optical microscopy

This page is under construction. Check back soon for new transmitted light photomicrographs of minerals. I’m not yet sure how many minerals I’ll feature here or even exactly how I’ll organize this, since I don’t want to inadvertently undermine potential interest in my optical mineralogy e-books. My feeling at the moment is that rather than just compile photomicrographs of minerals, it might be more worthwhile to highlight a few good examples of different optical properties. I’m also thinking about adding a few instructional photos (and maybe videos?) on useful topics that might be glossed over in an introductory optical mineralogy course, such as characterizing dispersion, estimating mineral compositions from optical properties, and assigning pleochroic colors to vibration directions.

In the meantime, this pair of temporary placeholder images (top photo in PPL, with lower polarizer oriented E-W; bottom photo in XP) demonstrate a corona texture of garnet + orthopyroxene (along with some brown pargasite), in a granulite facies meta-gabbro from Ontario (sample FKM-96). Due to its higher ferrosilite content (this opx is Fs43), the observed birefringence of this metamorphic orthopyroxene is higher than that observed for the more commonly-encountered-in-petrography-class enstatite-rich opx compositions typical of ultramafic & mafic igneous rocks.  But also note: the yellow cast of some of the An27 plagioclase in the XP image indicates the thin section may be thicker than the nominal 30 μm (perhaps closer to 40 μm).



Dispersion and anomalous birefringence

Testing out adding video. This video above shows a crystal of potassian fluoro-richterite (from thin section FKM-27; Kipawa complex, Québec, Canada) displaying marked brown to blue anomalous birefringence with stage rotation. This is a good example of extreme dispersion. The video below is the off-center BxA figure of the same crystal. The dispersion appears to be r > v.

Another example of dispersion. This video is a centered BxA figure of lamprophyllite (from thin section FKM-17; Lovozero massif, Kola Peninsula, Russia). The dispersion is r > v.