模拟表生环境水钠锰矿亚结构转化及钙锰矿的形成

Substructural Transformation of Birnessite and Formation of Todorokite in Simulated Surface Environment

水钠锰矿是土壤与沉积物中最为常见的氧化锰矿物,依据其MnO6层对称特点分为六方和三斜两种亚结构类型.六方水钠锰矿在表生环境中可通过Mn2+的化学或生物氧化形成,而环境中三斜水钠锰矿的形成及进一步转化为钙锰矿的途径尚不清楚.以两种六方水钠锰矿(酸性水钠锰矿和水羟锰矿)为前驱物,采用X射线吸收光谱(EXAFS)、X射线衍射(XRD)、电镜(FESEM/TEM)及化学组成分析等技术方法模拟表生环境研究了水钠锰矿从六方向三斜的亚结构转化及生成钙锰矿的化学条件和矿物学机制.结果表明,适当Mn(Ⅱ)浓度和弱碱性条件(pH≥8)可使六方水钠锰矿逐渐转化为三斜水钠锰矿,继而经Mg2+交换、常压回流得到了长纤维状的钙锰矿,其晶体生长以溶解-结晶为主.Mn(Ⅱ)与六方水钠锰矿MnO6八面体层内的Mn(Ⅳ)反应生成Mn(Ⅲ)并填充层内空位,使水钠锰矿对称型由六方向三斜转变.与酸性水钠锰矿相比,水羟锰矿结晶弱、层状堆积混乱度高,与Mn(Ⅱ)反应迅速,层结构向三斜水钠锰矿转化快.pH升高,促进六方水钠锰矿对Mn(Ⅱ)的吸附和Mn(Ⅱ)与Mn(Ⅳ)间的反应,六方水钠锰矿转化为三斜水钠锰矿的速率加快."六方水钠锰矿→三斜水钠锰矿"可能是环境中三斜水钠锰矿的重要来源,及进一步形成钙锰矿的重要化学生成机制.

Birnessite, one of the most common Mn oxide minerals in soils and sediments, has two types of substructures, hexagonal and triclinic, on the basis of the MnO6 layer symmetrical features. In the surface environment, hexagonal birnessite is formed through the chemical or biological oxidation of Mn2＋ , but the formation pathway of triclinic birnessite and further transformation into todorokite are still not clear. In the simulated surface environment, the chemical conditions and mineralogy mechanism of hexagonal birnessite （acid birnessite and vernadite） transformation to triclinic birnessite, and then into todorokite were investigated by EXAFS, XRD, FESEM/TEM and chemical composition analyses. The results show that hexagonal birnessite can be converted into triclinic birnessite under appropriate Mn（ III ） concentration and weak alkaline conditions （pH≥8）, and triclinic birnessite can be further converted into todorokite which consists of long fibers after Mg2＋ exchanged and refluxed under the atmospheric pressure. Long fibers of todorokite form mainly through a dissolution-recrystallization process. Reaction of aqueous Mn（ II ） with Mn（ IV ） in the hexagonal birnessite MnO6 octahedral layers causes transformation of hexagonal birnessite into triclinic birnessite via filling of yielded Mn（ II） into vacancy sites in the layers. Compared with acid birnessite, the transformation of vernadite into triclinic birnessite was much easier due to weak crystallization and turbostratic structure of vernadite. Higher pH facilitates adsorption of Mn（ II ） and reaction of Mn（ II ） with Mn（IV）, thus speeds up the transformation of hexagonal birnessite into triclinic birnessite. Therefore, one of the important sources of triclinic birnessite in nature can be denoted as= hexagonal birnessite → triclinic birnessite, which may be one of the important chemical formation mechanisms of todorokite in the surface environment.