Structure-Based Thermodynamic Properties of Feldspars

长石矿物占地壳的60%。因此，对它们的热力学和弹性性质的了解对于理解从微观到造山的所有尺度上的岩石学和地球化学过程是至关重要的。在之前NSF-EAR赠款的支持下，我们的研究小组已经确定了一组长石矿物-斜长石的弹性性质，其组成范围从NaAlSi3O8到CaAl2Si2O8。这些斜长石约占地壳中发现的长石的一半。另一半是碱性长石(NaAlSi3O8-KAlSi3O8)，其状态方程或弹性数据很少。确实存在的很少的数据大多是陈旧的、相互矛盾的，无论是组成还是结构状态都没有显示出明确的趋势。我们将通过确定关于碱和三元长石的高精度状态方程和结构数据来解决我们知识库中的这一关键差距。这一新数据与我们之前研究中提出的晶体化学概念相结合，将使我们能够建立第一个基于结构的长石热力学模型，该模型将对长石的整体固溶体和其稀释(微量元素)状态的描述统一为单一的、一致的描述。由此得到的模型将预测阳离子中心大小和骨架构型作为压力、温度和阳离子含量的函数。这反过来将允许计算任何压力、温度和组成的弹性、热膨胀和混合体积。这一模型的成功开发不仅可以预测长石的热力学性质，还将为控制长石、熔体和流体之间的阳离子分配的原子尺度机制以及地壳中地震波速度和各向异性的关键约束提供见解。因此，这样的模型将对范围广泛的地球科学家非常有用和感兴趣，包括矿物学家、岩石学家、地球化学家和地球物理学家。我们正在开发的描述长石对P、T和X的响应的晶体化学原理应该同样适用于其他骨架矿物，包括工业上重要的沸石矿物和技术上重要的钙钛矿。

Feldspar minerals comprise 60% of the Earth's crust. Therefore knowledge of their thermodynamic and elastic properties is essential for understanding petrological and geochemical processes at all scales from the microscopic to the scales of mountain building. With the support of a previous NSF-EAR grant, our research group has determined the elastic properties of one group of the feldspar minerals, the plagioclase feldspars, whose compositions range from NaAlSi3O8 to CaAl2Si2O8. These plagioclase feldspars comprise about half of the feldspars found in the Earth's crust. The other half are the alkali feldspars (NaAlSi3O8 to KAlSi3O8), for which there is very little equation of state or elasticity data. What little data that does exist is mostly old, contradictory, and exhibits no clear trends with either composition or structural state. We will to address this critical gap in our knowledge base by determining high-precision equation of state and structural data on alkali and ternary feldspars. This new data, combined with the crystal-chemical concepts developed in our previous research, will allow us to build the first structure-based thermodynamic model for feldspars that will unite the description of their bulk solid solutions and that of their dilute (trace element) regimes into a single, consistent description. The resulting model will predict cation site sizes and framework configuration as a function of pressure, temperature and cation content. This in turn will allow elasticity, thermal expansion and the volumes of mixing to be calculated for any pressure, temperature and composition. The successful development of this model will not only allow thermodynamic properties of feldspars to be predicted, but will also provide insights in to the atomic-scale mechanisms that control cation partitioning between feldspars, melts and fluids, and key constraints on seismic wave velocities and anisotropy in the Earth's crust. Such a model will therefore be of great use and interest to a broad range of earth scientists, including mineralogists, petrologists, geochemists and geophysicists alike. The crystal-chemical principles that we are developing to describe the response of feldspars to P,T, and X should be equally applicable to other framework minerals, including the industrially important zeolite minerals and technologically important perovskites.