Apatite, Magnesian Calcite, and Silica: An Experimental Approach to their Reactivity in Natural Environments

磷灰石、镁方解石和二氧化硅与粘土一起，是海洋沉积物中最丰富的相。如果我们要了解镁方解石的生物成因、海洋环境中的碳酸盐胶结物、镁方解石在地球二氧化碳收支中的作用以及它们在成岩过程中的命运，了解镁方解石的物理和化学性质是必要的。磷灰石的稳定性和在溶液中的反应速率是必需的，因为磷灰石在磷的沉积循环、磷灰石矿床的形成和控制海洋肥力方面具有重要作用。二氧化硅的稳定性和反应速率数据将有助于解释二氧化硅在成岩作用过程中所经历的一系列修饰，并有助于确定二氧化硅在沉积旋回中的作用。不幸的是，先前对这些系统的研究未能产生普遍接受的稳定性模型，部分原因是用于稳定性和动力学实验的材料不足。在评估天然相的稳定性和溶液行为之前，必须彻底了解合成样品的化学和固溶体性质。我们的方法是比较均匀的、良好结晶的合成相与天然材料的特性。我们将扩展我们以前对镁方解石的测量，以跨越天然样品的组成范围。我们也将扩展我们的合成到磷灰石和蛋白石。稳定性测定将在与成岩条件相对应的整个温度范围内进行。合成和天然样品的扫描电镜、x射线、电子探针、拉曼和红外分析将为解释这些矿物的溶液行为以及它们在自然环境中的反应提供基础。

Apatite, magnesian calcite, and silica are, along with clays, the most abundant phases in marine sediments. Knowledge of the physical and chemical properties of magnesian calcites is necessary if we are to understand their biogenic production, the carbonate cements in the marine environment, the role of magnesian calcites in the CO2 budget of the earth, and their fate during diagenesis. Apatite stabilities and reaction rates in solution are needed because of the importance of apatite in the sedimentary cycle of phosphorus, in the formation of phosphorite deposits, and in controlling the fertility of the ocean. Stability and reaction rate data on silica will help to explain the series of modifications silica undergoes during diagenesis, and help to define the role of silica in the sedimentary cycle. Unfortunately, previous investigations in these systems have failed to produce universally accepted models of their stabilities, partly because of the inadequacy of materials used in stability and kinetic experiments. A thorough understanding of the chemistry and solid solution nature of synthetic samples is necessary before the stabilities and solution behavior of natural phases can be evaluated. Our approach is to compare the characteristics of homogeneous, well- crystallized synthetic phases to natural materials. We will extend our previous measurements on magnesian calcites to span the composition range of natural samples. We will also extend our syntheses to apatite and opal. Stability determinations will be made throughout a temperature range corresponding to diagenetic conditions. SEM, x-ray, electron microprobe, Raman, and IR analyses of both synthetic and natural samples will provide a basis for interpretation of the solution behavior of these minerals, and thus their reactions in the natural environment.