Sometimes, radioactive minerals are included
in other minerals. Radioactive decay may then cause burn marks,
called pleochroic halos, or just halos. Two examples
are shown below.
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 to light green, high relief grains to the left and beneath the biotite are orthopyroxene. The large clear (PP) 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
Cordierite, Orthopyroxene and Biotite from near Kazabazua, Quebec
The photos above are of a spectacular rock
from near Kazabazua, Quebec. It contains primarily cordierite
and orthopyroxene with lesser amounts of
bioite. The cordierite
is twinned (and in XP looks a lot
like plagioclase) and contains
pleochoric halos around zircon inclusions.
The halos appear as "burn" marks in PP light. The orthopyroxene is blocky and fractured, shows
high relief (PP), and has first order
interference colors (XP). It
is pleochroic, with color
ranging from light pink to light green. Biotite
is also pleochroic and here exists as
flakes in various shades of brown (PP) and has upper 2nd order
interference colors (XP).