Today, we’re going to move closer to the Victory Pluton in the same contact aureole as the past two posts. The increase in temperature has increased the metamorphic grade of the sample and the rocks now contain: garnet + staurolite + sillimanite + biotite + quartz + plagioclase + cordierite. The latter is a very hard mineral to ID in thin section because it looks very similar to both quartz and plagioclase. In this case, though, the cordierite came with a calling card: pleochroic halos around radioactive inclusions.
For this rock, the radioactive mineral present is monazite (a similar halo would form around zircon, xenotime, or allanite). The cordierite is wrapped by fibrous sillimanite (there are also large prismatic sillimanite grains in this thin section, just not in this field of view). Biotite (the yellowish-brown more boxy mineral) is present as inclusions in some of the cordierite.
Pleochroic halos form as radioactive elements in one mineral decay and release alpha (or beta) particles. If the original mineral is small enough, the alpha particles travel into the surrounding mineral and the particles themselves cause the crystal lattice of the host mineral to be deformed. The deformation continues over time as more & more alpha particles are released and leads to the formation of an approximately circular halo around the included mineral. In cordierite, the ring of deformed crystal lattice around the inclusion is yellow in color. If the grain was plagioclase or quartz, the yellow halo would be absent, since these minerals don’t react in the same way to lattice deformation. (I looked on the internet a bit for an appropriate diagram, but couldn’t find anything–its at this point I normally would just draw on the chalkboard for my students!)
The cordierite itself adds a very interesting part of the overall story, since the mineral is stable at relatively high temperatures, but low pressures in metamorphic rocks. As it turns out, the texture of this sample was almost more interesting than the presence of cordierite within it, but we’ll get to that in the weeks to come.
(Oh, and for those budding gemologists out there, iolite is the jewelry form of cordierite.)