Got moonmilk? The characterization of moonmilk in Spider Cave, Carlsbad Caverns National Park, New Mexico
— Morgan Perrone-Vogt and Katherine Giles

Moonmilk from Spider Cave (known as “Crisco”) of Carlsbad Caverns National Park, NM was analyzed to determine the geochemistry, fabric, and depositional setting. Crisco moonmilk is composed of filamentous, calcitic carbonate that is associated with microbes and has a greasy texture. Moonmilk samples were analyzed using Scanning Electron  Microscopy (SEM) and Energy Dispersive X-Ray Spectrometry (EDS), thin sections stained with Alizarin Red S, and microprobe analysis with X-Ray maps. SEM examination showed an organic filamentous fabric with calcite coatings and calcite rhombohedrons. Two preparation techniques were used: one with and one without the use of a bicarbonate buffer. The use of a bicarbonate buffer prevents decalcification and the presence of sample preparation artifacts. A survey of the cave shows that moonmilk only forms below a certain depth (20 m below the entrance), and that the flood line (a detrital silt coating) follows this moonmilk line. The entrance of Spider Cave is in an arroyo and has been subjected to flooding throughout its history. The areas that contain moonmilk are at the cave’s lowest depths, where standing water can form. Petrographic analysis shows three types of moonmilk fabrics: Type 1 Crystalline—continuous, regular laminations that do not take on Alizarin Red S stain; Type 2 Diffuse— discontinuous, irregular laminations that stain; and Type 3 Recrystallized—no laminations or any distinguishing features and irregular staining. Within Spider Cave, Type 1 and Type 2 form in protected, higher elevation areas, whereas Type 3 forms at the lowest elevations where it would have remained under water the longest. EDS and microprobe analysis show that Type 1 and Type 2 samples have higher concentrations of elements (calcium, magnesium, aluminum, sulfur and silica); Type 3 samples contain smaller quantities of those elements, but have a higher abundance of carbon and oxygen. Type 2 samples contain the most aluminum and sulfur, as well as having more irregular layers, indicating that this type may be biotic. Type 1 has more regular, continuous layering, indicating that this type may be abiotic. Type 3 represents recrystallized Types 1 and 2, as suggested by the longer amount of time the Type 3 samples remained under water.