Structure and Thermoelastic Properties of Al- and Fe- Bearing High Pressure Silicate Minerals

虽然地球地幔矿物成分的大致轮廓是相当众所周知的，地球物理学仍然无法解释下地幔和过渡带横向不均匀性的地震剪切特性和层析成像图。 弹性波速度的实验室测量仍然局限于与上地幔相对应的压力。 此外，关于Al和Ca对（Mg，Fe）SiO3-钙钛矿性质的影响，可能是下地幔的主要成分，知之甚少。 最近的实验表明，少量的铝在硅酸盐钙钛矿显着提高其可压缩性，因此迫使重新检查的组成模型，完全忽略铝，或假设它对硅酸盐的弹性几乎没有影响。 已知Al的作用对Fe 3+的同时存在敏感，但其机制尚未完全理解。 铝和钙在稳定石榴石低于660公里的不连续性的作用也需要更好地理解。 人们对D "区的矿物学知之甚少，随着每一次新的地震检查，其性质变得更加复杂。我们建议开发一个模型的硅酸盐矿物，将允许一个准确的检查弹性性能相结合的低压数据与密度泛函理论。 这种半经验模型将基于"离子"材料中的成键的变分诱导呼吸（VIB）理论。 除了已证实的密度泛函能量，新模型将纳入参数共价贡献，这将与离子间电荷转移竞争，以响应电负性均衡。 这些参数将受到弹性速度和振动光谱的低压数据的约束。 初步调查表明，这是一种非常有前途的方法。 该方法的独特优势在于其相对于完全第一原理方法的高效率。 此外，该方法有望有效地引导高质量的低压弹性数据到地球深部条件，从而大大提高其实用性。 因此，将有可能有效地寻找矿物结构，并研究它们对各种地球物理条件的依赖性。充分发展的模型将用于产生声子谱，弹性常数和状态方程。 它也将被用来检查耦合Al和Fe 3+取代到（Mg，Fe）SiO3钙钛矿的影响。这将允许产生候选下地幔矿物组合的地震速度，并检查它们如何受到晶体结构和相对低丰度成分（如Al，Ca和Fe 3+）的影响。 还将尝试评估这些性能的温度依赖性。 待研究的矿物包括硅酸盐钙钛矿、石榴石、各种SiO2相以及可能存在于下地幔和D "带条件下的由MgO、SiO2、CaO、Al2O3和FeO组成的其他矿物。

BukowinskiEAR-0003456Although the gross outlines of the mineral composition of the Earth's mantle are reasonably well known, geophysics remains unable to interpret seismic shear properties and tomographic maps of lateral heterogeneity in the lower mantle and transition zone with much confidence. Laboratory measurements of elastic wave velocities are still confined to pressures corresponding to the upper mantle. In addition, little is known about the effects of Al and Ca on the properties of (Mg,Fe)SiO3-perovskite, likely the dominant component of the lower mantle. Recent experiments indicate that a small amounts of Al in silicate perovskite significantly enhances its compressibility, and hence forces a re-examination of compositional models that ignore Al altogether, or assume that it has little effect on the elasticity of silicates. The effect of Al is known to be sensitive to the concurrent presence of Fe3+, but the mechanism is not fully understood. The role of Al and Ca in stabilizing garnets below the 660 km discontinuity also needs to be better understood. Even less is known about the mineralogy of the D'' zone, whose properties gain in complexity with every new seismic examination. We propose to develop a model of silicate minerals that will allow an accurate examination of elastic properties by combining low pressure data with density functional theory. This semi-empirical model will be based on the Variationally Induced Breathing (VIB) theory of bonding in "ionic" materials. In addition to the proven density-functional energy, the new model will incorporate parametric covalent contributions, which will compete with inter-ionic charge transfer in response to electronegativity equalization. The parameters will be constrained with low pressure data on elastic velocities and vibrational spectroscopy. Preliminary investigations show this to be a very promising approach. The unique strength of the method lies in its high efficiency relative to fully first-principles approaches. Furthermore, the method promises to effectively bootstrap high quality low pressure elasiticity data into deep Earth conditions, thus greatly enhancing their utility. It will thus be possible to efficiently search for mineral structures and to examine their dependence on various geophysical conditions. The fully developed model will be used to generate phonon spectra, elastic constants and equations of state. It will also be used to examine the effects of coupled Al and Fe3+ substitution into (Mg,Fe)SiO3 perovskites. This will allow the generation of seismic velocities for candidate lower mantle mineral assemblages, and an examination of how they are affected by crystal structure and the presence of relatively low abundance components like Al, Ca and Fe3+. An attempt will also be made to evaluate the temperature dependence of these properties. The minerals to be investigated include silicate perovskites, garnets, various SiO2 phases, and other minerals composed of MgO, SiO2, CaO, Al2O3, and FeO that may be present at lower mantle and D'' zone conditions.