François Fröhlich

Spectroscopic characterization of biogenic silica

Amorphous biogenic silicas were studied by FTIR and MAS-NMR spectroscopy. Fossil diatom frustules and sponge spicules exhibit a highly condensed and well-organised silica whereas the silica frustule of living diatoms is much less condensed, suggesting that some condensation process still happens upon fossilisation during the diagenetic evolution of silica. Moreover, the silica network of living diatoms appears to be linked to the bio-organic components of the cell, in agreement with the biosilicification mechanisms suggesting that some proteins or polysaccharides favour the formation of silica.

Detailed FT near-infrared study of the behaviour of water and hydroxyl in sedimentary length-fast chalcedony, SiO2, upon heat treatment

Chalcedony is a porous spatial arrangement of hydroxylated nanometre sized α-quartz (SiO(2)) crystallites. Due to micro-structural transformations upon heat treatment, the optical and mechanical properties of the rock are modified. We investigated these transformations in sedimentary length-fast chalcedony through Fourier Transform near- and mid-infrared spectroscopy using direct transmission and the reflectivity. Chemical adsorption potential and absorption of H(2)O by pores was studied after heat treatment. We found that water held in open porosity is reduced upon heat treatment to temperatures above 150°C. Silanole is noticeably lost from 250 to 300°C upwards and new bridging Si-O-Si further reduces the surface of open pores, creating a less porous material. Molecular water, resulting from the reaction Si-OH HO-Si→Si-O-Si+H(2)O creates new isolated pores within the material. At temperatures above 500°C, the samples start internal fracturing, permitting water held in isolated pores to be evacuated. These results shed light on thermal transformations in chalcedony and allow for a better understanding of mechanical transformations after heat treatment.

Imaging fossil bone alterations at the microscale by SR-FTIR microspectroscopy

Diagenetic alterations modifying fossil bones over geological time can limit their use as archaeological and paleontological proxies. The understanding of fossilization processes and the evaluation of the extent of diagenetic alterations of bones therefore constitute major issues in current research. The complex hierarchical structure of bone tissue and the spatial heterogeneity of the diagenetic alterations induce significant chemical variations in fossil bones at the microscale. We adapted a sample preparation procedure based on PMMA impregnation that allowed us to obtain bone thin sections independent of their level of degradation. As a first step to investigate the variations in bone composition at the histological scale, synchrotron radiation FTIR micro-spectroscopy (FTIRM) was applied on two fossil bone samples displaying distinct preservation states: (a) a bone from Magdalenian layers dated to 15 ka of the Bize-Tournal cave (Aude, France) and (b) another from Song Terus site (Java island, Indonesia, 60 ka). The first one is partially recrystallized but still contains zones with locally preserved collagen and biogenic carbonates, whereas the other consists of a recrystallized mineral fraction without any remaining collagen fraction. These data obtained on the microscale were compared to bulk measurement data and the relationships between different IR diagenetic parameters explored. This analytical approach allowed the characterization of diagenetic alterations such as collagen loss, carbonate uptake and mineral recrystallization in heavily altered fossil bone tissue at the histological scale. Using the presented procedure, the histological bone structures could be studied, even for brittle samples altered by an extensive loss of their collagen matrix during diagenesis.

Geophagy: soil consumption enhances the bioactivities of plants eaten by chimpanzees

Geophagy, the deliberate ingestion of soil, is a widespread practice among animals, including humans. Although some cases are well documented, motivations and consequences of this practice on the health status of the consumer remain unclear. In this paper, we focused our study on chimpanzees (Pan troglodytes schweinfurthii) of the Kibale National Park, Uganda, after observing they sometimes ingest soil shortly before or after consuming some plant parts such as leaves of Trichilia rubescens, which have in vitro anti-malarial properties. Chemical and mineralogical analyses of soil eaten by chimpanzees and soil used by the local healer to treat diarrhoea revealed similar composition, the clay mineralogy being dominated by kaolinite. We modelled the interaction between samples of the two types of soil and the leaves of T. rubescens in gastric and intestinal compartments and assayed the anti-malarial properties of these solutions. Results obtained for both soil samples are similar and support the hypothesis that soil enhances the pharmacological properties of the bio-available gastric fraction. The adaptive function of geophagy is likely to be multi-factorial. Nevertheless, the medical literature and most of occidental people usually consider geophagy in humans as an aberrant behaviour, symptomatic of metabolic dysfunction. Our results provide a new evidence to view geophagy as a practice for maintaining health, explaining its persistence through evolution.

Temperature dependent crystallographic transformations in chalcedony, SiO2, assessed in mid infrared spectroscopy

Chalcedony consists of hydroxylated 50-100 nanometre measuring α-quartz (SiO(2)) crystallites that lose their surface silanole groups (Si-OH) upon heating between 350°C and 600°C. The loss of the chalcedonys ≈1% of silanole groups allows for the healing of water related defects in the crystallites. We investigated these crystallographic transformations using Fourier Transform mid Infrared Spectroscopy in direct transmission, Attenuated Total Reflection (ATR) and the reflectivity. We found that an absorption band that is specific for chalcedony at 555 cm(-1) disappears gradually upon heating between 350°C and 600°C. The reduction of the band is correlated to the loss of surface silanoles. This result leads to the assignment of the band to free Si-O vibrations in non bridging Si-OH groups that have a lower natural frequency than Si-O vibrations in bridging Si-O-Si. The recognition of a silanole signal in the mid infrared allows for an easy, cheap and rapid recognition of hydroxyl in chalcedony.