Surface Microanalysis of Staining on Vertebrate Fossil Bone from the Santa Fe River, Florida

Open Access
Author:
Foecke, Kimberly Kjaisa
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 20, 2016
Committee Members:
  • Peter J Heaney, Thesis Advisor
  • Russell W Graham, Thesis Advisor
  • Maureen Feineman, Thesis Advisor
Keywords:
  • Black staining
  • fossils
  • microanalysis
  • electron probe
  • EDS
  • WDS
Abstract:
Mammalian fossil bones excavated from the bed of the Santa Fe River in northern Florida exhibit dark black surface stains. Although such stains typically are attributed to secondary mineralization of manganese oxides, our microanalysis revealed little evidence for manganese. Instead, energy dispersive spectroscopy revealed that the surface stains consist of ~1 mm-thick bands of Fe oxide and sulfide. Wavelength dispersive spectroscopy and electron back-scattered diffraction identified the Fe sulfide as pyrite (FeS2), but the size and concentration of the crystallites of Fe oxide were too small for a definitive mineralogical determination. Fourier transform infrared spectroscopy (FTIR) offered evidence for ferric tannates within the black stains. We hypothesize that an initial stain of iron oxide provided a substrate for the subsequent formation of a ferric tannate complex, which is responsible for the black coloration. In some instances, sulfidation of iron oxide in the reducing micro-environment of interior pores promoted pyrite formation. These results conflict with the presumption that black surface stains on archaeological materials are necessarily attributable to Mn oxides. Further, in light of the instability of pyrite, stains on fossils may pose risks to collections if not properly identified, treated, and curated.