Structural, elemental and chemical characterization of apatite biominerals

Calcium phosphate minerals are frequently used in recent and fossil organisms, from unicellular to vertebrates. Apatite minerals offer in fact a wide range of biological adaptability in order to fulfil different uses, from protection to attack, from skeletal support to food grasping and processing. In addition, microbes are thought to play an active role in apatite precipitation. However, the term "bioapatite" has often been used improperly in the past to indicate apatite mediated by the intervention of organisms. Although development of mineralized parts seems controlled by specifically produced organic molecules that remain entrapped within the mineral units, the growth mechanisms and the diagenetic evolution of apatite fossils are still poorly understood. Conodonts represent an extinct group of jawless vertebrates that were the first among the group to experiment skeletal biomineralization with apatite elements in their feeding apparatus. We have recently applied microdiffraction techniques to conodont structural characterization on an Upper Ordovician (A. ordovicicus Zone) conodont fauna from Normandy (France). The conodont elements had a CAI of 4-5, indicating a heating up to 400°C, and were characterized by diagenetic neo-crystals arranged on the surface in distinctive large columnar, blocky or web-like microtextures. By the integration of ?XRD and chemical analyses, we have revealed that diagenetic apatite neo-crystals exhibit the same chemical composition as the original fossil structure, and that no significant difference in unit cell parameters appears to exist between the newly formed apatite crystals and those of the smooth (with no crystal overgrowth) conodont surfaces. In order to better constrain variability of apatite cell parameters, the analysis has been extended to encompass conodont elements of different age and having diverse CAI as well as apatites documented in other fossil and living organisms. Apatite crystals were analyzed in terms of size, morphology, composition, geometry and spatial arrangement by integration of optical and scanning electron microscopy (SEM), environmental scanning electron microscopy coupled with chemical microanalyses (ESEM-EDX), RAMAN analysis, and X-ray microdiffraction (?XRD). In particular, the microdiffraction method proved to be a reliable tool in describing mineralogical features that otherwise cannot be resolved by the use of microscopic methods alone. Microdiffraction measurements were carried out using collimators with different beam diameters. ?XRD method allows for small volumes of material to be probed: X-rays are collimated to form a small beam before irradiating a sample, giving the possibility to check for local "micro" environment such as defects or preferred orientations of the crystallites.