Investigation on Modulated Structures and Subsolidus Phase Relations of Intermediate Plagioclase Feldspars Using Integrated Experiment and Simulation Methods

斜长石是地壳中最丰富的矿物。它们是许多商用花岗岩台面和地砖的主要矿物。像橄榄岩、月长石和虹彩拉布拉长石这样的宝石也是斜长石。自1940年首次发现斜长石以来，其晶体结构和形成机制几十年来一直是个谜。提出的研究将有助于地质学家对其结构、稳定性、形成机制和演化历史有定量的了解。研究结果将被纳入研究人员教授的入门级和研究生级课程。研究人员团队致力于让不同的研究生和本科生参与他们的研究。研究人员和研究生将继续通过地质博物馆为K-12社区开展外展活动，指导高中生利用暑期机会，并开发开源学习软件，如AtomTouch，一种用于教育目的的3D交互式移动平台分子模拟软件工具。由于所研究的中间斜长石中的不相称调制发生在一个连续变化的宽组成范围内，因此所提出的研究也将有利于其他非常规（非周期）晶体材料（如具有（3+1）D结构的金属间化合物）的不相称调制的研究，这些研究可能为未来应用的新材料奠定基础。研究小组最近的像差校正z对比成像结果揭示了富钠和富钙电子斜长石中Ca-Na有序和非中心对称性。多学科团队将研究来自不同地质环境的中斜长石，这些中斜长石跨越了整个组成范围（从~An25到~An75）。利用非中心对称性和超空间群特征方面的新突破，对电子斜长石进行结构细化，以破译电子斜长石的复杂性质。本研究包括以下3个方面的工作：(1)利用像差校正扫描透射电子显微镜对中斜长石进行z -对比度成像；(2)单晶x射线衍射法测定e斜长石晶体结构；(3)利用基于密度泛函理论的相图模型原子计算斜长石二元体系的次固相关系。中斜长石的晶体结构和亚固相关系是矿物学和岩石学中非常基础和重要的课题。解决这些神秘的非共度调制结构及其亚固相关系问题，将有助于更好地理解斜长石及其在经历不同演化历史的变质岩和火成岩中的组合/共生行为。

Plagioclase feldspars are the most abundant minerals in the Earth's crust. They are major minerals in many commercial granite countertops and floor tiles. Precious stones like peristerite, moonstone, and iridescent labradorite are plagioclase feldspars too. The crystal structures and formation mechanisms of the plagioclase feldspars have been an enigma for decades beginning with their first discovery in 1940. The proposed study will help geologists to have quantitative understanding of their structures, stabilities, formation mechanisms, and evolution histories. The research results will be incorporated into both introductory level and graduate level courses taught by the investigators. The team of researchers is committed to the engagement of diverse graduate and undergraduate students in their research. The investigators and graduate students will continue outreach activities through the Geology Museum for K-12 community, mentoring high school students through summer reach opportunity, and developing open source software for learning, such as, AtomTouch, a 3D interactive mobile platform molecular simulation software tool for educational purposes. Because the studied incommensurate modulations in the intermediate plagioclase feldspars occur in a wide compositional range with continuous variation, the proposed study will also benefit the study of incommensurate modulations in other non-conventional (aperiodic) crystalline materials like intermetallic coumpounds with (3+1)D structures that may underlie new materials for future applications. Recent aberration-corrected Z-contrast imaging results from the investigator's group reveal Ca-Na ordering and non-centrosymmetricity in both Na-rich and Ca-rich e-plagioclase feldspars. The multidisciplinary team will investigate the intermediate plagioclase feldspars that span the entire composition range (from ~An25 to ~An75) from different geological environments. Structural refinements of e-plagioclase feldspars will be carried out using new breakthroughs regarding non-centrosymmetricity and super space group characteristics to decipher the intricate nature of e-plagioclase feldspars. The proposed study includes the following 3 tasks: (1) Z-contrast imaging of intermediate plagioclase feldspars using an aberration-corrected scanning transmission electron microscope; (2) Crystal structure determination of e-plagioclase feldspars by single crystal X-ray diffraction method; and (3) Atomistic calculation of the subsolidus phase relations of the plagioclase binary system using density functional theory based phase diagram modeling. Both crystal structure and subsolidus phase relation of the intermediate plagioclase feldspars are very basic and important topics in mineralogy and petrology. Solving the problems of the mysterious incommensurately modulated structures and their subsolidus phase relations will provide improved understanding about behaviors of the plagioclase feldpars and their associations / intergrowths in metamorphic and igneous rocks that have experienced different evolution histories.