Metamorphic Minerals

Texture and Microstructure

Metamorphic Rocks

Anthophyllite - (Mg, Fe)₂(Mg, Fe)₅Si₈O₂₂(OH, F)₂

Gedrite - (Mg;Fe²⁺)₂[(Mg;Fe²⁺)₃Al₂](Si₆Al₂)O₂₂(OH)₂

Anthophyllite

Anthophyllite is a common component of some metamorphic and metasomatic rocks. Its name comes from the Latin word for clove and is an allusion to its typical and distinct clove-brown color. Anthophylite is the product of metamorphism of magnesium-rich rocks, especially ultrabasic igneous rocks and impure dolomitic shales. It also forms as a retrograde product rimming relict orthopyroxenes and olivine, and as an accessory mineral in cordierite-bearing gneisses and schists. Anthophyllite also occurs as a retrograde metamorphic mineral derived from ultramafic rocks along with serpentinite. Anthophyllite is formed by the breakdown of talc in ultramafic rocks in the presence of water and carbon dioxide as a prograde metamorphic reaction. The partial pressure of carbon dioxide (XCO₂) in aqueous solution favors production of anthophyllite.

Ultramafic rocks in purely hydrous, CO₂-free environments will tend to form serpentinite-antigorite-brucite-tremolite assemblages (dependent on MgO content) or at amphibolite to granulite metamorphic grade, metamorphic pyroxene or olivine. Thus, metamorphic assemblages of ultramafic rocks containing anthophyllite are indicative of at least greenschist facies metamorphism in the presence of carbon dioxide bearing metamorphic fluids.

The typical metamorphic assemblage reactions for low-magnesian (<25% MgO) and high-magnesian (>25% MgO) ultramafic rocks are:

• olivine + tremolite + talc → olivine + tremolite + anthophyllite
• talc + tremolite + magnesite → tremolite + anthophyllite + magnesite
• talc + magnesite + tremolite → anthophyllite + tremolite + magnesite

Retrogressive anthophyllite is relatively rare in ultramafic rocks and is usually poorly developed due to the lower energy state available for metamorphic reactions to progress and also the general dehydration of rock masses during metamorphism. The usual metamorphic assemblage of retrograde-altered ultramafic rocks is thus usually a serpentinite or talc-magnesite assemblage. Retrograde anthophyllite is present most usually in shear zones where fracturing and shearing of the rocks provides a conduit for carbonated fluids during retrogression.

Gedrite

Gedrite occurs in contact and medium to high grade metamorphic rocks in association with garnet, cordierite, anthophyllite, cummingtonite, sapphirine, sillimanite,kyanite, quartz, staurolite and biotite. Gedrite was first described for an occurrence in Gèdres, Héas Valley, Francein 1836.

Optical Properties of antophyllite and gedrite:

• Colour: Colorless, Pale brown to pale yellow.
• Pleochroism: Gedrite have a stronger pleochorism than Antophyllite (pale brown to darker brown).
• Habit: Elongate to fibrous crystals.
• Relief: Moderate.
• Birifrangence: Low-moderate (antophyllite), moderate (Gedrite).
• Extinction: Straight.

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Radial yellowish brown to light green acicular anthophyllite crystals. Bastnas mines, Sweden.

Radial yellowish brown to light green acicular anthophyllite crystals. Bastnas mines, Sweden.

Radial black acicular gedrite crystals. Schisshyttan, Sweden.

Radial black acicular gedrite crystals. Schisshyttan, Sweden.

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.

