Pyrite and sphalerite. Cornwall, Britain
In these low grade metamorphosed shales (the 'killas') framboidal pyrite (yellow-white, centre) has not extensively recrystallized. The variation in reflectance between framboids is a measure of the pyrite to non-pyrite ratio within an individual framboidal aggregate which is partly a measure of the amount of recrystallization. Sphalerite (light grey, bottom right) is present. The matrix is fine-grained phyllosilicate.
Polished thin section, plane polarized light, x 80, air