. Modeling the attenuated total reflectance infrared (ATR-FTIR) spectrum of apatite. Physics and Chemistry of Minerals, Springer Verlag, 2016, 43 (9) 3

The chemical and isotopic compositions of biogenic apatite are important geochemical markers, which can suffer modifications during fossilisation. Compared with modern ones, fossil apatites generally exhibit variations in carbonate content, enrichment in fluorine, incorporation of trace elements and an increase in crystallinity parameters. Detailed understanding of these transformations induced by fossilisation should help assess the preservation of geochemical records in apatites. In this contribution, we investigate the transformation of modern bone altered under controlled conditions. Modern bone samples were soaked in aqueous solutions of neutral to alkaline pH (9-10) for one and three weeks at various temperatures (20 and 70 °C) and experiments were duplicated in fluorine-free and in 10-2 M NaF solutions. Bone transformation was monitored through the modifications of chemical (F, Ca, P) and isotopic (δ 13 C, δ 18 Oc, δ 18 Op) composition as well as using vibrational (ATR-FTIR, Raman) and solidstate (1 H, 13 C, 19 F) NMR spectroscopies. The observed modifications sustain a transformation mechanism through partial dissolution of biogenic apatite and precipitation of secondary apatite. This transformation occurs irrespective of the presence or absence of fluorine and leads to the formation of carbonate-bearing fluorapatite or carbonate-bearing hydroxylapatite, respectively. The fraction of secondary apatite seems to be limited to ~60 %, suggesting that its formation has a protecting role against further dissolution of the primary apatite. The observation of clumped (CO 3 2-, F-) defect in the structural B-site of some samples as well as perturbation of the isotopic compositions attest to carbonate incorporation in the secondary apatite. Although the incorporation of fluoride ions can serve as a probe revealing dissolutionrecrystallisation of bone, the present study also underlines the difficulty to systematically relate mineralogical transformations to an open-system behaviour in bioapatite.

Water exchange around a free magnesium ion and magnesium paired with carbonate in aqueous solution was studied using free energy methods. Both a rigid-ion and a polarizable force field based on the AMOEBA model were examined. The parameters were adjusted to accurately reproduce the hydration structures of magnesium and carbonate in aqueous solution. The magnesium carbonate ion pairing free energies calculated with both force fields were found to be in excellent agreement with experimental data. Metadynamics simulations of the water exchange conducted with both models revealed that the formation of a contact magnesium carbonate ion pair significantly decreases the energy barrier for water exchange relative to the free magnesium ion in solution. This finding suggests that the presence of carbonate could accelerate the water exchange around magnesium and constitutes a first step toward a better understanding of the atomic-scale mechanisms involved in the nucleation of magnesium-bearing carbonate minerals.

Secondary minerals in soils can record climatic changes affecting continental surfaces over geological times. Their dating should refine our present knowledge about their potential periods of formation as well as their relations with the ongoing change of climate and erosion/weathering regimes. In the present study, twenty kaolinite samples from two lateritic profiles of the Karnataka plateau, an intensively studied area in the southern India, have been dated using electron paramagnetic resonance (EPR) spectroscopy. Kaolinite ages vary between 0.229 ±0.24 Ma to 40.73 ±15.37 Ma. Four different groups of age can be identified with ages clustered around 1.0, 3.5, 9.0 and 39.0 Ma. These groups of age indicate local preferential weathering periods that coincide with distinct Indian climatic events described in independent studies, such as monsoon strengthening. Thus, regional or subcontinental factors likely prevailed over global forcing in the imprint of climatic events in the regolith profiles. These results confirm that despite their simple mineralogy, laterites can contain several relictual and coexisting generations of secondary minerals and that EPR dating of kaolinite contributes to unraveling the complex history of continental surfaces over geological periods.

The interaction of organic molecules with mineral systems is relevant to a wide variety of scientific problems both in the environment and minerals processing.

The adsorption of small molecules containing two different organic functional groups at terrace and step sites on the {101̅4} surface of calcite at the interface with aqueous solution was studied using free energy methods. For comparison, the adsorption free energies of the component ions of calcium carbonate were also determined at the same sites. Polarizability was taken into account through using a force field developed for calcium carbonate based on the AMOEBA model that contains static multipoles and self-consistent induced dipoles. The influence of including polarization was examined by comparing to data obtained with a fixed charge rigid-ion model. The strong hydration layers above the basal plane of calcite were shown to hinder the direct attachment of the small species studied, including the constituent ions of the mineral. Only the species bearing an amino group, namely, methylammonium and glycine, demonstrated favorable adsorption free energies. The ability of amino groups to more readily pass through the hydration layers than carboxylate and carbonate groups can be explained by their weaker solvation free energies, while the carbonate ions within the calcite surface with which they bind are also less strongly hydrated than calcium ions. Acetate, glycine, and methylammonium were all found to be able to directly bind to one growth site at the acute step of calcite. This is at variance with results obtained with a rigid-ion model in which all binding free energies are endergonic. Thus, including polarization allows for a description of the adsorption process that is more consistent with experimental observations, particularly at calcite steps, and for determination of more reliable atomic-scale mechanisms for calcite growth and its modification by organic additives. Even with polarization, the organic functional groups considered only exhibit moderate binding to calcite steps with adsorption free energies not exceeding −13 kJ/mol.

Bones mostly consist of composite materials based on almost equivalent volume fractions of mineral (apatite) and organic (collagen) components. Accordingly, their infrared spectroscopic properties should reflect this composite nature. In this letter, we show by theory and experiment that the variability of the strong phosphate bands in the ATR-FTIR spectra of a series of modern and archeological bone samples can be related to electrostatic interactions affecting apatite particles and depending on the bone collagen content. Key parameters controlling the shape of these bands are the mineral volume fraction and the dielectric constant of the embedding matrix. The magnitude of these effects is larger than the one related to microscopic changes of the apatite structure. Consequently, the interplay of microscopic and macroscopic parameters should be considered when using FTIR spectroscopy to monitor the preservation state of bioapatite during diagenetic and fossilization processes, especially during the degradation of the organic fraction of bone.

Bone reactivity offers a potential way to record local physical–chemical conditions prevailing in fossilization environments and archaeological sites. In the present study, a series of fossil bone samples from the karstic environments of the Bolt's Farm cave system (Cradle of Humankind, South Africa) and from fluvio‐lacustrine environments of the Tugen Hills (Gregory Rift, Kenya) is analysed. The chemical composition and infrared and nuclear magnetic resonance (NMR) spectroscopic properties of fossil samples point to a transformation of the biogenic apatite and formation of secondary apatite. Depending on the sample, the secondary apatite corresponds to a carbonate‐bearing hydroxy‐ or fluor‐apatite. The maximum fraction of secondary apatite is close to 60%, coinciding with previous observations in experimental alteration of bone in aqueous solutions and suggesting that a fraction of pristine biological apatite is likely to be preserved. The present results also suggest that the acetic acid treatment of fossil samples moderately increases their average crystallinity but may dissolve carbonate‐rich domains of secondary apatite.