Smith College - Geology 222b - Petrology

Petrographic Data File

Formula Mg3Si4O10(OH)2 Very little solid solution.
Crystal System Monoclinic Beta = 100°
Crystal Habit Massive, fine-grained; tabular books .
Cleavage (001) perfect .
Hardness 1 Low hardness and greasy feel can be used to distinguish talc and other sheet silicates.
Specific Gravity 2.58-2.83  
Color/Pleochroism Colorless in thin section No pleochroism; colorless to slightly brown in thin section
Optic Sign Biaxial (-) .
2V 0-30° .
Optic Orientation Y=b 
X^c ~10o
O.A.P. perp (010)
Refractive Indices
alpha = 
beta = 
gamma = 
delta = 

Max Birefringence 0.05 Beyond 2nd order blue
Elongation Not prismatic  
Extinction Can be parallel  
Dispersion r > v .
Distinguishing Features

Can look similar to micas in cross-polarized light.
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.

Deer WA, Howie RA, Zussman J Talc. In: Rock-Forming Minerals, 6th edn. Longmans, pp 121–130.
"" Talc: The mineral Talc information and pictures. N.p., n.d. Web. 29 Jan. 2017.

Editors Heather Upin ('16), Emily Grote ('18)

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.)

Petrographic image of talc showing the variation of color in plain-polarized light verses cross-polarized light. The talc is pastel-colored in cross-polarized light, mainly 2rd order colors due to a maximum birefringence of 0.050. This image is also from a massive talc deposit in SW Montana.

Photomicrograph of talc in cross-polarized and plane polarized light.  The talc has golden, second order colors and is lenticular in shape. It is surrounded by quartz and calcite. This photomicrograph was taken from slide 46-6A, a low-grade marble from Vermont.  (Click for roll over image.)

Image of off-centered bxa figure for talc; Click on the image to see a rollover image that shows insertion of the quartz plate. This image shows that talc is biaxial negative as the colors nearest the NE isogyre (concave up to the NE) changes to yellow, resulting from a subtraction of interference colors. Conversely the colors closest the SW facing isogyre turn blue, similarly showing the mineral is biaxial negative.
WWW Images

Union College [under amphiboles]
Oxford University [image 107]
University of North Carolina

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