New Mexico Mineral Symposium — Abstracts

Museum specimens of copper sulfide, notably chalcocite and bornite, sometimes develop brown or black, powdery coatings that degrade their aesthetic quality. Over a period of months to years, a beautiful specimen can develop an unattractive sooty coating that resembles black fungal smut that forms on plants. However, the mineral specimens are typically stored in display cabinets or drawers in ambient humidity and temperature, certainly not the type of environment one associates with fungal infestations. Although the coating can be washed off, it will usually reappear after a few years. Such coatings can raise havoc with museum mineral specimens, particularly in humid localities, where some specimens have to be removed and cleaned on a regular basis.

Several afflicted specimens from the Flambeau mine in Wisconsin were originally provided by the Harvard Mineralogical Museum in 2001. Specimens from this locality seem to be especially prone to quickly developing a brown coating while in display cabinets. The specimens were examined by scanning electron microscopy (SEM) and were also used to inoculate enrichment cultures on growth media containing various copper sulfides. Under the SEM, these samples exhibit an abundance of micrometer-sized, arborescent copper oxide growths. This finding led us to examine specimens from other localities and from different storage and display facilities. Many specimens displayed microscopic hyphae, filaments, and visible cells of fungi and bacteria.

The cultures from the original specimens responded slowly, but after several years, developed visible growth of blue-green or brown to black colonies that precipitate brilliant blue and green copper oxide minerals. Samples from the cultures were collected for DNA analysis and examination by SEM and electron microprobe. Several types of microorganisms that may be oxidizing copper and/or sulfur in these specimens have been identified from the cultures.

The level of humidity in certain regions may enhance microorganism growth on copper sulfides. In this case, specimens stored in Boston, on the eastern seaboard of the U.S., exhibit thick powdery coatings, whereas similar specimens from the mineral collection at the University of New Mexico, stored under much lower ambient humidity, display a thin sooty black coating. It is possible that handling the specimens provides a small amount of organic skin oil that my feed some of the organisms. In order to remove the coating, specimens should be cleaned and disinfected, where possible, with a bleach solution to kill the microorganisms. Specimens should also be stored under as dry conditions as possible to prevent growth.