italy. XPL image, 10x (Field of view = 2mm)
sillimanite2015g(2).jpg

Colorless, high Relief, sillimanite crystals in association with biotite in a sialic granulite, Calabria, Italy. PPL image, 2x (Field of view = 7mm)
sillimanite2015g(3).jpg

Colorless, high Relief, sillimanite crystals in association with biotite in a sialic granulite, Calabria, Italy. XPL image, 2x (Field of view = 7mm)
sillimanite2015g(4).jpg

Big sillimanite crystals, with microfractures perpendicular to long axis, in association with biotite in a sialic granulite, Calabria, Italy. PPL image, 2x (Field of view = 7mm)
sillimanite2015g(5).jpg

Big sillimanite crystals, with microfractures perpendicular to long axis, in association with biotite in a sialic granulite, Calabria, Italy. XPL image, 2x (Field of view = 7mm)
sillimanite2015g(6).jpg

Big sillimanite crystals, with microfractures perpendicular to long axis, in association with biotite in a sialic granulite, Calabria, Italy. PPL image, 2x (Field of view = 7mm)
sillimanite2015g(7).jpg

Big sillimanite crystals, with microfractures perpendicular to long axis, in association with biotite in a sialic granulite, Calabria, Italy. XPL image, 2x (Field of view = 7mm)
sillimanite2015g(8).jpg

Big sillimanite crystals, with microfractures perpendicular to long axis, in association with biotite in a sialic granulite, Calabria, Italy. PPL image, 2x (Field of view = 7mm)
sillimanite2015g(9).jpg

Big sillimanite crystals, with microfractures perpendicular to long axis, in association with biotite in a sialic granulite, Calabria, Italy. XPL image, 2x (Field of view = 7mm)
sillimanite2015g(10).jpg

Big sillimanite crystals, with microfractures perpendicular to long axis, in association with biotite in a sialic granulite, Calabria, Italy. PPL image, 2x (Field of view = 7mm)
sillimanite2016(1).jpg

sillimanite crystals in association with biotite in a kinzigite, Calabria, Italy. PPL image, 2x (Field of view = 7mm)
sillimanite2016(2).jpg

sillimanite crystals (yellow-orange interference colors) in association with biotite in a kinzigite, Calabria, Italy. XPL image, 2x (Field of view = 7mm)
sillimanite2016(3).jpg

sillimanite crystals in association with biotite in a kinzigite, Calabria, Italy. PPL image, 2x (Field of view = 7mm)
sillimanite2016(4).jpg

sillimanite crystals (yellow-orange Interference colors) in association with biotite in a kinzigite, Calabria, Italy. XPL image, 2x (Field of view = 7mm)
sillimanite2016(5).jpg

sillimanite crystals in association with biotite in a kinzigite, Calabria, Italy. PPL image, 2x (Field of view = 7mm)
sillimanite2016(6).jpg

sillimanite crystals in association with biotite in a kinzigite, Calabria, Italy. PPL image, 2x (Field of view = 7mm)
sillimanite2016(7).jpg

sillimanite crystals in association with biotite in a kinzigite, Calabria, Italy. PPL image, 2x (Field of view = 7mm)
sillimanite2016(8).jpg

sillimanite crystals in association with biotite in a kinzigite, Calabria, Italy. PPL image, 2x (Field of view = 7mm)
sillimanite2016(9).jpg

sillimanite crystals (yellow-orange interference colors) in association with biotite in a kinzigite, Calabria, Italy. XPL image, 2x (Field of view = 7mm)