Optical Microscopy of Meteoritic Metal

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The study of metals and other opaque minerals requites the use of reflected-light microscopy.  The Polyvar microscope manufactured by Reichert-Jung and Leica is considered by many to be the "Rolls Royce" of microscopes.  I own a Polyvar-SC equipped with both reflected and transmitted-light illumination.  The "supermodule" slider allows easy transitioning between brightfield, polarized, and darkfield modes.  The microscope is also equipped with differential interference contrast (DIC, also known as Nomarski), and this mode of illumination is effective in amplifying minor topographic variations of the specimen's surface.

The following images show a troilite inclusion in the iron meteorite NWA6259, viewed using the three modes of reflected-light illumination (FOV = 688 microns).

 

The photo on the left was taken in plane-polarized light and shows troilite nodule surrounded by cloudy zone taenite. The thin white mineral at the top of the troilite is schreibersite (isotropic). The center photo, taken in cross-polarized light, reveals the presence of troilite twinning.  The third photo was taken using differential interference contrast.

For reflected-light work, it is important that the specimen's surface is precisely perpendicular to the microscope objective.  This can be accomplished using a metallographic specimen press (shown next to the microscope in the top photo).  Plasticine is sandwiched between the specimen and glass slide, and the press assures parallelism between the glass slide and specimen.

For photography, I use a Canon T3i coupled to the microscope using a Zeiss camera adapter of 0.25x magnification (Zeiss # 45 60 29) and Wild-Herzburg eyepiece of 5x.  This combination yields a total camera adapter magnification of 1.25x.  The Canon T3i is an excellent choice for microscopy because it has "Liveview" capability.