Aragonite Crystallography and Diagenesis: An Old Problem Viewed at a New Scale

Haywick 9527103 文石成岩作用是造成与大气孔隙水接触的碳酸盐沉积物锂化的大部分方解石胶结作用的原因。 长期以来，沉积学家和晶体学家一直在研究文石，并且现在存在关于矿物的物理性质、介稳性和成岩作用的大量数据库。 使用高分辨率透射电子显微镜（HRTEM）在微观结构水平上检查文石是一项新技术，部分原因是矿物对电子束的不稳定性。 迄今为止，已经做的大部分工作都集中在所有矿物的典型微观结构特征上（例如，堆垛层错、孪生）。 一些证据表明，生物成因的文石可能具有更普遍的微观结构。 白垩双壳类的初步HRTEM成像显示，5-15 nm尺度的微区（定向晶格条纹），包括大部分的文石。 这意味着，双壳类，也许所有的生物文石，是异质的微观结构尺度。 文石溶解和成岩作用经常沿着贝壳和其他骨架内的结构弱点沿着发生。 微区可能代表另一个结构弱点，引发成岩蚀变。 或者，微区可能由于成岩过程而形成。 该提案寻求资金来解决这些问题，以及其他有关微结构域起源的相关问题，以及它们在生物成因文石中的普遍存在。 该研究主要针对来自亚拉巴马、佛罗里达和新西兰研究地点的双壳类。 之所以选择这三个地区，是因为它们含有丰富的年龄相似的双壳类（上新世-更新世至现代），居住在相似的沉积环境（浅碳酸盐陆架）。 最重要的是，佛罗里达和新西兰的网站，有很好的记录，文石驱动的成岩历史。 为了评价溶解和蚀变在微域形成中的作用，有必要解析成岩作用的等级。 新西兰的研究地点特别适合这样做。 在这里，相对原始特征的贝壳几乎出现在部分溶解（“白垩质”）、完全溶解（铸模或方解石晶石填充铸模）或重结晶为方解石晶石的贝壳旁边。 这项研究将使用各种已建立的和相对较新的技术来实现其目标。 通过高分辨率视频成像和扫描电子显微镜增强的薄片岩相学将解析贝壳组构、溶解程度（白垩度）和晶内和晶间孔隙度。 将采用X射线衍射确定体相晶体学和样品纯度。 地球化学分析（电子探针、电感耦合等离子体分光光度法、稳定同位素）将用于确定薄片和粉末样品的微量元素和同位素组成以及成岩等级。 这些数据将用于选择合适的样品进行HRTEM成像。 最终，该项目的结果将产生关于文石晶体学的新信息。 此外，它将为今后研究碳酸盐矿物的超微结构和碳酸盐成岩作用开辟新的途径。

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.