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。它们是许多商业花岗岩台面和地砖中的主要矿物。蠕石、月光石和彩虹色拉布拉多石等宝石也是斜长石。斜长石的晶体结构和形成机制自1940年首次发现以来，几十年来一直是一个谜。这项拟议的研究将帮助地质学家对它们的结构、稳定性、形成机制和演化史有一个定量的了解。研究结果将被纳入调查人员教授的入门和研究生课程。研究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K-12社区地质博物馆开展外联活动，通过夏季接触机会指导高中生，并开发用于学习的开源软件，如用于教育目的的3D互动移动平台分子模拟软件工具tom Touch。由于所研究的中间斜长石中的无公度调制发生在一个连续变化的宽组成范围内，该研究也将有助于研究其他非常规(非周期)晶体材料中的无公度调制，如具有(3+1)D结构的金属间化合物，这可能是未来应用的新材料的基础。研究小组最近的Z像差校正成像结果显示，富钠和富钙的e斜长石中都存在Ca-Na有序性和非中心对称性。这个多学科小组将研究来自不同地质环境的横跨整个成分范围(从~An25到~An75)的中间斜长石。将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错综复杂的性质。该研究包括以下三个方面的工作：(1)利用像差校正的扫描电子显微镜对中间斜长石进行Z衬度成像；(2)用单晶X射线衍射法测定e-斜长石的晶体结构；(3)采用基于密度泛函理论的相图模型对斜长石二元系的固相线下相关系进行原子计算。中间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本和重要的课题。解决神秘的无公度调制结构及其固相线下相关系的问题，将有助于更好地理解斜长石的行为及其在经历了不同演化历史的变质岩和火成岩中的组合/共生。

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.