Metamorphic Minerals

Texture and Microstructure

Metamorphic Rocks

Talc - Mg₃Si₄O₁₀(OH)₂

Allegedly named in 1546 by Georgius Agricola (Georg Bauer) from Arabic "talq", pure, probably alluding to the color of its powder. Talc has the chemical formula - Mg₃Si₄O₁₀(OH)₂. It is probably best know for its low hardness, although it has a micaceous structure, it is so easily deformed, that crystals are rarely seen. Talc is a metamorphic mineral resulting from the metamorphism of magnesian minerals such as serpentine, pyroxene, amphibole, olivine, in the presence of carbon dioxide and water. This is known as talc carbonation or steatization.

Talc can be formed in different geological environments or processes. Several types of talc deposits may be distinguished according to the present composition and parent rock from which they are derived:

• Alteration of sedimentary magnesium carbonate rocks

The talc is formed by the alteration of sedimentary magnesium carbonate rocks (dolomite and magnesite) at elevated temperature and pressure. While the magnesium is fixed in situ, the silica is transported by silica-containing hot fluids to react with the Magnesium- bearing carbonates to form talc. These altered rocks are then talc-rich dolomites or magnesites. This type of deposits usually deliver the massive talcs accompanied by carbonates, chlorite and some quartz. The mineral composition is generally 30 - 100% talc, 0 - 70% chlorite/carbonates and 0.1 - 0.5% quartz.

Formation of talc rock from dolomite.

• Alteration of magnesium-rich ultramafic rocks

These talc deposits result from hydrothermal alteration of magnesium-rich ultramafic rocks. Alteration process is two-fold: first hydration of these mafic minerals such as olivine or pyroxene by H₂O influx into serpentine which is a hydrated Magnesium-silicate. Second step is alteration of serpentine into talc and magnesite by CO₂- addition:

2Mg₃Si₂O₅(OH)₄ (serpentine) + 3CO₂ = Mg₃Si₄O₁₀(OH)₂ (talc) + 3MgCO₃ (magnesite) + 3H₂O.

These rocks consist of talc, magnesite, chlorite, sulfides and other minerals with no or very low quartz content.

Talc may also be derived from alumino-silicate rocks. In this case, talc is stable under high-pressure conditions up to a temperature of about 700°C in association with phengite, Mg-garnet, Mg-chloritoid and kyanite in meta pelites:

chlorite + quartz = kyanite + talc + water

In this reaction, the ratio of talc and kyanite is dependent on aluminium content, with more aluminous rocks favoring production of kyanite. Such rocks are typically white, friable, and fibrous, and are known as whiteschist.

Optical Properties:
• Form: Fibrous, scaly or dense, randomly oriented or subparallel to radiating aggregates.
• Cleavage: Perfect (001).
• Color: Colorless.
• Interference colors: third-order colors, similar to muscovite.

talc2020(2).jpg

Fine-grained massive talc crystals. Murphy, North Carolina, USA. From RRUFF.

Micaceous talc crystals. Old Talc mine, Chester, Vermont, USA. From RRUFF.

Micaceous talc crystals. St. Lawrence County, New York, USA. From RRUFF.

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
Talc veins (colorless) in a serpentinite from Livorno, Italy. PPL image, 2x (Field of view = 7mm)	Talc veins (third-order color) in a serpentinite from Livorno, Italy. XPL image, 2x (Field of view = 7mm)	Talc veins (colorless) in a serpentinite from Livorno, Italy. PPL image, 2x (Field of view = 7mm)
Talc veins (third-order color) in a serpentinite from Livorno, Italy. XPL image, 2x (Field of view = 7mm)	Talc veins (colorless) in a serpentinite from Livorno, Italy. PPL image, 2x (Field of view = 7mm)	Talc veins (third-order color) in a serpentinite from Livorno, Italy. XPL image, 2x (Field of view = 7mm)
Talc veins (third-order color) and serpentine veins in a serpentinite from Livorno, Italy. XPL image, 2x (Field of view = 7mm)	Talc veins (third-order color) and serpentine veins in a serpentinite from Livorno, Italy. XPL image, 2x (Field of view = 7mm)	Talc veins (third-order color) and serpentine veins in a serpentinite from Livorno, Italy. XPL image, 2x (Field of view = 7mm)
Talc vein (colorless) in a serpentinite from Livorno, Italy. PPL image, 2x (Field of view = 7mm)	Talc vein (third-order color) in a serpentinite from Livorno, Italy. XPL image, 2x (Field of view = 7mm)	Talc vein (third-order color) in a serpentinite from Livorno, Italy. XPL image, 2x (Field of view = 7mm)
Talc veins (colorless) in a serpentinite from Livorno, Italy. PPL image, 10x (Field of view = 2mm)	Talc (third-order color) and carbonates (in a cavity) in a serpentinite from Livorno, Italy. XPL image, 2x (Field of view = 7mm)	Talc (third-order color) in a serpentinite from Livorno, Italy. XPL image, 2x (Field of view = 7mm)
Talc vein (third-order color) in a serpentinite from Livorno, Italy. XPL image, 2x (Field of view = 7mm)	Talc vein (third-order color) in a serpentinite from Livorno, Italy. XPL image, 10x (Field of view = 2mm)	Talc vein (third-order color) in a serpentinite from Livorno, Italy. XPL image, 2x (Field of view = 7mm)
Olivine pseudomorphosed by talc. XPL image, 10x (Field of view = 2mm)	Olivine pseudomorphosed by talc. XPL image, 10x (Field of view = 2mm)	Olivine pseudomorphosed by talc. XPL image, 10x (Field of view = 2mm)
Olivine pseudomorphosed by talc. XPL image, 10x (Field of view = 2mm)	Olivine pseudomorphosed by talc. XPL image, 10x (Field of view = 2mm)	Olivine pseudomorphosed by talc. XPL image, 10x (Field of view = 2mm)
Olivine pseudomorphosed by talc. XPL image, 10x (Field of view = 2mm)	Olivine pseudomorphosed by talc. XPL image, 10x (Field of view = 2mm)	Olivine pseudomorphosed by talc and Calcite (high relief). PPL image, 10x (Field of view = 2mm)
Olivine pseudomorphosed by talc. XPL image, 10x (Field of view = 2mm)	Olivine pseudomorphosed by talc. XPL image, 10x (Field of view = 2mm)	Olivine pseudomorphosed by talc. XPL image, 10x (Field of view = 2mm)
Olivine pseudomorphosed by talc. PPL image, 10x (Field of view = 2mm)	Olivine pseudomorphosed by talc. XPL image, 10x (Field of view = 2mm)	Olivine pseudomorphosed by talc. XPL image, 10x (Field of view = 2mm)
Olivine pseudomorphosed by talc. XPL image, 10x (Field of view = 2mm)	Olivine pseudomorphosed by talc. XPL image, 10x (Field of view = 2mm)	Olivine pseudomorphosed by talc. XPL image, 10x (Field of view = 2mm)

Metamorphic Minerals

Texture and Microstructure

Metamorphic Rocks

Talc - Mg3Si4O10(OH)2

Talc - Mg₃Si₄O₁₀(OH)₂