A SIMPLE METHOD TO CREATE MINERAL MOUNTS IN THIN SECTION TO TEACH COMPARATIVE RELIEF, COLOR, PLEOCHROISM, AND BIREFRINGENCE IN OPTICAL MINERALOGY

Mineral separates mounted in thin section help introduce mineralogy students to concepts of relief, color, pleochroism, and birefringence, and serve as reference materials. Here, a simple method is described to produce materials that can be sent to thin section manufacturers to construct sets of mineral mounts. The method is relatively cheap, requires only simple equipment and few skills, and allows mounting of multiple minerals (easily 20) in a single thin section. The main “trick” is to first create a slurry of 500-1000 mg of each crushed mineral plus ~300 mg (500 µl) of epoxy and use a micro spoon spatula to spoon each slurry into a 4 cm long tube of 6 mm (1/4") diameter, standard wall, glass tubing. After epoxy sets, tubes can be bundled together and shipped to thin section makers, who can set the bundles in epoxy (ends down) and make 10 or more duplicate sections.

Production details: slurries are created by first aliquoting epoxy into microcentrifuge tubes, weighing out each mineral separately, then pouring on top of the epoxy. Gravity settles grains without creating bubbles. Ideal grain sizes are between ~150 and ~750 µm. The slurry is spooned into a mound on top of an open tube, and gravity and surface tension draw the slurry slowly down the tube. Filled tubes laid on their sides will set up without leaking. This method requires only a source of mineral, a mortar and pestle, a set of 4” sieves, microcentrifuge tubes, epoxy and means for mixing, transfer pipettes, a balance, and a micro spoon spatula. Total processing time for each mineral from crushing through completely filling a tube is ~45 minutes. An optional wafering saw allows oriented strips of minerals to be mounted, e.g., tourmaline to illustrate reverse pleochroism. The strip is first inserted into the tube, and epoxy in pipetted into the top; surface tension and gravity downfill epoxy into the tube around the mineral strip with minimal bubbles. A wafering saw also allows more efficient use of each tube: 1 mm slices are cut; mounted on double-stick tape; surrounded with a 1x2” aluminum sleeve (1/4” wall thickness; coated with release agent); and backfilled with epoxy. This approach conserves resources (25 thin sections), but requires an additional ~45 minutes effort per thin section.