Smith College - Geology 222b - Petrology

Petrographic Data File

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

The variation in biotite usually occurs in what elements occupy the octahedral sites. However, variations can also occur in what elements are found in the tetrahedral and hydroxyl sites. Examples of these are siderophyllite, oxybiotite, and ferriannite.

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 that may be bent (especially in metamorphic rocks). Thin folia are elastic.
Physical Properties

H = 2.5 - 3
G = 2.7 - 3.3

The color of biotite in hand sample is brown to black (sometimes greenish). Its streak is white or gray, and it has a vitreous luster.

The physical properties of biotite are affected by the amount of iron present. The specific gravity (G) is greater with increasing iron content, while the color in hand sample is darker with an increase in iron.
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. Pleochroic halos around radioactive minerals (zircon or allanite) are common.
Optic Sign Biaxial (-) Some biotite may have a 2V of 0o, is sensibly uniaxial.
2V 0-25o  


Twins with 001 composition planes are possible, but usually not observed
Optic Orientation Y=b 
Z^a = 0 - 9o
X^c = 0 - 9o
optic plane (010)
Refractive Indices
alpha = 
beta = 
gamma = 

Refractive indices increase with increasing iron content. The diversity in biotite's composition makes it hard to use optical properties as an indicator for composition. Ferriannite may have alpha and gamma indices as high as 1.677 and 1.721 respectively.
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. Bent grains show wavy extinction.
Dispersion v > r (weak) Less commonly, r > v for Mg rich varieties.
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 Biotite is common in a variety of igneous and metamorphic rocks. In igneous rocks, it is found more commonly in silicic and alkalic rocks, e.g. granties, diorites, gabbros and peridotites. It is important in metamorphic rocks including schists, gneisses, phyllites, and hornfels.  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), Lily Seidman (11), Ngozika Onuzo (12)

First In plain polarized light, biotite displays perfect cleavage in one direction. 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.

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 in a granite 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. There are several small opaque grains as well. Under polarized light biotite appears green with pink undertones. It is surrounded by quartz and feldspars.


This is an image of a thin section containing biotite. It is seen now in plane polarized light as a brown to tan color. Under cross-polarized light, the biotite thin section appears darker brown in areas as well as a pink-yellow color in others.

Biotite, usually dark green, brown, or black, has a pale brown color under plane-polarized light in this sample. It exhibits high interference colors in cross-polar light just as muscovite, but has a lower birefringence than muscovite.
Biotite shows parallel extinction in cross polarized light, and has perfect cleavage in one direction. The biotite mineral shown here is exhibits bird's eye texture during extinction, and the mineral is surrounded by quartz, muscovite, and feldspars.

UCLA Petrographic workshop
USGS Rare Earth elements containing biotite
North Dakota Optical Mineralogy

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