Natural and Experimental Weathering of Pyroxenes and Pyroxenoids

本提案的研究旨在通过将两组反应的高分辨率电镜表征与溶解动力学数据相结合，弥合自然和实验矿物风化研究之间的差距。研究涉及链状硅酸盐矿物。之所以选择链状硅酸盐，是因为它们在地球化学上的重要性，而且它们的结构与粘土制品非常相似，这使我们能够预测，在某些条件下，转化可能涉及一些结构继承。此外，通过合作安排，可以选择合适的实验反应材料和溶解速率数据。我们通过比较天然和实验风化的辉石矿物来测试所提出的方法。结果揭示了实验和自然反应表面的亚纳米尺度细节，并提供了对反应机制的见解。初步结果表明，在实验反应的类辉石上有富硅浸出层。因此，解释测量的同溶速率的模型必须考虑到进入溶液的Si可能主要来自浸出层-溶液界面这一事实。我们发现了凝胶层再结晶的初步证据。因此，速率也可能受到凝胶的传输特性、溶解度和结晶度的限制。自然风化反应的初始数据与蒙脱石反应产物的聚合单元至少部分继承是一致的。因此，我们假设辉石的速率可能反映了在实验室和自然条件下硅酸盐链的不完全解聚。拟议的工作有两个主要目标。首先，我们将描述天然辉石反应的组成、体积、矿物学和结构方面的特征，并检查微观结构（例如，脱溶片层、晶界、反相界、链宽缺陷、位错）对相对反应速率的影响。其次，我们将对实验反应的辉石和类辉石进行表征，并将这些数据与速率测量相结合，以阐明可能的溶解动力学模型的细节。通过这种方法，我们希望加强目前对风化反应的机制和产物的理解，以及实验测量速率适用于自然过程的条件和程度。

9317082 Banfield The research in this proposal aims to bridge the gap between natural and experimental mineral weathering studies by combining high-resolution electron microscopic characterization of both sets of reactions with dissolution kinetic data. Research involves chain silicate minerals. Chain silicates have been selected because of their geochemical importance and because their structures are sufficiently similar to those of clay products to allow us to predict that transformations might, under some conditions, involve some structural inheritance. Furthermore, through collaborative arrangements, a suitable selection of experimentally reacted materials and dissolution rate data are available. We have tested the proposed approach by comparing naturally and experimentally weathered pyroxenoid minerals. Results reveal sub-nanometer-scale details of both experimentally and naturally reacted surfaces and provide insights into reaction mechanisms. Initial results show Si-rich leached layers on experimentally reacted pyroxenoids. Thus, models to explain measured congruent dissolution rates must account for the fact that Si entering solution may be largely derived from the leached-layer-solution interface. We find preliminary evidence for recrystallization of the gel layer. Thus, rates may also be limited by transport properties, solubility, and crystallinity of the gel. Initial data for natural weathering reactions are consistent with at least partial inheritance of polymerized units by smectite reaction products. Thus, we hypothesize that rates for pyroxenes may reflect incomplete depolymerization of the silicate chains under both laboratory and natural conditions. The proposed work has two major goals. First, we will characterize the compositional, volumetric, mineralogical, and structural aspects of natural pyroxene reactions and examine the influence of microstructures (e.g., exsolution lamellae, grain boundaries, antiphase bounda ries, chain width defects, dilocations) on relative reaction rates. Second, we will characterize experimentally reacted pyroxenes and pyroxenoids and combine these data with rate measurements to clarify details of possible dissolution kinetic models. Through this approach we hope to enhance current understanding of the mechanisms and products of weathering reactions and the conditions under, and extent to which, experimentally measured rates apply to natural processes.