Christopher J. Stefano

Connoisseur's Choice: Mimetite Epimorphs after Cerussite, Death Valley, California

This Connoisseur’s Choice column is a departure from the usual format. Instead of focusing on a single mineral species, as has been the custom since the column began in 1993, it deals with a specific occurrence that produced beautiful mimetite epimorphs* after cerussite. For a detailed profile of mimetite, see the Connoisseur’s Choice column by Robert B. Cook in the March/April 2001 issue (pages 114–117), and for an in-depth look at cerussite, see Cook’s column in the May/June 1997 issue (pages 176–179). Discussions of connoisseur-quality mineral specimens often overlook one of the most interesting groups of minerals—pseudomorphs and related specimens. The word pseudomorph comes from two Greek words: pseudo (“false”) and morphē (“shape” or “form”). A pseudomorph is a mineral that shows the form of another species rather than its own. Pseudomorphs occur in a variety of ways, the most common being by replacement where one species has completely replaced another. Another type is an incrustation pseudomorph or epimorph where one species coats the other, and the other species later dissolves away (Hurlbut 1971). Because the replacement minerals are typically very fine grained, pseudomorphs are typically not lustrous and are rarely transparent. This can often result in specimens that are not particularly attractive; however, there are some noteworthy exceptions. One such notable, if obscure, exception is the mimetite epimorphs after cerussite from the Fred Glaze claim in the Panamint Range, Death Valley, California. These specimens consist of fine-grained orange mimetite that forms a thin shell in the shape of an earlier-formed V-twinned cerussite crystal, which in most cases has subsequently dissolved away. They also often have small red wulfenite crystals on them, adding additional color. Not only are these specimens beautiful, but they are also quite rare. There are probably fewer than twenty examples extant (Dave Bunk, pers. comm., 2012). Beyond their beauty, these specimens also tell an interesting story. Connoisseurship of fine minerals is not just appreciation of specimen beauty; it is also appreciation of the stories about Earth that every specimen tells—some more clearly than others. In the case of these pseudomorphs, a cerussite crystal had to grow first. Cerussite is a secondary mineral, so it probably grew as a weathering product of primary galena. At some point after the cerussite formed, the mimetite coating developed on the surface. The cerussite subsequently became unstable (one specimen still has undissolved cerussite), causing it to dissolve away. At each growth stage the chemistry of the fluid would have to have changed. The fluid would have become more acidic perhaps from sulfuric acid from pyrite weathering in the deposit, mobilizing arsenic, depositing the mimetite, and dissolving the cerussite. Following the dissolution of cerussite, wulfenite crystals grew on many of the casts. Because wulfenite crystals appear on both the inside and outside of the casts, the wulfenite growth must have occurred last. This means the cerussite was already dissolved before the wulfenite grew. This story focuses on multiplestage chemical weathering of a primary mineral deposit and shows the complexity of such a process in a microcosm. Death Valley, California CHRISTOPHER J. STEFANO A. E. Seaman Mineral Museum 1404 E. Sharon Avenue Michigan Technological University Houghton, Michigan 49931 cjstefan@mtu.edu