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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 is the most common manganese oxide mineral in soil and sediment, and is divided into hexagonal and triclinic substructures according to its symmetry characteristics of MnO6 layer. Hexagonal birnessite can be identified by the chemistry or biology of Mn2 + The formation of tridanhydride birnessite in the environment and its further conversion to calcium-manganese ore are not clear.With two kinds of hexagonal birnessite (acidic birnessite and hydro-manganese ore) as precursors, X-ray absorption The chemical structure of birnessite from six directions triclinic substructure and the chemical composition of calcareous manganese ore were studied by means of exponential spectroscopy (EXAFS), X-ray diffraction (XRD), electron microscopy (FESEM / TEM) and chemical composition analysis And mineralogical mechanism.The results show that the appropriate Mn (Ⅱ) concentration and weak alkaline conditions (pH≥8) can gradually convert the hexagonal birnessite into trisanaline birnessite, and then exchange the Mg2 + Fibrous calcium and manganese ore, the crystal growth mainly dissolves and crystallizes, and Mn (Ⅱ) reacts with Mn (Ⅳ) in MnO6 octahedral layer of hexagonal birnessite to form Mn (Ⅲ) and fill the vacancy in the layer, Manganese ore symmetry by the three directions of the six changes. And acidic Compared with birnessite, the hydromagnesite has weak crystallization, high chaos accumulation in layered sediments and rapid reaction with Mn (Ⅱ), and the layer structure transforms rapidly to dipalmitic bimodal. ) And the reaction between Mn (Ⅱ) and Mn (Ⅳ), the rate of conversion of hexagonal birnessite to trisanaline bimodal is accelerated. “The hexagonal birnessite → triclinic birnessite” may be in the environment Important sources of trisodium bilineate bilirudite, and further formation of calcium manganese ore important chemical mechanism.