Crystal growth of illitic minerals in open systems

本研究旨在从晶体生长和地球化学角度阐明自然开放体系下伊利石及其相关矿物的形成机制。(1)利用X射线衍射仪和透射电子显微镜研究了水热环境下蒙脱石向伊利石转化过程中伊利石的生长机理。研究表明，伊利石是通过前驱体的溶解和新相的沉淀生长形成的。新相的生长是通过螺旋生长机制实现的。换言之，水热环境代表了一个相对较高的温度梯度和较低的岩水比的系统。在这样的体系中，伊利石的晶粒度分布符合稳态对数正态分布，与不同的时间、结构和位置无关。通过X射线衍射、透射电子显微镜和溶液饱和指数的研究，发现高岭石在北海道Ohyunuma火山口湖悬浮液中原位生长。在透射电子显微镜和原子力显微镜下，高岭石的(001)面上没有明显的生长螺旋，但具有小晶体聚集体的特征。这表明高岭石晶体可能在Ohyunuma的对流流体循环系统下以二维生长机制和晶体聚结方式生长。(3)风化过程中的高岭石反应系列是遵循Ostwald阶跃规律的天然矿物反应。反应系列可以用不稳定相和较稳定相的连续溶解和沉淀来描述，也可以用蒙脱石-伊利石反应系列来描述。考虑到各矿物溶解度的大小依赖关系，对风化反应路径进行了半定量模拟。

The purpose of this study is to clarify the formation mechanism of illite and related minerals under natural open systems from crystal growth and geochemical viewpoints. The following results were obtained during the term of project :(1) Growth mechanism of illite during the smectite-to-illite conversion in hydrothermal environments was studied by means of XRD and TEM.These studies indicated that illite is formed via dissolution of precursors and precipitation and growth of new phase. The growth of new phases is donimated by spiral growth mechanism. The hydrothermal environments, in other words, is representative of a system with relatively high temperature gradient and low rock/water ratio. In such systems the crystal size distribution of illite is characterized by a steady-state log-normal distribution independent of different times, structures, and locations. The habit change of growing illite crystals can be distinguishable from those in systems with relatively high rock/water ratios.(2) It was found that kaolinite is in-situ growing in the suspension of Ohyunuma crater lake, Hokkaido, based on studies of XRD,TEM,and saturation index of solution. Growth spirals were not visible on the (001) face of kaolinite under TEM and AFM but coalescent aggregates of small crystals were characteristic. It suggests that the kaolinite crystals may be growing by 2-dimensional growth mechanism and crystal-coalescence under the convectional fluid circulation system of Ohyunuma.(3) A reaction series, allophane * halloysite * kaolinite, in weathering is representative of a natural mineral reaction following to the Ostwald's step rule. The reaction series can be described by consecutive dissolution and precipitation of less stable and more stable phases as well as in smectite * illite reaction series. The reaction path in weathering was simulated semiquantitatively taking into account of the size-dependence of solubility of each minerals.