Smith College - Geology 222b - Petrology

Petrographic Data File

Biotite
Property
Value
Comments
Formula K(Mg,Fe)3AlSi3O10(OH,O,F)2

Shows substantial variability in composition that can be represented with the four end members:

Annite= KFe3AlSi3O10(OH)2

Phlogopite= KMg3AlSi3O10(OH)2

Siderophyllite KFe2AL(Al2Si2O10)(OH)2

Unnamed end member (formerly known as eastonite)= KMg2Al(Al2Si2O10)(OH)2
Crystal System Monoclinic (2/m) Beta = 99.3o
Crystal Habit Pseudo-hexagonal prisms or lamellar plates without crystal outline. Micaceous or tabular grains, also irregular grains. Thin folia are elastic.
Cleavage (001) perfect Easily seen in thin section
Color/Pleochroism Typically brown, brownish green or reddish brown Usually strongly pleochroic so grains are darker when the trace of cleavage is parallel to the lower polarizer. colors are X = colorless, light tan, pale greenish brown, pale green; Y~Z = brown, olive brown, dark green, dark red-brown. Intensity of color generally increases with increase in iron content.  Parallel to (001) yields darker colors with little pleochroism.
Optic Sign Biaxial (-) Some biotite may have a 2V of 0o, is sensibly uniaxial.
2V 0-25o  
Optic Orientation Y=b 
Z^a = 0 - 9o
X^c = 0 - 9o
optic plane (010)		  
Refractive Indices
alpha = 
beta = 
gamma = 		 1.522-1.625 
1.548-1.672
1.549-1.696 		 Refractive indices increase with increasing iron content.
Max Birefringence 0.03-0.07 Strong mineral color will often mask the interference colors (3rd to 4th order). Flakes on the cleavage and sections that are cut parallel with {001}show low birefringence.
Elongation Yes  Along cleavage
Extinction  Parallel or close to parallel Has a "birds eye" texture seen at extinction.
Dispersion v > r Less commonly, r > v for Mg rich varities
Distinguishing Features Resembles muscovite but has smaller 2V and darker color. Mottled "birds-eye" extinction helps distinguish from similar minerals outside the mica family.  Has moderate to high relief in thin section. Has micaceous habit and dark color.
Occurrence In igneous rocks, more commonly silicic and alkalic, e.g. granties, diorites, gabbros and peridotites. Important in metamorphic rocks including shists, gneisses, phyllites, and hornfelses.  Also found in immature sedimentary rocks, but will alter to clay minerals when weathered.
Editors Mary Hawkins (01J), Raycine Hodo (02J), Lisa Berrios (02), Jennifer Fitzsimmons (03), Rachel Grandpre (05)

 
In plain polarized light, biotite displays perfect cleavage in one direction. It appears as brown or green platy forms.  The maximum absorption color is shown when the polarizer is parallel to the cleavage.
Under crossed polars biotite displays a distinctive mottled texture as it nears extinction. In extinction position small spots fail to extinguish. These traits are charicteristic of birdseye extinction which is common in micas.
Under crossed polars biotite displays a distinctive mottled texture as it nears extinction. In extinction position small spots fail to extinguish. These traits are charicteristic of birdseye extinction which is common in micas.
Biotite
 Under polarized light biotite appears green with pink undertones. It is surrounded by quartz and feldspars. This thin section is from a granitic rock. 
In plane polarized light, biotite is seen as as dark brown to grey against the surrounding mostly colorless minerals. Under crossed polars "bird's eye extinction" can easily be seen when the mineral is nearly extinct. Often, the mineral color masks the interference colors when the mineral is not extinct.

In plain light, biotite appears a slightly drab brown. Radiation damage from zircon can leave pleochroic spots that fade to extinction at a different rate that the surrounding, undamaged material. Most of the surrounding minerals appear colorless. There are several small opaque grains as well.
WWW   U.C.L.A. Petrographic Workshop

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