Anthophyllite
Colorless, Fibrous anthophyllite crystals in a alterd harzburgite, Elba island, Italy. PPL image, 2x (Field of view = 7mm)	Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 2x (Field of view = 7mm)	Colorless, Fibrous anthophyllite crystals in a alterd harzburgite, Elba island, Italy. PPL image, 2x (Field of view = 7mm)
Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 2x (Field of view = 7mm)	Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 2x (Field of view = 7mm)	Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 2x (Field of view = 7mm)
Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 2x (Field of view = 7mm)	Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 2x (Field of view = 7mm)	Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)
Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)	Basal section of Anthophyllite in a alterd harzburgite, Elba island, Italy. PPL image, 10x (Field of view = 2mm)	Fibrous anthophyllite with Straight extinction in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)
Basal section of Anthophyllite in a alterd harzburgite, Elba island, Italy. PPL image, 10x (Field of view = 2mm)	Basal section of Anthophyllite in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)	Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)
Fibrous anthophyllite crystals (with moderate interference colors) note the E-W Anthophyllite with Straight extinction. XPL image, 10x (Field of view = 2mm)	Basal sections of Anthophyllite in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)	Fibrous anthophyllite crystals (with low interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)
Fibrous anthophyllite crystals (with low interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)	Fibrous anthophyllite crystals (with low interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)	Fibrous anthophyllite crystals (colorless) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)
Fibrous anthophyllite crystals (colorless) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)	Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)	Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)
Fibrous anthophyllite crystals (colorless) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)	Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)	Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)
Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)	Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)	Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)
Fibrous anthophyllite crystals (colorless) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)	Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)	Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)
Fibrous anthophyllite crystals (colorless) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)	Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)	Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)
Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)	Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)	Fibrous anthophyllite crystals (with moderate interference colors) in a alterd harzburgite, Elba island, Italy. XPL image, 10x (Field of view = 2mm)

Gedrite
Fibrous gedrite crystals (gray-green) in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. PPL image, 10x (Field of view = 2mm)	Fibrous gedrite crystals in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. XPL image, 10x (Field of view = 2mm)	Fibrous gedrite crystals (gray-green) in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. PPL image, 10x (Field of view = 2mm)
Fibrous gedrite crystals in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. XPL image, 10x (Field of view = 2mm)	Fibrous gedrite crystals in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. XPL image, 10x (Field of view = 2mm)	Fibrous gedrite crystals (gray-green) in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. PPL image, 10x (Field of view = 2mm)
Fibrous gedrite crystals in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. XPL image, 10x (Field of view = 2mm)	Fibrous gedrite crystals in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. XPL image, 10x (Field of view = 2mm)	Fibrous gedrite crystals in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. XPL image, 10x (Field of view = 2mm)
Fibrous gedrite crystals in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. XPL image, 10x (Field of view = 2mm)	Fibrous gedrite crystals in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. XPL image, 10x (Field of view = 2mm)	Fibrous gedrite crystals in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. XPL image, 10x (Field of view = 2mm)
Fibrous gedrite crystals in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. XPL image, 2x (Field of view = 7mm)	Fibrous gedrite crystals (gray-green) in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. PPL image, 2x (Field of view = 7mm)	Fibrous gedrite crystals in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. XPL image, 2x (Field of view = 7mm)
Fibrous gedrite crystals in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. XPL image, 2x (Field of view = 7mm)	Fibrous gedrite crystals (gray-green) in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. PPL image, 2x (Field of view = 7mm)	Fibrous gedrite crystals in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. XPL image, 2x (Field of view = 7mm)
Fibrous gedrite crystals (gray-green) in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. PPL image, 2x (Field of view = 7mm)	Fibrous gedrite crystals (gray-green) in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. PPL image, 2x (Field of view = 7mm)	Fibrous gedrite crystals (gray-green) in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. PPL image, 10x (Field of view = 2mm)
Fibrous gedrite crystals in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. XPL image, 10x (Field of view = 2mm)	Fibrous gedrite crystals in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. XPL image, 10x (Field of view = 2mm)	Fibrous gedrite crystals in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. XPL image, 10x (Field of view = 2mm)
Fibrous gedrite crystals in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. XPL image, 10x (Field of view = 2mm)	Fibrous gedrite crystals (gray-green) in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. PPL image, 2x (Field of view = 7mm)	Fibrous gedrite crystals in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. XPL image, 10x (Field of view = 2mm)
Fibrous gedrite crystals (gray-green) in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. PPL image, 2x (Field of view = 7mm)	Fibrous gedrite crystals in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. XPL image, 10x (Field of view = 2mm)	Fibrous gedrite crystals (gray-green) in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. PPL image, 2x (Field of view = 7mm)
Fibrous gedrite crystals (gray-green) in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. PPL image, 2x (Field of view = 7mm)	Fibrous gedrite crystals in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. XPL image, 10x (Field of view = 2mm)	Fibrous gedrite crystals (gray-green) in a metamorphosed aplite. Schisshyttan Väster - Silvberg Dalarna, Sweden. PPL image, 2x (Field of view = 7mm)