Crystal growth of illitic minerals in open systems

本研究的目的是从晶体生长和地球化学的角度阐明自然开放系统下伊利石及相关矿物的形成机制。项目期间取得了以下成果：(1)利用XRD和TEM研究了热液环境中蒙脱石向伊利石转化过程中伊利石的生长机理。这些研究表明，伊利石是通过前驱体的溶解和新相的沉淀和生长而形成的。新相的生长是由螺旋生长机制决定的。换句话说，热液环境代表了具有相对较高的温度梯度和较低的岩水比的系统。在此类系统中，伊利石的晶体尺寸分布的特征在于与不同时间、结构和位置无关的稳态对数正态分布。伊利石晶体生长习性的变化可以与岩水比较高的体系中的晶体生长习性变化区分开来。(2)通过X射线衍射、透射电镜和溶液饱和指数的研究，发现北海道大汤沼火山口湖悬浮液中高岭石是原位生长的。在 TEM 和 AFM 下，高岭石的 (001) 面上看不到生长螺旋，但小晶体的聚结聚集体是其特征。这表明高岭石晶体可能是在大汤沼对流流体循环系统下通过二维生长机制和晶体聚结生长的。(3)风化过程中水铝英石*埃洛石*高岭石的反应系列是遵循奥斯特瓦尔德阶跃规则的天然矿物反应的代表。该反应系列可以通过不太稳定相和较稳定相的连续溶解和沉淀以及蒙脱石*伊利石反应系列来描述。考虑到每种矿物溶解度的尺寸依赖性，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.