Structure and thermal elastic properties of calcium silicate perovskite

钙硅酸盐钙钛矿（CaPv）是地球下地幔中第三丰富的相，仅次于（Mg,Fe）SiO3桥菱矿和（Mg,Fe）O。它被认为是解释一些下地幔地震观测和异常的候选矿物，也是下地幔中大型阳离子的主要宿主。然而，由于文献中经常存在相互矛盾的理论和实验预测和确定，地幔条件下CaPv的地震特性仍然缺乏约束。实验和计算揭示了CaPv的多种晶体结构，但其相变边界和与地幔相关的压力-温度-组成（P-T-X）空间的弹性特性仍存在争议。该项目将有助于解决以下基本问题：(1)俯冲洋壳的命运及其在下地幔矿物学和地震学中的作用；(2)地幔地震异常和非均质性的成因。该项目将直接有助于在内部实验室和大型同步加速器设施培训一名研究生，并将积极参与两名早期职业科学家。研究小组将继续指导本科生或高中生，特别是传统上代表性不足的少数民族，即夏威夷土著和其他太平洋岛民，通过积极参与正在进行的研究来研究地球科学和行星科学。拟议项目的结果将通过出版物、国家和国际会议、公开演讲和新闻媒介传播。本项目拟采用激光加热和外加热金刚石砧池（EHDAC）技术以及同步x射线衍射技术，在地幔P-T-X空间精确测量含（Al,Ti） CaSiO3钙钛矿的晶体结构，确定其相变边界。该团队将通过开发原位生长不可淬灭硅酸盐单晶的方法，并将单晶/多晶x射线衍射测量与新开发的EHDAC装置相结合，以达到1200 K以上的温度，从而实现这一目标。本研究旨在(1)确定纯和含（Ti,Al）的CaPv的晶体结构和相变；(2)测量与地球过渡带和下地幔相关的压力-温度条件下CaPv的状态热方程。对CaSiO3晶体相变和热弹性性质的测量将打破地幔矿物学研究的瓶颈，从而为CaPv在解释下地幔地震观测和异常方面的贡献提供新的认识。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Calcium silicate perovskite (CaPv) occurs as the third most abundant phase in the Earth’s lower mantle, next to (Mg,Fe)SiO3 bridgmanite and (Mg,Fe)O. It is considered a candidate mineral to explain some lower-mantle seismic observations and anomalies, as well as the main host for large cations in the lower mantle. Seismic properties of CaPv at mantle conditions, however, remain poorly constrained, as there are often contradictory theoretical and experimental predictions and determinations in the literature. Experiments and calculations have revealed multiple crystal structures of CaPv, but its phase transition boundary and elastic properties in the pressure-temperature-composition (P-T-X) space of relevance to the mantle are still under debate. This project will contribute to the following fundamental questions: (1) the fate of subducted oceanic crust and CaPv’s roles in lower mantle mineralogy and seismology, and (2) the origin of mantle’s seismic anomalies and heterogeneity. The project will directly contribute to the training of one graduate student at in-house laboratory and large synchrotron facilities and will also actively involve two early-career scientists. The research team will continue to mentor undergraduate students or high school students, particularly traditionally underrepresented minorities, i.e., Native Hawaiians and other Pacific Islanders, to study geosciences and planetary sciences by active involvement in ongoing research. The results from the proposed project will be disseminated via publications, national and international meetings, public lectures and news media.The proposed project aims to accurately measure the crystal structure and identify the phase transition boundary of (Al,Ti)-bearing CaSiO3 pervoskite in the P-T-X space of Earth’s mantle, employing laser-heated and externally-heated diamond anvil cell (EHDAC) and synchrotron X-ray diffraction techniques. The team will approach this goal by developing in-situ methods of growing non-quenchable silicate single crystals, and combining single-crystal/multigrain X-ray diffraction measurements with a newly developed EHDAC setups to reach temperatures above 1200 K. The proposed research aims to (1) determine the crystal structure and phase transformations of pure and (Ti,Al)-bearing CaPv, and (2) measure the thermal equation of state of CaPv at pressure-temperature conditions relevant to Earth's transition zone and lower mantle. The proposed measurements on the crystallographic phase transitions and thermal elastic properties of CaSiO3 will break open a bottleneck on the quest into the mantle mineralogy, and thus offer new understanding of CaPv’s contribution in elucidating the lower-mantle seismic observations and anomalies.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.