ALBERT

Description: In this paper, we discuss information on crystal structure and morphology available from nuclear magnetic resonance (NMR) spectroscopy of 207Pb for the mineral family [Pb(4f)]2[Pb(6h)]3(AO4)3Cl with A = V (vanadinite), P (pyromorphite), and As (mimetite). The isotropic chemical shift of the 207Pb atoms at Wyckoff positions 4f and 6h was (re-)determined from either static single-crystal or magic angle spinning NMR experiments. This isotropic shift can be linearly correlated to the unit-cell volume within the mineral family, and in the wider context of lead-bearing minerals, to the shortest Pb-O distance for position 4f, in which 207Pb is solely coordinated by oxygen. By evaluating the number of resonances and their respective line widths in the 207Pb-NMR spectra of these three naturally grown minerals, it could be established that vanadinite forms single-domain macroscopic crystals with very small mosaicity, whereas pyromorphite crystals show NMR characteristics, which can be interpreted as being caused by significant mosaicity. In some instances, this mosaic spread could be quantitatively approximated by a Gaussian distribution with a standard deviation angle of σ = 5°. In contrast, our mimetite specimen was composed of multiple sub-crystals with a very high variability of orientations, going beyond mere mosaicity effects. By extending the NMR methodology presented here to other minerals, it may be possible to gain new insights about structure-property relationships and the morphology of natural grown minerals.

Description: The assessment of diagenetic overprint on microstructural and geochemical data gained from fossil archives is of fundamental importance for understanding palaeoenvironments. The correct reconstruction of past environmental dynamics is only possible when pristine skeletons are unequivocally distinguished from altered skeletal elements. Our previous studies show (i) that replacement of biogenic carbonate by inorganic calcite occurs via an interface-coupled dissolution–reprecipitation mechanism. (ii) A comprehensive understanding of alteration of the biogenic skeleton is only given when structural changes are assessed on both, the micrometre as well as on the nanometre scale. In the present contribution we investigate experimental hydrothermal alteration of six different modern biogenic carbonate materials to (i) assess their potential for withstanding diagenetic overprint and to (ii) find characteristics for the preservation of their microstructure in the fossil record. Experiments were performed at 175°C with a 100 mM NaCl + 10 mM MgCl2 alteration solution and lasted for up to 35 days. For each type of microstructure we (i) examine the evolution of biogenic carbonate replacement by inorganic calcite, (ii) highlight different stages of inorganic carbonate formation, (iii) explore microstructural changes at different degrees of alteration, and (iv) perform a statistical evaluation of microstructural data to highlight changes in crystallite size between the pristine and the altered skeletons. We find that alteration from biogenic aragonite to inorganic calcite proceeds along pathways where the fluid enters the material. It is fastest in hard tissues with an existing primary porosity and a biopolymer fabric within the skeleton that consists of a network of fibrils. The slowest alteration kinetics occurs when biogenic nacreous aragonite is replaced by inorganic calcite, irrespective of the mode of assembly of nacre tablets. For all investigated biogenic carbonates we distinguish the following intermediate stages of alteration: (i) decomposition of biopolymers and the associated formation of secondary porosity, (ii) homoepitactic overgrowth with preservation of the original phase leading to amalgamation of neighbouring mineral units (i.e. recrystallization by grain growth eliminating grain boundaries), (iii) deletion of the original microstructure, however, at first, under retention of the original mineralogical phase, and (iv) replacement of both, the pristine microstructure and original phase with the newly formed abiogenic product. At the alteration front we find between newly formed calcite and reworked biogenic aragonite the formation of metastable Mg-rich carbonates with a calcite-type structure and compositions ranging from dolomitic to about 80mol % magnesite. This high-Mg calcite seam shifts with the alteration front when the latter is displaced within the unaltered biogenic aragonite. For all investigated biocarbonate hard tissues we observe the destruction of the microstructure first, and, in a second step, the replacement of the original with the newly formed phase.