Bone diagenesis: New data from infrared spectroscopy and X-ray diffraction Elizabeth T. Stathopoulou a, ⁎, Vassilis Psycharis b , Georgios D. Chryssikos c , Vassilis Gionis c , George Theodorou a a Department of Historical Geology and Palaeontology, Subfaculty of Geology & Geoenvironment, University of Athens, Panepistimiopolis, 15784, Zografou, Athens, Greece b Institute of Materials Science, N.C.S.R. “Demokritos”, 15310, Aghia Paraskevi, Attiki, Greece c Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, Athens, 11635, Greece ABSTRACT ARTICLE INFO Article history: Accepted 26 March 2008 Keywords: Fossil bones Biological apatites Infrared spectroscopy Attenuated total reflectance Near-infrared spectroscopy X-ray diffraction Rietveld method Pikermi Chalkoutsi Aghia Napa This paper combines non-destructive high-resolution Fourier transform infrared spectroscopic techniques (attenuated total reflectance in the mid-infrared — ATR, and diffuse reflectance in the near-infrared — NIR) with X-ray diffraction and Rietveld analysis, in the study of bone diagenesis. Sixty fossil bones from two Upper Miocene sites in Greece (Pikermi and Chalkoutsi) and one Upper Pleistocene site in Cyprus (Aghia Napa) are investigated in comparison to various mineral and biological apatites. Diagenetic trends, common to all these sites include a subtle but systematic decrease of the unit cell volume and a-axis of carbonate hydroxylapatite, as well as a parallel increase of the coherence length along the c-axis. Chemometric modelling reveals that the changes in the unit cell and the coherence length are highly correlated to (and can be predicted on the basis of) the ATR spectra. Besides using chemometrics as a convenient predictive tool, we have been able to identify that the correlation with the XRD data is primarily based on the intensity of infrared bands at 577, 865 and 1092 cm - 1 , as well as on the position of the ν 1 phosphate mode at ca. 960 cm - 1 . These structural changes constitute the vibrational signature of diagenesis throughout our set of bone samples and can be accounted for by the stabilization of a distorted CO 3 2- species in the B-sites of apatite, and to a lesser extent by the substitution of OH - by F - . NIR spectroscopy allowed for the identification of a well-defined H 2 O species, absorbing at 5318 and 7240 cm - 1 . This species is labile, appears to characterize mostly biogenic apatite, and is therefore considered to be chemisorbed on the surface of the crystallites. © 2008 Published by Elsevier B.V. 1. Introduction Bone diagenesis leading to the preservation of skeletal material over geological time is a highly complex phenomenon involving the physical, chemical, histological and mechanical alterations that occur at different time scales from the time of death and depend on the local geochemical conditions (Clarke and Barker, 1993; Hedges and Millard, 1995; Hedges, 2002; Reiche et al., 2003; Trueman et al., 2004). The geochemical aspects of bone diagenesis have been studied by a variety of physicochemical techniques including optical and electronic microscopy (Jans et al., 2004), X-ray diffraction (Chipera and Bish, 1991; Person et al., 1995), vibrational spectroscopy (Surovell and Stiner, 2001; Lee-Thorp and Sponheimer, 2003) and chemical analysis (Trueman and Tuross, 2002). Bone itself is a heterogeneous, composite material, constructed from an intimate association of organized collagen fibres and plate-like inorganic crystallites (Weiner and Price, 1986; Weiner and Traub, 1992). The latter are analogous to poorly crystallized carbonate hydroxylapatite, and similar to the mineral earlier known as “dahlite” (Ca 10 (PO 4 , CO 3 ) 6 (OH) 2 )(Posner, 1985; Person et al., 1995). In turn, apatite is a very stable, yet versatile, hexagonal crystal (P6 3 /m, a =9.4 Å, c =6.9 Å, V cell =530 Å 3 ) with many non-biogenic, mineral or synthetic, members (White et al., 2005). In chemical terms, bone diagenesis has been associated with a variety of mostly anionic substitutions (e.g. halide for OH - ; CO 3 2- for PO 4 3- or OH - ) as well as with changes in crystallinity (Shemesh, 1990; Surovell and Stiner, 2001; Trueman et al., 2004). Despite serious research efforts, a detailed scenario for bone diagenesis remains elusive. Often, the proposed models are based on limited numbers of specimens from very specific localities, or lack a broader interdisci- plinary perspective. This work contributes towards understanding bone diagenesis over a large collection of fossilized specimens from three locations of different geological age in Greece and Cyprus. All samples reported here are taken from long bones, while a broader study including dental tissues from the same sites is in preparation. The fossilized specimens as well as a set of mineral and biogenic reference compounds are studied by new crystallographic and vibrational techniques. Emphasis is given in the identification of common structural features underlying bone diagenesis in the three sites Palaeogeography, Palaeoclimatology, Palaeoecology 266 (2008) 168–174 ⁎ Corresponding author. Tel.: +30 210 7274178. E-mail addresses: estathop@geol.uoa.gr (E.T. Stathopoulou), vpsychar@ims.demokritos.gr (V. Psycharis), gdchryss@eie.gr (G.D. Chryssikos), vgionis@eie.gr (V. Gionis), gtheodor@geol.uoa.gr (G. Theodorou). 0031-0182/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.palaeo.2008.03.022 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo