Clausthalite is a very rare mineral in nature, but a relatively common trace mineral in West Virginia coals. Clausthalite and suspected chalcopyrite/clausthalite solid solutions were observed in 12 of 24 coal samples in an unpublished SEM study by the WVGES. Clausthalite generally was observed as very small submicron grains distributed throughout the coals. These occurred in partings and within the organic parts of the coal in vitrinite and on occasion filling voids in semifusinite as shown in this SEM photomicrograph. In this semifusinite ("fossil charcoal") band clausthalite fills some voids with sphalerite, chalcopyrite and clay minerals filling other voids. Clausthalite is classified as part of the galena group of minerals, but in coal is rarely associated with galena, and is instead often associated with chalcopyrite and in the observed solid solution between clausthalite and chalcopyrite. This is borne out in other fields with clausthalite occurring in association with copper sulfides in nature 1. The left photograph shows a typical SEM view produced by secondary electrons. A technique for identifying very small trace minerals uses backscatter electrons as in the image at the right. Backscatter electrons are reflected off of the surface in proportion to the atomic weight of the surface substrate. Coal (carbon) is low atomic weight and reflects very few electrons and appears dark, heavy minerals, especially those containing lead, appear very bright and are easily spotted, even when very small, while most common silicate minerals appear very faint (dots at the bottom of the photograph are each 10 µm apart).
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