固体核磁光谱法研究镁铝复合氧化物的水热转化机制

Hydrothermal transformation mechanism of Mg-Al mixed oxides revealed by ^(29)Si and ^(27)Al MAS NMR

复合金属氧化物广泛分布于自然界,其中镁铝复合氧化物(简称MMO)则大量分布于基性-超基性岩浆岩、蒸发河湖相和海相沉积岩等之中。本研究以镁铝水滑石(HT)煅烧产物MMO为研究对象,采用水热合成方法,通过改变反应时长,考察并揭示MMO在富硅、碱性和160℃水热条件下的结构复原特性及其转化机制。X射线衍射结果显示,该条件下MMO首先复水形成水滑石相,进而水滑石相逐渐向皂石转化,并伴有钠霞石等杂相的形成与溶解。29Si和27Al魔角旋转核磁共振谱(MAS NMR)的结果证明,在皂石结晶过程中,Al3+优先占据四面体片层的四配位位置。该研究对揭示水滑石稳定性、水滑石与硅酸盐矿物异相成核与生长、热液类型的天然皂石矿床的形成过程等具有重要指示意义。

Mixed metal oxides constitute a common kind of material in nature, in which Mg-Al mixed oxide （MMO） is widely distributed in the basic-ultrabasic magmatic rocks, evaporates of rivers-lakes and marine sedimentary rocks. In this study, MMO was obtained by calcining of Mg-Al hydrotalcite. Its rehydration features and the related transformation mechanism in silicic alkaline solution under hydrothermal condition were investigated by changing reaction time. X-ray diffraction （XRD） patterns showed that MMO reconstructed to form hydrotalcite firstly, after that hydrotalcite transformed to saponite gradually, along with the formation and disappearance of impure phase （e.g., natrodavyn）. 29Si and 27Al MAS NMR results suggested that Al3＋ cations tended to occupy the tetrahedral sites on tetrahedral sheets. The new findings of this research have great significance for understanding the stability of hydrotalcite, heterogeneous nucleation and growth of hydrotalcite and silicate minerals as well as the formation processes of the hydrothermal saponite deposits.