Talc

Talc
Property	Value	Comments
Formula	Mg₃Si₄O₁₀(OH)₂	Very little solid solution.
Crystal System	Monoclinic	Beta = 100°
Crystal Habit	Massive, fine-grained; tabular books	.
Cleavage	(001) perfect	.
Color/Pleochroism	Colorless in thin section	.
Optic Sign	Biaxial (-)	.
2V	0-30^°	.
Optic Orientation	Y=b Z~a X^c ~10^o O.A.P. perp (010)	.
Refractive Indices alpha = beta = gamma = delta =	1.539-1.550 1.589-1.594 1.589-1.596 0.050-0.046	.
Max Birefringence	0.05	Beyond 2nd order blue
Elongation	Not prismatic
Extinction	Can be parallel
Dispersion	r > v	.
Distinguishing Features	Muscovite(35-50), paragonite(40-50), and pyrophyllite (53-62) have larger 2V's. Brucite and gibbsite are (+).
Occurrence	With calcite and tremolite in low-grade marbles. With chlorite or magnesite in blackwall reaction zones around serpentinite bodies.
Editors	John Brady (00)

	Photomicrograph in crossed polarized light of polycrystalline talc replacing dolomite. The talc appears to be replacing dolomite along fractures as well as along its edges. The talc close to the dolomite tends to be more fine-grained. This sample is from the Sweetwater View Mine, Ruby Range, SW Montana.
	Thin section of massive fine-grained talc replacing coarse dolomite in crossed polarized light. The talc is brightly colored (1st to 2nd order colors). The dolomite appears in the washed-out beige characteristic of the high birefringence of carbonates. (Field of view is 5 mm.)
WWW Images	U.C.L.A. Petrographic Workshop University of North Carolina