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Talc is a clay mineral composed of hydrated magnesium silicate with the chemical formula H2Mg34 or Mg3Si4O102. In loose form, it, along with corn starch, was one of the most widely used substances known as baby powder It occurs as foliated to fibrous masses, and in an exceptionally rare crystal form. It has a perfect basal cleavage, and the folia are not elastic, although slightly flexible.

Talc
Talc.jpg
Crystals of talc
General
Category Silicate mineral
Formula
Mg3Si4O10(OH)2
Strunz classification 9.EC.05
Crystal system Monoclinic or Triclinic
Crystal class Either prismatic (2m) or pinacoidal (1)
Unit cell a = 5.291 Å, b = 9.173 Å
c = 5.290 Å; α = 98.68°
β = 119.90°, γ = 90.09°; Z = 2 or
a = 5.287 Å, b = 9.158 Å
c = 18.95 [Å], β = 99.3°; Z = 4
Identification
Color Light to dark green, brown, white, grey
Crystal habit Foliated to fibrous masses, rare as platey to pyramidal crystals
Cleavage Perfect on {001} basal cleavage
Fracture Flat surfaces (not cleavage), fracture in an uneven pattern
Tenacity Sectile
Mohs scale hardness 1 (defining mineral)
Luster Waxlike or pearly
Streak White to pearl black
Diaphaneity Translucent
Specific gravity 2.58 to 2.83
Optical properties Biaxial (-)
Refractive index nα = 1.538 – 1.550
nβ = 1.589 – 1.594
nγ = 1.589 – 1.600
Birefringence δ = 0.051
Pleochroism Weak in dark varieties
Ultraviolet fluorescence Short UV=orange yellow, long UV=yellow
References

Talc is a clay mineral composed of hydrated magnesium silicate with the chemical formula H2Mg3(SiO3)4 or Mg3Si4O10(OH)2. In loose form, it, along with corn starch, was one of the most widely used substances known as baby powder (in the case of talc, often called simply talcum powder; see litigation.) It occurs as foliated to fibrous masses, and in an exceptionally rare crystal form. It has a perfect basal cleavage, and the folia are not elastic, although slightly flexible.

Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 1 as the hardness of talc. As such, talc can easily be scratched by a fingernail. Talc has a specific gravity of 2.5–2.8, a clear or dusty luster, and is translucent to opaque. Talc is not soluble in water, but is slightly soluble in dilute mineral acids. Its color ranges from white to grey or green and it has a distinctly greasy feel. Its streak is white.

Soapstone is a metamorphic rock composed predominantly of talc.

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