Disorder and Dynamics in Silicate and Aluminosilicate Liquids, Glasses, and Crystals: Nuclear Magnetic Resonance Studies

本提案旨在为地球科学感兴趣的晶体、玻璃状、熔融硅酸盐和氧化物的原子尺度结构和动力学研究项目提供新的资助。该课程强调核磁共振（NMR）技术的应用，以获得基础数据，以帮助更好地理解和预测地球化学、岩石学和地球物理学中感兴趣的过程。核磁共振是一种特定元素的实验技术，可以提供晶体和非晶固体和液体中H、Li、B、C、O、F、Na、Mg、Al、Si、P、K（等）等原子周围的局部结构的定量数据，可以达到第二个原子邻居的距离，有时甚至更远。它对相对缓慢的时间尺度（秒到纳秒）的原子运动也非常敏感，这在扩散、相变和粘性流动等过程中特别有趣。这项研究工作的一个不寻常的能力是原位，高温核磁共振温度高达1500摄氏度。在之前的资助下，该项目已经产生了广泛的结果，包括玻璃和熔体中阳离子和阴离子的配位环境，成分，温度和压力如何改变结构，以及这些变化如何影响热力学和传输性质。特别强调了液体中结构无序程度的量化，并将其与构型熵和粘度联系起来；晶体硅酸盐和铝硅酸盐中无序性的相关问题也受到高度重视。在目前的资助期内，在确定铝硅酸盐玻璃和熔体中Al-Si有序程度方面取得了重大进展，包括首次使用17O“三量子”核磁共振（“3QMAS”）直接计算Al-O-Al氧的比例。这些结果很好地支持了一个独立于29Si核磁共振数据的有序统计热力学模型，并可以与性质数据进行预测和比较。使用这种方法的其他研究为我们理解铝硅酸盐熔体中水溶解的复杂过程和检测沸石中严格的“免铝”的变化做出了新的贡献。对于后者，包括“五量子”技术在内的新核磁共振方法已经显示出显著提高的光谱分辨率。新安装的14.1特斯拉光谱仪（600 MHz 1H频率）和斯坦福大学的18.8 T仪器（800 MHz）使分辨率和灵敏度得到了显着提高，这在研究微小（1至10 mg）高压样品以确定熔体和地幔矿物（如含al的MgSiO3钙钛矿）的结构变化方面特别有用。如果该提案获得资助，其中许多项目将继续并延长。一个特别的重点将是量化温度对熔体结构的影响，这是理解熔体性质的一个关键和知之甚少的问题。这项工作的一个主要工具将是最近建造的快速淬火装置，它将允许在广泛的“有效温度”范围内对熔体结构进行采样，即使是对小的、昂贵的、同位素富集的样品。在现场，高温核磁共振将直接用于表征铝硅酸盐熔体的动力学。将继续研究晶体硅铝酸盐（包括框架和硅酸片矿物）的无序性，以及它对框架氧和水之间特定位点交换速率的影响。高压玻璃的合作将继续进行，探索正离子和阴离子配位的同时变化和不同的P/T途径，以记录压力对熔体结构的影响；还将扩大关于场地占用和上下地幔矿物的有序/无序的合作。新的核磁共振方法和技术将继续发展，这将在地球科学内外具有广泛的适用性。

StebbinsEAR-0104926 This proposal is for renewed funding for a program of research on atomic-scale structure and dynam-ics of crystalline, glassy, and molten silicates and oxides of interest to the Earth Sciences. This program emphasizes the application of techniques of Nuclear Magnetic Resonance (NMR) to obtain fundamental data to help better understand and predict processes of interest in geochemistry, petrology, and geophys-ics. NMR is an element-specific experimental technique that can provide quantitative data on local struc-ture around atoms such as H, Li, B, C, O, F, Na, Mg, Al, Si, P, K (etc.) in crys-talline and amorphous sol-ids and liquids, out to distances of second atom neighbors and some-times beyond. It is also uniquely sen-sitive to atomic motions at relatively slow time scales (seconds to nanoseconds), that are particularly in-teresting in processes such as diffusion, phase transitions, and viscous flow. An unusual capability of this research effort is in situ, high tempera-ture NMR to temperatures up to 1500 degrees C. Under previous funding, this project has produced a wide range of results on coordination en-vironments of both cations and anions in glasses and melts, how composition, temperature and pressure change the structure, and how these changes affect thermodynamic and transport prop-erties. In particular, quantifying the extent of structural disorder in liquids, and relating this to configurational entropy and viscosity, have been emphasized; related questions of disorder in crystalline silicates and aluminosilicates have also been of high priority. During the current funding period, significant progress has been made in determining the degree of Al-Si ordering in aluminosilicate glasses and melts, including for the first time the use of 17O "triple quantum" NMR ("3QMAS") to directly count the proportions of Al-O-Al oxygens. A statistical thermodynamic model of the ordering, formulated independently from 29Si NMR data, is well supported by these results and allows prediction and comparison with data on properties. Other studies using this method have made new contributions to our understanding of the complex process of water dis-solution in aluminosilicate melts and to detect variations from strict "aluminum avoidance" in zeolites. For the latter, new NMR methods including "five quantum" techniques have shown dramatically en-hanced spectral resolution. A newly installed 14.1 Tesla spectrometer (600 MHz 1H frequency), and ac-cess to an 18.8 T instrument at Stanford (800 MHz), have allowed dramatic improvements in resolution and sensitivity, which have been especially useful in studying tiny (1 to 10 mg) high pressure samples to determine structural changes in melts and in mantle minerals such as Al-bearing MgSiO3 perovskite. Many of these projects will continue and be extended if this proposal is funded. A particular emphasis will be on quantifying the effects of temperature on melt structure, a critical and poorly understood issue that is at the heart of understanding melt properties. A major tool in this effort will be a recently con-structed fast quench apparatus that will allow sampling of the melt structure from a wide range of "fictive temperatures", even for small, expensive, isotopically enriched samples. Further in situ, high temperature NMR will be used to characterize dynamics in aluminosilicate melts directly. Studies will continue of disorder in crystalline aluminosilicates (including framework and sheet silicate minerals), and its effects on the site-specific rates of exchange among framework oxygens and H2O. Collaborations on high pres-sure glasses will continue, to explore simultaneous changes in both cation and anion coordination and dif-ferent P/T pathways for recording pressure effects on melt structure; collaborations on site occupancy and order/disorder in upper and lower mantle minerals will also be extended. New NMR methods and tech-nologies will continue to be developed which should have a wide range of applicability both within and outside of the Earth Sciences.