Haywick 9527103 Aragonite diagenesis is responsible for much of the calcite cementation that lithifies carbonate sediment in contact with meteoric porewater. Sedimentologists and crystallographers have long studied aragonite, and a considerable data base now exists concerning the mineral's physical properties, metastability and diagenesis. Examination of aragonite at microstructural levels using high-resolution transmission electron microscopy (HRTEM) is a comparably new technique, in part, because of the minteral's instability to the electron beam. Much of what has been done to date, has focused on microstructural features typical of all minerals (e.g., stacking faults, twinning). Some evidence now suggests that biogenic aragonite may be characterized by a more pervasive microstructure. Preliminary HRTEM imaging of chalky bivalves has revealed that 5-15 nm-scale microdomains (variably orientated lattice fringes), comprise much of the aragonite. The implication is that bivalves, perhaps all biogenic aragonite, is heterogeneous at the microstructural scale. Aragonite dissolution and diagenesis frequently initiates along structural weaknesses within shells and other skeletal frameworks. Microdomains may represent yet another structural weakness to initiate diagenetic alteration. Alternatively, microdomains may form due to diagenetic processes. The proposal seeks funds to address these, and other pertinent questions regarding the origin of the microdomains, and their ubiquity within biogenic aragonite. The study primarily targets bivalves from study sites in Alabama, Florida and New Zealand. These three areas were selected because they contain abundant bivalves of similar age (Plio-Pleistocene to modern), that inhabited similar depositional environments (shallow carbonate shelf). Most importantly, the Florida an New Zealand sites, have well documented, aragonite-driven diagenetic histories. It will be necessary to resolve the grade of diagenesis in order to assess the role of diss olution and alteration on the formation of the microdomains. The New Zealand study site is particularly well suited to do this. Here, shells of relatively pristine character occur virtually alongside those that are partially dissolved ("chalky"), completely dissolved (molds or calcite spar-filled molds), or recrystallized to calcite spar. This study will use a variety of established and relatively new techniques to achieve its goals. Thin-section petrography, enhanced by high-resolution video imaging, and scanning electron microscopy will resolve shell fabrics, degree of dissolution (chalkiness) and intra-and intercrystalline porosity. X-ray diffraction will be employed to establish bulk crystallography and sample purity. Geochemical analysis (electron microprobe, inductively coupled plasma spectrophotometry, stable isotopes) will be used to establish trace elemental and isotopic compositions, and diagenetic grade of thin-sectioned and powdered specimens. These data will be used to select suitable samples for HRTEM imaging. Ultimately, the results of this project will yield new information about the crystallography of aragonite. Moreover, it will open up new avenues for future research projects examining the ultrastructure of carbonate minerals, and carbonate diagenesis.
