About Talc
Talc is a mineral composed of hydrated magnesium silicate with the chemical formula Mg3Si4O10(OH)2, and is usually found in metamorphic rocks with abundant carbonate minerals associated. It is best known among geologists for its low hardness (H =1). Allegedly named in 1546 by Georgius Agricola (Georg Bauer), name of ancient origin, probably derived from Arabic talq.
Talc hand-specimen
Formula: Mg3Si4O10(OH)2
System: Triclinic
Color: Colorless, white, pale green, bright emerald-green to dark green, brown, gray
Lustre: Sub-Vitreous, Resinous, Waxy, Greasy, Pearly
Hardness: 1
Density: 2.58–2.83
System: Triclinic
Color: Colorless, white, pale green, bright emerald-green to dark green, brown, gray
Lustre: Sub-Vitreous, Resinous, Waxy, Greasy, Pearly
Hardness: 1
Density: 2.58–2.83
XPL
PPL
Rutile #1 thin section (hFOV 2mm)
Talc PPL properties
Relief: Low-Moderate positive
Habit/Form: Talc is common as foliated, radiating, or randomly oriented aggregates of irregular flakes or fibers resembling the micas. Individual grains may be bent.
Color: Colorless
Pleochroism: –
Cleavage: Perfect basal cleavage {001} fails to produce extensive flat surfaces, due to bent and contorted sheets.
Habit/Form: Talc is common as foliated, radiating, or randomly oriented aggregates of irregular flakes or fibers resembling the micas. Individual grains may be bent.
Color: Colorless
Pleochroism: –
Cleavage: Perfect basal cleavage {001} fails to produce extensive flat surfaces, due to bent and contorted sheets.
Talc XPL properties
Isotropy/Anisotropy: Anisotropic
Interference color: Order III yellow
Extinction angle: 0 – 3°
Twins: Absent
Uniaxial/Biaxial: Biaxial (-)
Optic axial angle (2V): 2V 0 – 30°
Interference color: Order III yellow
Extinction angle: 0 – 3°
Twins: Absent
Uniaxial/Biaxial: Biaxial (-)
Optic axial angle (2V): 2V 0 – 30°
Talc distinguishing features under the microscope
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References
- Deer, W. A., Howie, R. A., & Zussman, J. (2013). An introduction to the rock-forming minerals (pp. 498). Mineralogical Society of Great Britain and Ireland, London.
- mindat.org – The Mineral Database
