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
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.
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
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.