Synthesis of large and homogeneous single crystals of water-bearing minerals by slow cooling at deep-mantle pressures
Synthesis of large and homogeneous single crystals of water-bearing minerals by slow cooling at deep-mantle pressures
复制标题
DOI：
10.2138/am-2015-5237
复制
发表时间：
2015-07-01
影响因子：
3.1
通讯作者：
Yurimoto, Hisayoshi
中科院分区：
文献类型：
--
作者：
Okuchi, Takuo;Purevjav, Narangoo;Yurimoto, Hisayoshi
地球深部地幔中是否存在水是高压矿物学领域越来越感兴趣的问题。进一步推进该领域研究的一项重要任务是创建1毫米或更大尺寸的候选深地幔含水矿物的均匀单晶，这是将其应用于使用第三代中子仪器的飞行时间（TOE）单晶劳厄衍射方法所需的。在这项研究中，我们进行了几个实验，以证明一种改进的方法，通过非常缓慢的冷却超过1天的最长时间，在相关的过渡区和下地幔条件下生长如此大尺寸的含水晶体。使用该方法成功合成的晶体包括致密水合硅酸镁（DHMS）相E、水合瓦氏石、水合灵伍德石和桥镁石（硅酸盐钙钛矿）。通过X射线旋进照相、单晶X射线衍射（SCXRD）、场发射扫描电子显微镜（FE-SEM）、电子探针显微分析仪（EPMA）、X射线衍射（XRD）、X射线二次离子质谱（西姆斯）、粉末X射线衍射（PXRD）和TOF中子粉末衍射（TOF-NPD）。产物晶体经确认无内含物且结晶学均质。主要元素和氢同位素丰度的组成和同位素差异分别低于1%和3%，在每个回收的样品胶囊内的晶内和晶间。在1100 ℃的恒温下保持3小时生长最大尺寸达600 μ m的E相晶体。使用E相的晶格参数与温度的关系，用于E相合成的样品胶囊中的热梯度已被评估为20 ℃/mm。在加热10小时期间，在1390 ℃下以70 ℃的温度降低生长最大尺寸高达1100 gm的含水wadsleyite晶体。最大尺寸达1000 μ m的含水林伍德石晶体在约1400 ℃下生长，在加热12小时期间温度降低110 ℃。最大尺寸达600 μ m的硼镁石晶体在1700 ℃下生长，在加热12小时期间温度降低30 ℃。一个TOF单晶衍射仪已成功地用于分析的含水wadsleyite晶体之一，这表明，适合其预期用途的单晶创建使用本研究中提出的方法。
The presence of water in the Earth's deep mantle is an issue of increasing interest in the field of high-pressure mineralogy. An important task for further advancing research in the field is to create homogeneous single crystals of candidate deep-mantle water-bearing minerals of 1 mm or larger in size, which is required for applying them for the time-of-flight (TOE) single-crystal Laue diffraction method with a third-generation neutron instrument. In this study, we perform several experiments to demonstrate an improved methodology for growing hydrous crystals of such large sizes at relevant transition zone and lower-mantle conditions via very slow cooling over a maximum period of 1 day. Successfully synthesized crystals using this methodology include dense hydrous magnesium silicate (DHMS) phase E, hydrous wadsleyite, hydrous ringwoodite, and bridgmanite (silicate perovskite). It is also demonstrated that these hydrous crystals can be grown from deuterium enriched starting materials in addition to those having a natural hydrogen isotope ratio.Magnitudes of chemical and crystallographic heterogeneities of the product crystals were characterized by comprehensive analysis of X-ray precession photography, single-crystal X-ray diffraction (SCXRD), field-emission scanning electron microscope (FE-SEM), electron probe microanalyzer (EPMA), secondary ion mass spectroscopy (SIMS)(,) powder X-ray diffraction (PXRD), and TOF neutron powder diffraction (TOF-NPD). The product crystals were confirmed to be inclusion free and crystallographically homogeneous. Compositional and isotopic differences of major elements and hydrogen isotope abundances were lower than 1 and 3%, respectively, among intracrystals and intercrystals within each recovered sample capsule. Phase E crystals up to 600 mu m in the largest dimension were grown at a constant temperature of 1100 degrees C kept for 3 h. Using a lattice parameter-to-temperature relation of phase E, the thermal gradient in the sample capsules for the phase E synthesis has been evaluated to be 20 degrees C/mm. Hydrous wadsleyite crystals up to 1100 gm in the largest dimension were grown at 1390 degrees C with a temperature reduction of 70 degrees C during heating for 10 h. Hydrous ringwoodite crystals up to 1000 mu m in the largest dimension were grown at around 1400 degrees C with a temperature reduction of 110 degrees C during heating for 12 h. Bridgmanite crystals up to 600 mu m in the largest dimension were grown at 1700 degrees C with a temperature reduction of 30 degrees C during heating for 12 h. A TOF single-crystal diffraction instrument has been successfully used for analyzing one of the hydrous wadsleyite crystals, which demonstrated that single crystals appropriate for their expected usage are created using the method proposed in the present study.