How phosphate is incorporated into carbonate minerals and its dependence on crystal growth conditions

溶解无机磷酸盐与碳酸盐矿物方解石和文石的相互作用对许多低温地球化学和古环境系统具有重要意义。磷酸盐是一种必需的，有时是生物限制性的营养素，但在土壤和自然沃茨中，碳酸盐矿物的生长可以去除磷酸盐。磷酸盐强烈吸附在方解石和文石的表面，并在晶体生长过程中作为不相容的微量元素掺入这些矿物中。这些过程从环境中去除磷，使其无法被生物体利用，但可能会记录溶解的磷含量。例如，高空间分辨率下方解石洞穴沉积物中磷酸盐浓度的变化被用于古气候研究，以标记洞穴沉积物中的年度生长层，并指示古水文条件。最近有人提出，某些深海珊瑚的磷含量可以作为生长时海水中溶解磷酸盐浓度的代表。虽然已经有许多研究磷酸盐在碳酸盐矿物中的吸附和结合，但结合物种的化学性质仍然不确定。最近，我们已经表明，固态31 P NMR光谱可以用来确定在方解石中的P形态原位在自然界中发现的浓度。在方解石洞穴沉积物磷酸盐被纳入至少三个不同的过程中，涉及至少两个不同的结晶inclusions.We提出了一个实验方案，调查磷酸盐纳入方解石和文石的途径，以及它如何随晶体生长条件而变化。方解石和文石将在精心控制的条件下制备，具有不同的溶解磷酸盐、生长速率、pH值和与自然沉淀相关的其他重要参数。磷酸盐在矿物相中的分布将通过固态NMR光谱和P K边XANES和光谱显微镜方法的组合来确定，这也将允许鉴定任何结晶磷酸盐相。特别强调的是确定的条件下，溶液中的磷酸盐浓度可以推断出的P分布的矿物沉淀。该实验计划将与其他研究人员合作，对天然碳酸盐矿物进行研究，包括方解石和文石洞穴沉积物以及文石深海珊瑚。智力成果：该项目将提供关于碳酸盐矿物中P的化学形式如何随晶体生长条件而变化的信息，从而为磷酸盐在普通矿物中的掺入提供基础性的新知识。需要了解的P掺入机制，以促进知情使用微量P含量的古环境重建。这些数据也将是有用的，为构建更一般的模型，将大的不相容的含氧阴离子在碳酸盐。实验数据将提供限制条件，导致表面沉淀物的形成，这是目前知之甚少，因为这样的沉淀物是非常难以检测。更广泛的影响：该项目的主要目标是提供相关的研究机会，为研究生的教育，并介绍本科生在一个结构化的环境中进行科学研究。该研究计划是专为学生获得材料表征和实验低温地球化学，矿物学和光谱学，适用于广泛的科学问题的各种技能。这项研究将涉及本科生的方式，为研究生提供指导经验。该项目将通过支持固态核磁共振设施和支持操作这些光谱仪进行研究合作的研究生，为地球化学研究基础设施做出贡献。

The interaction of dissolved inorganic phosphate with the carbonate minerals calcite and aragonite is important to many low temperature geochemical and paleoenvironmental systems. Phosphate is an essential, sometimes bio-limiting nutrient, but in soils and natural waters growth of carbonate minerals can remove phosphate. Phosphate is sorbed strongly to the surface of calcite and aragonite and incorporated into these minerals during crystal growth as an incompatible trace element. These processes remove P from the environment, making it unavailable to organisms, but potentially creating a record of the dissolved P content. For example, variation in phosphate concentration in calcite cave deposits at high spatial resolution is used in paleoclimate studies to mark annual growth layers in speleothems and to indicate paleohydrologic conditions. Recently it has been proposed that the P content of certain deep-sea corals can serve as a proxy for the dissolved phosphate concentration in seawater at the time of growth. Although there have been many studies of the sorption and incorporation of phosphate in carbonate minerals, the chemical nature of the incorporated species remains uncertain. Recently, we have shown that solid-state 31P NMR spectroscopy can be used to determine the P speciation in calcite in situ at concentrations found in nature. In calcite speleothems phosphate is incorporated by at least three different processes, involving at least two distinct crystalline inclusions.We propose an experimental program to investigate the pathway for phosphate incorporation in calcite and aragonite and how it varies with crystal growth conditions. Calcite and aragonite will be prepared under carefully controlled conditions with varying dissolved phosphate, growth rate, pH, and other important parameters relevant to precipitation in nature. The distribution of phosphate in the mineral phase will be determined by a combination of solid-state NMR spectroscopy and P K-edge XANES and spectromicroscopic methods, which will also allow identification of any crystalline phosphate phases. Particular emphasis will be placed on determining the conditions under which the phosphate concentration of the solution can be inferred from the P distribution of the mineral which precipitates. The experimental program will be guided by investigation of natural carbonate minerals in collaboration with other researchers, including calcite and aragonite cave deposits and aragonitic deep-sea corals.Intellectual Merit: This project will provide fundamental new knowledge of phosphate incorporation in common minerals, by providing information on the chemical form of the P in carbonate minerals how it varies with crystal growth conditions. An understanding of the P incorporation mechanism is needed to promote informed use of trace P content for paleoenvironmental reconstruction. These data will also be useful for constructing more general models for incorporation of large incompatible oxyanions in carbonates. The experimental data will provide constraints on the conditions that lead to formation of surface precipitates, which is at present poorly understood because such precipitates are very difficult to detect.Broader Impacts: A primary goal of this project is to provide relevant research opportunities for the education of graduate students and to introduce undergraduate students to scientific research in a structured environment. The research plan is designed specifically for students to acquire a diverse set of skills in materials characterization and experimental low-temperature geochemistry, mineralogy, and spectroscopy that are applicable to a wide range of scientific problems. The research will involve undergraduate students in a manner that provides mentoring experience for a graduate student. This project will contribute to research infrastructure in geochemistry through support of solid-state NMR facilities and support for graduate students who operate these spectrometers for research collaborations.