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

虽然地幔矿物组成的大致轮廓已经相当为人所知，但地球物理学仍然无法很有把握地解释地震剪切特性和下地幔和过渡带的横向非均质层析成像图。弹性波速的实验室测量仍然局限于与上地幔相对应的压力。此外，Al和Ca对（Mg,Fe） sio3 -钙钛矿（可能是下地幔的主要成分）性质的影响知之甚少。最近的实验表明，少量的铝在硅酸盐钙钛矿显著提高其压缩性，因此迫使重新检查成分模型完全忽略铝，或假设它对硅酸盐的弹性几乎没有影响。已知Al的效应对Fe3+的同时存在很敏感，但其机制尚不完全清楚。Al和Ca在稳定660 km间断层以下石榴石中的作用也需要更好地了解。人们对D”带的矿物学了解更少，其性质随着每次新的地震检查而变得更加复杂。我们建议开发一种硅酸盐矿物模型，通过将低压数据与密度泛函理论相结合，可以准确地检查弹性特性。这种半经验模型将基于“离子”材料中键合的可变诱导呼吸（VIB）理论。除了已证实的密度-功能能外，新模型还将纳入参数共价贡献，这将与响应电负性均衡的离子间电荷转移相竞争。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它相对于完全第一性原理方法的高效率。此外，该方法有望有效地将高质量的低压弹性数据引导到深部地球条件下，从而大大提高了它们的实用性。这样就有可能有效地寻找矿物结构，并检查它们对各种地球物理条件的依赖。充分开发的模型将用于生成声子谱、弹性常数和状态方程。它还将用于检查耦合Al和Fe3+取代成（Mg,Fe）SiO3钙钛矿的影响。这将允许生成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Fe3+）的影响。还将尝试评估这些性质对温度的依赖性。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.