"CRISCO", A CALCITIC, FIBROUS MOONMILK DEPOSIT FROM SPIDER CAVE, NEW MEXICO: DISTINCTIVE MINERAL FORMS AND THE ROLE OF MICROORGANISMS

"Crisco" is a white cave deposit with the appearance of cottage cheese curds and a greasy, slippery, claylike texture. It is one of a class of globally distributed cave deposits collectively known as "moonmilk." Microscopic moonmilk grains consist mainly of calcite, but in relatively warm caves with magnesium-rich dolomitic bedrock, hydromagnesite, magnesite, huntite, nesquehonite, and altered dolomite are common constituents. Some workers have concluded that moonmilk is biogenic while others have identified nonbiological formation mechanisms. Specific temperature and other conditions may determine which mechanism predominates.

SEM examination of Crisco reveals filaments of bimodal size distribution and putative bacteria possibly responsible for filament formation. EDS analysis shows only elemental abundances consistent with calcium carbonate. Compared to a pure calcite standard, we observe a small excess of carbon, perhaps organic material. When dissolved with weak acid (3% HCl), a transparent glob of organic material remains (4.5% of total mass). Of this, 1.4% is organic carbon. Isotopic analyses of different fractions show isotopically light del 13C values for the organic carbon compared to the calcite. Staining of intact Crisco with DNA-binding fluorescent dyes shows that calcite is precipitated on filaments that are, or recently were, living. Broken filament ends also stain while the calcite does not. The fibers themselves appear to be continuous slender crystals but larger crystals resembling stacked rhombohedrons are also present. The range of bacteria and fungi culturable from this material demonstrate the presence of an active biological community (e.g. Actinomyces spp., Bacilli spp, Micromonas spp., Streptomyces spp., and other unidentified isolates).

If biological and abiological formation mechanisms are both possible, it is critical to distinguish between the two. The two distinctive morphologies within the Crisco (larger stacked rhombahedrons and smaller smooth filaments) may indicate abiotic vs. biotic origin; however, evidence is still sketchy. Microcosm studies (6 month- years) and other experiments are underway in an attempt to reproduce filament growth and subsequent calcite mineralization.