ELECTRON MICROSCOPY CAPTURES CRITICAL MINERAL DIVERSITY THAT POWDER X-RAY DIFFRACTION DOES NOT DETECT IN CAVE SEDIMENTS: AN EXAMPLE FROM A CAVE FILL IN NORTHERN KENTUCKY

The mineralogical diversity of cave sediments is often investigated with basic powder X-ray diffraction techniques and rarely is detailed electron microscopy work done as part of the investigation. Sediment from a small cave near Maysville Kentucky in the Upper Ordovician Grant Lake Formation with known basic mineralogical data was investigated in detail to determine the nature of mineralogical complexity. The cave is approximately 1.5 meters in height and has a sinuous passage approximately 1 meter wide that is filled with finely laminated clay sediment. In the middle of the stratigraphy of the sediment there are well developed deposits of iron and manganese oxides. Going down in section the sediment darkens in color from top to bottom and then lightens slightly again, ranging from 2.5Y 6/4 to 2.5Y 2.5/1 at its darkest on the Munsell color chart. Powder X-ray diffraction indicates phyllosilicate mineralogy is dominated by vermiculite and illite with lesser amounts of chlorite and kaolinite. Diffraction patterns of some samples have peaks that are consistent with todorokite or other tunnel structure Mn-oxides. Scanning electron microscopy indicates sediment is dominated by clay size particles and fine silt. Transmission electron microscopy of grain mounts of sediment indicates highly crystalline Fe-oxides are present. Some of these Fe-oxides are single delicate crystals with six-fold symmetry indicated by selected area electron diffraction. Discrete prismatic single crystals of apatite which are approximately 500 to 300 nm in length are also common. Illite and vermiculite crystals are moderately crystalline.

Electron microscopy data enabled the identification of minerals critical for geochemical cycles previously missed by simple powder X-ray diffraction. This is attributed to the detection limit of powder XRD of a few weight percent. Although much of the mineralogical assemblage is consistent with a soil-derived sediment source, textures of Fe-oxides and apatite suggest an in situ, authigenic origin for some or all Fe-oxides and apatite. The complex Mn-oxides observed may be recrystallized or reworked soil concretions, authigenic minerals derived from limestone alteration or both. Results demonstrate the value of basic TEM analysis in cave sediment studies.