The term "bioapatite" has often been used improperly in the past and in different scientifi c fi elds to characterize apatite mediated by the intervention of organisms. Conodonts, for a long time considered enigmatic, represent an extinct group of jawless vertebrates that were the fi rst among the group to experiment skeletal biomineralization with tooth-like elements in their feeding apparatus. Their use of apatite is shared with many other vertebrates that have applied calcium-phosphate biominerals to grow their skeletal structure and to shape their teeth. However, microbes are thought to play a role in apatite precipitation (e.g., Crosby and Bailey, 2012). Although development of mineralized parts seems controlled by specifi cally produced organic molecules that remain entrapped within the mineral units, the growth mechanisms and the diagenetic evolution of apatite fossils are still poorly understood. Ferretti et al. (2016) recently described peculiar diagenetic apatite overgrowths on Late Ordovician (A. ordovicicus Zone) conodonts from Normandy. The conodont specimens exhibit a CAI of 4-5, indicating a heating up to 400°C. Diagenetic neo-crystals observed on the surface of conodont elements show distinctive large columnar, blocky or web-like microtextures. 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) and X-ray microdiffraction (?XRD). X-ray diffraction technique had been used in the past to characterize lattice parameters in apatite crystals (e.g., Ellisson, 1944; Pietzner et al., 1968; Nemliher and Kallaste, 2012). Microdiffraction, applied to conodont structural characterization, proved to be a reliable tool in describing overgrowths that otherwise cannot be resolved by the use of microscopic methods alone. In fact ?XRD method allows for small volumes of material to be probed: X-rays are collimated to form a very small beam (up to 10 ?m in diameter) before irradiating a sample, giving the possibility to check for local "micro" environment such as defects or preferred orientations of the crystallites. Microdiffraction measurements were carried out on various points of the surface using a 50 ?m collimator and changing specimen orientation (fi xed Omega revolution angle and varying Phi rotation angle). The integration of ?XRD with chemical analyses allowed Ferretti et al. (2016) to reveal that diagenetic apatite neo-crystals exhibit the same chemical composition as the original fossil structure, and that no signifi cant 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 other words, diagenesis has strictly replicated the unit cell signature of the older crystals. The application of this approach, coupled with RAMAN analysis, has been extended to encompass conodont elements of different age and having diverse CAI in order to better constrain variability of apatite cell parameters. These results have been compared with those derived from apatite documented in other fossil and living organisms.
Mineralogical charcaterization of apatite biominerals: preliminary results
L Medici;
2017
Abstract
The term "bioapatite" has often been used improperly in the past and in different scientifi c fi elds to characterize apatite mediated by the intervention of organisms. Conodonts, for a long time considered enigmatic, represent an extinct group of jawless vertebrates that were the fi rst among the group to experiment skeletal biomineralization with tooth-like elements in their feeding apparatus. Their use of apatite is shared with many other vertebrates that have applied calcium-phosphate biominerals to grow their skeletal structure and to shape their teeth. However, microbes are thought to play a role in apatite precipitation (e.g., Crosby and Bailey, 2012). Although development of mineralized parts seems controlled by specifi cally produced organic molecules that remain entrapped within the mineral units, the growth mechanisms and the diagenetic evolution of apatite fossils are still poorly understood. Ferretti et al. (2016) recently described peculiar diagenetic apatite overgrowths on Late Ordovician (A. ordovicicus Zone) conodonts from Normandy. The conodont specimens exhibit a CAI of 4-5, indicating a heating up to 400°C. Diagenetic neo-crystals observed on the surface of conodont elements show distinctive large columnar, blocky or web-like microtextures. 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) and X-ray microdiffraction (?XRD). X-ray diffraction technique had been used in the past to characterize lattice parameters in apatite crystals (e.g., Ellisson, 1944; Pietzner et al., 1968; Nemliher and Kallaste, 2012). Microdiffraction, applied to conodont structural characterization, proved to be a reliable tool in describing overgrowths that otherwise cannot be resolved by the use of microscopic methods alone. In fact ?XRD method allows for small volumes of material to be probed: X-rays are collimated to form a very small beam (up to 10 ?m in diameter) before irradiating a sample, giving the possibility to check for local "micro" environment such as defects or preferred orientations of the crystallites. Microdiffraction measurements were carried out on various points of the surface using a 50 ?m collimator and changing specimen orientation (fi xed Omega revolution angle and varying Phi rotation angle). The integration of ?XRD with chemical analyses allowed Ferretti et al. (2016) to reveal that diagenetic apatite neo-crystals exhibit the same chemical composition as the original fossil structure, and that no signifi cant 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 other words, diagenesis has strictly replicated the unit cell signature of the older crystals. The application of this approach, coupled with RAMAN analysis, has been extended to encompass conodont elements of different age and having diverse CAI in order to better constrain variability of apatite cell parameters. These results have been compared with those derived from apatite documented in other fossil and living organisms.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
