斜硅石(moganite)高温晶体结构和相变的固体光谱学研究

Moganite is a polymorph of microcrystalline silica which was approved as a distinct and valid mineral species in 1999 by International Mineralogical Association's Commission on New Minerals and Mineral Names. However, there were little researches on this mineral in China. Moganite is found to commonly coexist with varieties of microcrystalline quartz in evaporite and hydrothermal environments. It is considered to be an intermediate phase in the transformation of opal-A to quartz, whereas the presence of high moganite contents is proposed as an indicator or evidence for evaporitic regimes. Gaining a better understanding of thermal dynamics and stability of moganite would be important for evaluating its role in geology process, however, these physical properties of moganite remain poorly studied. It has recently been found that moganite undergos a structural phase transition from space group I2/a to Imab at the temperature of 570 K. The transition is proposed to be displacive and associated with soft modes, like the α-βphase transition in quartz. However, the transition mechanism or model is not fully consistent with available experimental data. The applicants of this project and their co-workers have recently carried out a primary investigation of moganite at high temperatures using in-situ high-temperature infrared spectroscopy. The infrared results confirmed a structural change near 570 K. In addition, the infrared data indicated an extra structural phase transition. This proposed study will aim to carry out, through comparing with quartz and cristobalite, a systematic analysis of moganite using various experimental methods, to gain better insights into thermal dynamics and stability of moganite, and other important scientific issues such as the number of transitions in the studied temperature region, the nature and driving forces of the changes, the factors determining the transition temperatures, potential soft modes and the reason why one crystal phase occurs in a certain temperature region in preference to another potential phase. In addition, the study is expected to improve the comprehension of the role of moganite in diagenetic and structural geology and offer experimental data for future theoretical modeling.

斜硅石是1999年才在国际上被正式承认为二氧化硅的新矿物种类，而国内对它的研究鲜有开展。地质学上，它被认为是水热环境中从非晶二氧化硅向石英变化的中间相； 高含量的斜硅石被认为是蒸发岩环境下结晶生长的地质证据。理解斜硅石在这些地质过程中的价值，需要了解其高温热力学特性和稳定性，而该方面的研究目前有待深入。在晶体结构方面，近年研究表明斜硅石在570K会发生I2/a-Imab可逆的结构相变，类似石英中由软模驱动的α-β位移型相变。但实验证据仍然对有关相变模型支持不足或有矛盾。申请人最近进行了前期红外光谱实验，结果证实了其在570K有结构变化，并预示还有新的高温相变存在。本项目拟通过与石英和方石英对比的方式，研究斜硅石的热稳定性能、高温物理性能和晶体结构。认识其可能的高温相变数目、出现相变的原因、是否存在光学软模、影响相变的因素等矿物物理学问题, 加深对斜硅石物理特性在成矿和构造地质学中意义的理解。

本研究在斜硅石（moganite）的晶体结构、光学特性、以及高温热力学行为方面进行了一系列的分析和研究，获得了高质量的光谱新数据和原创的分析结果，澄清了斜硅石高温相变机理的重要争议。首次在FIR-MIR-NIR波段（20-5600 cm-1）获得了目前国际上最为完整的斜硅石的红外吸收光谱，以及多声子过程和H2O/OH有关的振动模式。通过红外反射实验和Kramers–Kronig分析获得了不同含量斜硅石样品的室温的介电常数（ε = ε' + i ε''）、能量损失函数（energy loss function, –Im（1/ε））、以及横波光学振动模（transverse optical, TO）和纵波光学振动模（longitudinal optical, LO）。首次利用变温原位红外光谱、 软膜拉曼光谱、29Si核磁共振等光谱实验技术、朗道理论（Landau Theory)、以及基于密度泛函理论的第一性原理，分析和研究了斜硅石的高温I2/a-Imab结构相变、斜硅石向微石英（micro quartz）的转变过程中的结构变化、以及与岩石年龄的关联。本研究利用高温原位硬模光谱（Hard Mode Spectroscopy）的原理和朗道理论分析，利用红外的峰位、峰宽和峰强为序参量Q，证实了斜硅石的高温I2/a-Imab相变是二级相变（临界指数= 0.5），该相变应该与软膜（soft mode）有关；基于朗道相变理论对实验获得序参量Q的分析给出相变温度为Tc = 568 K；实验数据表明序参量序参量Q在相变温度Tc并没有减少至零，而是出现异常，在Tc以上区域出现一个拖尾，继续存在直到620 K或者以上；实验数据支持在620K以下只有一个相变，本项目的数据表明前人报道中斜硅石高温相变的异常，可能与序参量Q的拖尾有关，而非真正的结构相变；而该拖尾可能是起源于目前所有研究采用的天然样品中的晶体缺陷。 研究通过第一性原理方法，首次计算了低温I2/a相和高温Imab相的振动光谱，并预测在低频存在拉曼激发的软膜；高温原位软膜拉曼分析观测到soft-mode-like 振动模。近红外、中红外和核磁共振29Si数据表明含斜硅石的二氧化硅样品（年龄在13-3486 Ma）的半峰宽（即结晶度）与样品的岩石年龄以及样品中斜硅石含量有关联。DAC静高压拉曼实验显示斜硅石存在高压结构异常。