Metamorphic Minerals

Texture and Microstructure

Metamorphic Rocks

Andalusite (chiastolite)- Al₂SiO₅

Andalusite, named by Jean-Claude Delamétherie in 1798 for the supposed type locality, "Andalusia region" (Spain), is an aluminium mineral with the chemical formula Al₂SiO₅. Andalusite was first analysed by Werner on specimens coming from Spain. Both Werner and Delamétherie believed that the specimens came from Andalusia, which is why the latter named it "andalusite". However, it seems that the analyzed specimens came from El Cardoso (Guadalajara), a town they erroneously thought to be in Andalusia. Consequently the type locality must be considered El Cardoso, probably the outcrop located in Zahurdon. The error was perpetuated by other later authors, who considered that the origin was the province of Almeria, and remained so in the majority of books, even in modern ones.

Andalusite is a common regional metamorphic mineral which forms under low pressure and low to high temperatures. The minerals kyanite and sillimanite are polymorphs of andalusite, 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. Andalusite form al low pressures (<1.5 Kb) and low moderate temperature in thermal aureoles and regional metamorphism of Buchan type (high T and low P). At higher temperature its invert in sillimanite.

The variety chiastolite commonly contains dark inclusions of carbon or clay which form a checker-board pattern when shown in cross-section, while viridine, or manganandalusite, is a Mn-rich variety of andalusite, with deep green colors.

Optical properties

• Color: Colourless or pale pink
• Form: Usually occurs as elongate prisms with square cross section.
• Relief: Moderate.
• Cleavage: (001) good appearing as traces parallel to the prism edge in prismatic sections but intersecting at right angle in basal section.
• Interference colors: Low, first order (like quartz)
• Alteration: Commonly in sericite

Al₂SiO₅ phase diagram

Andalusite porphyroblasts. Clooney, Donegal. Keele collection. From G. Stimpson.

Andalusite (chiastolite). From Wikipedia.

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.

Chiastolite crystals in a slate from Karakorum. PPL image, 1x (Field of view = 9mm)	Chiastolite crystals in a slate from Karakorum. XPL image, 1x (Field of view = 9mm)	Chiastolite crystals in a slate from Karakorum. XPL image, 1x (Field of view = 9mm)
Chiastolite crystals in a slate from Karakorum. PPL image, 1x (Field of view = 9mm)	Chiastolite crystals in a slate from Karakorum. PPL image, 1x (Field of view = 9mm)	Chiastolite crystals in a slate from Karakorum. XPL image, 1x (Field of view = 9mm)
Chiastolite crystals in a slate from Karakorum. PPL image, 1x (Field of view = 9mm)	Chiastolite crystals in a slate from Karakorum. XPL image, 1x (Field of view = 9mm)	Chiastolite crystals in a slate from Karakorum. PPL image, 1x (Field of view = 9mm)
Chiastolite crystals in a slate from Karakorum. PPL image, 1x (Field of view = 9mm)	Chiastolite crystals in a slate from Karakorum. PPL image, 1x (Field of view = 9mm)	Chiastolite crystals in a slate from Karakorum. PPL image, 1x (Field of view = 9mm)
Chiastolite crystals in a slate from Karakorum. PPL image, 1x (Field of view = 9mm)	Chiastolite crystals in a slate from Karakorum. XPL image, 1x (Field of view = 9mm)	Chiastolite crystals in a slate from Karakorum. PPL image, 1x (Field of view = 9mm)
Chiastolite crystals in a slate from Karakorum. PPL image, 1x (Field of view = 9mm)	Chiastolite crystals in a slate from Karakorum. XPL image, 1x (Field of view = 9mm)	Chiastolite crystal in a slate from Karakorum. PPL image, 2x (Field of view = 7mm)
Chiastolite crystal in a slate from Karakorum. PPL image, 2x (Field of view = 7mm)	Chiastolite crystal in a slate from Karakorum. PPL image, 2x (Field of view = 7mm)	Chiastolite crystal in a slate from Karakorum. PPL image, 2x (Field of view = 7mm)
Chiastolite crystal in a slate from Karakorum. PPL image, 2x (Field of view = 7mm)	Chiastolite crystal in a slate from Karakorum. PPL image, 2x (Field of view = 7mm)	Chiastolite crystals in a slate from Karakorum. PPL image, 2x (Field of view = 7mm)
Chiastolite crystals in a slate from Karakorum. PPL image, 2x (Field of view = 7mm)	Chiastolite crystals in a slate from Karakorum. PPL image, 2x (Field of view = 7mm)	Chiastolite crystal in a slate from Karakorum. PPL image, 2x (Field of view = 7mm)
Chiastolite crystal in a slate from Karakorum. XPL image, 2x (Field of view = 7mm)	Chiastolite crystal in a slate from Karakorum. PPL image, 2x (Field of view = 7mm)	Chiastolite crystal in a slate from Karakorum. XPL image, 2x (Field of view = 7mm)
Chiastolite crystal in a slate from Karakorum. PPL image, 2x (Field of view = 7mm)	Chiastolite crystal in a slate from Karakorum. XPL image, 2x (Field of view = 7mm)	Chiastolite crystal in a slate from Karakorum. PPL image, 2x (Field of view = 7mm)
Chiastolite crystals in a slate from Caldew Valley, Skiddaw Cumbria, England. PPL image, 2x (Field of view = 7mm)	Chiastolite crystals in a slate from Caldew Valley, Skiddaw Cumbria, England. PPL image, 2x (Field of view = 7mm)	Chiastolite crystals in a slate from Caldew Valley, Skiddaw Cumbria, England. XPL image, 2x (Field of view = 7mm)
Chiastolite crystals in a slate from Caldew Valley, Skiddaw Cumbria, England. PPL image, 2x (Field of view = 7mm)	Chiastolite crystals in a slate from Caldew Valley, Skiddaw Cumbria, England. XPL image, 2x (Field of view = 7mm)	Chiastolite crystals in a slate from Caldew Valley, Skiddaw Cumbria, England. PPL image, 10x (Field of view = 2mm)
Chiastolite crystals in a slate from Caldew Valley, Skiddaw Cumbria, England. XPL image, 10x (Field of view = 2mm)	Chiastolite crystals in a slate from Caldew Valley, Skiddaw Cumbria, England. PPL image, 10x (Field of view = 2mm)	Chiastolite crystals in a slate from Caldew Valley, Skiddaw Cumbria, England. XPL image, 10x (Field of view = 2mm)
Chiastolite crystals in a slate from Caldew Valley, Skiddaw Cumbria, England. PPL image, 10x (Field of view = 2mm)	Chiastolite crystals in a slate from Caldew Valley, Skiddaw Cumbria, England. PPL image, 10x (Field of view = 2mm)

Metamorphic Minerals

Texture and Microstructure

Metamorphic Rocks

Andalusite (chiastolite)- Al2SiO5

Andalusite (chiastolite)- Al₂SiO₅