Natural and Experimental Weathering of Pyroxenes and Pyroxenoids

9317082班菲尔德，这项建议中的研究旨在通过将两组反应的高分辨率电子显微镜表征与溶解动力学数据相结合，弥合自然矿物风化研究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。链状硅酸盐之所以被选中，是因为它们具有重要的地球化学意义，而且它们的结构与粘土产品的结构非常相似，因此我们可以预测，在某些条件下，转化可能涉及一些结构遗传。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度数据。我们通过比较自然风化的辉石矿物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级细节，并提供了对反应机理的见解。初步结果表明，在实验反应的辉石上形成了富硅的浸出层。因此，解释测量的一致溶解速率的模型必须考虑这样一个事实，即进入溶液的硅可能主要来自浸出层-溶液界面。我们发现了凝胶层重结晶的初步证据。因此，速率也可能受到凝胶的传输性质、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对聚合单元的至少部分遗传一致。因此，我们假设辉石的速率可能反映了实验室和自然条件下硅酸链的不完全解聚。拟议的工作有两个主要目标。首先，我们将表征天然辉石反应的组成、体积、矿物学和结构方面的特征，并考察微结构(例如，出溶片、晶界、反相界、链宽缺陷、离位)对相对反应速率的影响。其次，我们将对实验反应的辉石和辉石类化合物进行表征，并将这些数据与速率测量相结合，以澄清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理和产物的了解，以及实验测量的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.