Diagenesis does not invent anything new: Precise replication of conodont structures by secondary apatite

Conodont elements are important archives of sea/pore water chemistry yet they often exhibit evidence of diagenetic mineral overgrowth which may be biasing measurents. We decided to investigate this phenomenon by characterising chemically and crystallographically, the original biomineral tissue and the diagenetic mineral nature of conodont elements from the Ordovician of Normandy. Diagenetic apatite crystals observed on the surface of conodont elements show distinctive large columnar, blocky or web-like microtextures. We demonstrate that these apatite neo-crystals exhibit the same chemical composition as the original fossil structure. X-ray microdiffraction has been applied herein for the first time to conodont structural investigation. Analyses of the entire conodont element surface of a variety of species have revealed the existence of a clear pattern of crystal preferred orientation. No significant difference in unit cell parameters was documented between the newly formed apatite crystals and those of the smooth conodont surfaces, thus it emerges from our research that diagenesis has strictly replicated the unit cell signature of the older crystals

Mineralogy and crystallization patterns in conodont bioapatite from first occurrence (Cambrian) to extinction (end-Triassic)

Bioapatite represents an important acquisition in the evolution of life, both in the seas and on land. Vertebrates applied calcium-phosphate biominerals to grow their skeletal support and to shape their teeth, while some invertebrates sheltered their soft parts within apatite shells. Conodonts were the first among vertebrates to experiment with skeletal biomineralization of tooth-like elements in their feeding apparatus. Spanning a time record of over 300 million years, they offer a unique tool to test possible variation in bioapatite structure from the experimentation of a very primitive biomineralization type to a more evolute pattern just before going extinct. X-ray microdiffraction carried out through an X-ray micro-diffractometer, integrated with environmental scanning electron microscopy coupled with chemical microanalyses (ESEM-EDX), has been applied in this study to investigate conodont element crystal structure throughout the entire stratigraphic range of these organisms. In particular, bioapatite crystallographic cell parameters have been calculated for about one hundred conodont elements ranging from the late Cambrian to the Late Triassic. Resulting data clearly indicate two distinct distribution plots of cell parameters for paraconodonts and euconodonts. In contrast, age, taxonomy, geographic provenance and CAI do not affect the dimension of the bioapatite crystal cells. Conodont bioapatite crystallographic cell parameters have been compared with cell parameters resulting from phosphatic/phosphatized material (ostracodes, brachiopods, bryozoans, and fish teeth) present in the same residues producing conodonts. Resulting values of the cell parameters are, in general, mainly correlated with the type of organisms even if, for some of them, a correlation also with age cannot be completely ruled out. According to our data, primary bioapatite appears to imprint a key signature on fossil crystal-chemistry (crystal structure and major chemical element contents), while the contribution of fossilization and diagenetic processes seems less relevant

HOW DID VERTEBRATES SHARPEN THEIR TEETH? A NEW PERSPECTIVE IN BIOAPATITE ANALYSIS

HOW DID VERTEBRATES SHARPEN THEIR TEETH? A NEW PERSPECTIVE IN BIOAPATITE ANALYSI

Crystal structure and crystal chemistry of fluorannite and its relationships to annite

This contribution deals about the crystal chemical characterization of fluorannite from Katugin Ta-Nb deposit, Chitinskaya Oblast’, Kalar Range, Transbaikalia, Eastern-Siberian Region, Russia. The mineral chemical formula is (K0.960Na0.020Ba0.001) (Fe2+2.102Fe3+0.425Cr3+0.002Mg0.039Li0.085Ti0.210Mn0.057) (Al0.674 Si3.326) O10 (F1.060OH0.028O0.912). This mica belongs to 1M polytype (space group C2/m) with layer parameters a = 5.3454(2) Å, b = 9.2607(4) Å, c = 10.2040(5) Å, beta = 100.169(3)°. Structure refinement, using anisotropic displacement parameters, converged at R = 0.0384. When compared to annite, fluorannite shows a smaller cell volume (Vfluorannite = 497.19 Å3; Vannite = 505.71 Å3), because of its smaller lateral dimensions and its reduced c parameter. The flattening of the tetrahedral basal oxygen atoms plane decreases with F content, together with the A-O4 distance (i.e., the distance between interlayer A cation and the octahedral anionic position) because of the reduced repulsion between the interlayer cation and the anion sited in O4

Zooming in REE and Other Trace Elements on Conodonts: Does Taxonomy Guide Diagenesis?

Conodont elements are calcium phosphate (apatite structure) mineralized remains of the cephalic feeding apparatus of an extinct marine organism. Due to the high affinity of apatite for rare earth elements (REE) and other high field strength elements (HFSE), conodont elements were frequently assumed to be a reliable archive of sea-water composition and changes that had occurred during diagenesis. Likewise, the crystallinity index of bioapatite, i.e., the rate of crystallinity of biologically mediated apatite, should be generally linearly dependent on diagenetic alteration as the greater (and longer) the pressure and temperature to which a crystal is exposed, the greater the resulting crystallinity. In this study, we detected the uptake of HFSE in conodont elements recovered from a single stratigraphic horizon in the Upper Ordovician of Normandy (France). Assuming therefore that all the specimens have undergone an identical diagenetic history, we have assessed whether conodont taxonomy (and morphology) impacts HFSE uptake and crystallinity index. We found that all conodont elements are characterized by a clear diagenetic signature, with minor but significant differences among taxa. These distinctions are evidenced also by the crystallinity index values which show positive correlations with some elements and, accordingly, with diagenesis; however, correlations with the crystallinity index strongly depend on the method adopted for its calculation

Layer charge and heavy metals structures in hydrated 2 : 1 silicates:state of the art and new advances on cadmium

This study will discuss how layer charge can affect chemical speciation and topology of heavy metals adsorbed to 2:1 layer silicates, by providing: i) an overview of literature data; ii) experimental data on Cd complexes adsorbed by 2:1 layer silicates with different layer charge (montmorillonite and vermiculite); iii) a comparison between our results and literature data. This study will also be supported by several different experimental techniques such as chemical and thermal analyses, X-ray powder diffraction and X-ray absorption spectroscopy.Based on our data Cd atoms were found to complex water molecules in both clay minerals and to show four-fold coordination in montmorillonite (CdO distances of 2.24 Å) and six-fold coordination in vermiculite (CdO distances of 2.16 and 2.28 Å). Furthermore our models clearly suggest that Cd mainly bonds to interlayer water, without neglecting the more limited, but still significant, Cd multinuclear surface complexes at the octahedral broken edges. Both clay minerals show H2O/Cd ratio, as evidenced by thermal analyses, drastically higher than expected from X-ray adsorption spectroscopy data, thus implying that most of the water molecules are only loosely coordinated to interlayer cations