Biotite


Occurrence
Biotite is widespread and common in a wide variety of igneous and metamorphic rocks and, to a much lesser extent, in some immature sedimentary rocks.

Composition
Biotite's chemistry is variable, so optical properties vary as well. The most significant end members are annite (KFe3AlSi3O10(OH)2) and phlogopite (KMg3AlSi3O10(OH)2).

Identification
In general, keys to identifying biotite are its color and pleochrism, cleavage, optical texture and characteristic extinction, and habit. Biotite's features generally make identification unambiguous. However, in some medium-grade metamorphic rocks and volcanic rocks, the features are less prominent.

Important properties
·Color - generally pleochroic, typically in shades of brown, but also can be yellow, green or red. Common biotite is one of the most strongly colored minerals seen in thin section. For biotite with low Fe-content (phlogopite), coloration is more subtle (clear, light browns and tans).
·Habit and cleavage - often appears as tabs or long skinny flakes which may be bent. It shows one excellent cleavage, depending on grain orientation. Rarely, it forms hexagonal crystals.
·Birds-eye or pebbly extinction or wavey pattern of interference colors, especially when looking down on flat side of flakes.
·Extinction angle is 0 to a few degrees from cleavage.
·Interference colors may range up to 2nd order red, but color of mineral may mask them.
·Often contains pleochroic halos around small included zircons.
·Biaxial (-), 2V= 0-25o .
·Often alters to cholrite.

Similar minerals
·Light colored phlogopite may be confused with muscovite but has a smaller 2V.
·Brown or green hornblende or other amphiboles (images at right) may occasionally be mistaken for biotite, but amphibole has 2 cleavages, often a diamond shaped cross section, and lacks birds-eye/pebbly extinction.  PP  XP
·Stilpnomelane, a brown brittle mica, has higher relief, lacks birds-eye/pebbly extinction, has poorer cleavage, and has a pseudouniaxial interference figure.
·Brown or green tourmaline lacks biotite's characteristic extinction, cleavage and habit. It also appears darker/stronger colored in some orientations.
·Chlorite has lower relief and birefringence (often showing anomalous colors)


Biotite With Anthophyllite

This sample is a biotite-anthophyllite-cordierite rock from near Guffey, Colorado. In this view, one large flake of biotite dominates the top half of the photos. It is light brown (PP). In XP light it shows a pebbly texture and mostly second order green interference color (XP). Most of the clear mineral matter beneath the biotite is anthophyllite, an amphibole. Minor cordierite cannot be easily picked out at this magnification. The anthophyllite shows low second order interference colors (XP) and has a tabular to bladed appearance.

The field of view is about 2 mm.


Biotite With a Halo Around a Zircon Inclusion

This sample is a biotite-cordierite-orthopyroxene gneiss from near Sioux Lookout, Ontario. The field of view, about 2 mm across, is dominated by olive-green-brown biotite.

The largest grain of biotite contains a pelochroic halo (burn mark) around an included zircon. Although hard to identify in this view, the clear, high relief grains to the left and beneath the biotite are orthopyroxene. The large clear grain to the lower right is cordierite. which, in this view, looks like quartz.

The biotite grain in the lower left shows one good cleavage, but the large grain in the center does not because the view is more or less perpendicular to the cleavage. It does, however, show a partially developed "birds eye" pattern in XP light.



 Biotite in a Mica Schist

The views above show mostly biotite; PP light on the left, XP light on the right.

The large flake near the center and a few other grains, clear in PP light and having high birefringence, are muscovite. Minor quartz is also present in the lower right.

The long flakey habit is typical of biotite (and muscovite) as are the mottled extinction and interference colors (XP view).

Note pleochroism (various shades of brown in PP light).

The rock shown is a mica schist from Manhattan, New York. Field of view is 4.5 mm across.


Biotite in an Andesite

 

The pictures above show a 2 mm wide biotite grain in an andesite from Mineral County, Nevada. PP on the left, XP on the right. The field of view is 4.5 mm.

Note the variable interference colors (XP), caused by variable thickness of the biotite grain. This biotite is armored (surrounded) by magnetite (black, opaque mineral) and lots of quartz and plagioclase.

In this view, we are looking more or less down on a flake, so biotite's cleavage and typical elongate habit do not show well.


Biotite - showing alteration to chlorite
    The photos to the left show biotite with an alteration patch of chlorite through the center (PP on far left, XP on immediate left). Some additional chlorite is just to the right of the bottom right corner of the biotite. Note the light green color of the chlorite and the anomalous (dark inky blue) interference colors. The surrounding minerals are mostly quartz and feldspar, but the two cannot be distunguished in this view. The field of view is about 2.5 mm.

Biotite in a Mica Schist from Western Massachusetts

Here we see mostly biotite flakes. The biotite is pleochroic from nearly clear to medium brown (PP) and shows mica's characteristic cleavage. It also displays many halos around radiation burns, probably centered on included zircon grains. The clear mineral (PP) is quartz and the opaque mineral is graphite.

The field of view is about 2.5 mm.