A number of cave systems host colorful deposits of what has been termed “corrosion residue” (CR), material that appears to be the breakdown product of bedrock minerals. The CR may be red, pink, orange, ocher, brown gray, or black and usually occurs in a variety of places within caves (ceilings, walls, tops and sides of boulders, etc.). Geologists have hypothesized that CR is the long-term result of upwelling corrosive air. However, discovery of evidence of microbial activity has led to a complimentary explanation that microbes could be active participants in the production of the corrosion residue. These deposits have been cursorily examined in several caves, including Jewel Cave (SD), Lechuguilla and Spider Caves (NM), and Cueva de Villa Luz (Tabasco, Mexico) using microscopy techniques of SEM and TEM, along with EDS and WDS analysis.

In all cases, the CR is a complex mixture of iron and/or manganese oxides, clays, quartz, and corroded bedrock material. In several cases, rare earth element (REE) phosphate minerals and other unusual minerals have been observed. The REE minerals are believed to result from recrystallization of apatite present in the bedrock; rare earth elements are commonly present in trace amounts within the apatite minerals. X-ray diffraction and bulk chemical analysis has shown that Fe3+ makes up the bulk if not all of the Fe-oxides in the CR while microprobe analysis of thin sections of the bedrock has found Fe2+ in dolomite as high as 700 ppm.

Within much of examined material, evidence of microbial life is present in the form of structures shaped like coccoid or filamentous bacteria. There is often a close association of presumptive bacteria with small dissolution pits in corroded fragments of bedrock minerals in the CR (Figure 1). In addition to the putative bacteria, the mineral residue often hosts small star-shaped minerals containing Fe-oxide (Figure 2).