水铝英石中铁赋存对其可见-近红外光谱特征的影响和机理

Identification of mineralogy on Mars surface is the basis for analyzing the type of geological environment therein and further deducing its formation and evolution. Recently, based on the comparative study on the visible/near-infrared spectral features of minerals existing on Earth, allophane has been confirmed to occur widely on Mars surface, and part of it is considered to be closely associated with iron. However, even for Earth allophane, the mechanism of iron occurrence is ambiguous, and studies on how iron occurrence affects the visible/near-infrared spectral features of allophane have not yet been reported, which hinder the precise interpretation of information about the mineralogical compositions on Mars surface and the related palaeoenvironment. Given this, this proposed project aims to study the effects of iron occurrence on the visible/near-infrared spectral features of allophane. Typical natural allophane samples and synthesized allophane samples with iron of varied valences and contents will be comparatively studied. A combination of advanced techniques such as synchrotron radiation based X-ray absorption fine structure spectroscopy, Mössbauer spectroscopy and cryo-transmission electron microscopy are to be used to study the mechanism of iron occurrence in allophane, and to uncover the changes in the structure and surface moieties of allophane resulted from iron occurrence and the effects of these changes on the visible/near-infrared spectral features of allophane. It is anticipated that the findings of this proposed project will provide mineralogical basis for the identification of poorly crystallized hydrated minerals on Mars surface and deduction of the related local palaeoenvironment, and lay the foundation for understanding the surface-interface reactivity of natural iron-rich allophane and its environmental impacts.

火星表面的矿物组成分析，是判析火星表面地质环境类型、进而推演其形成和演化的重要基础。近年来，基于与地球矿物可见-近红外光谱特征的对比研究，确认了水铝英石在火星表面的广泛分布，且认为部分水铝英石可能含铁。然而，即使对于地球水铝英石，其铁赋存机制尚不清楚，有关铁赋存如何影响其可见-近红外光谱特征的研究更是尚未开展，这制约了对火星表面矿物组分及其所反映的环境信息的准确解读。鉴于此，本项目拟研究水铝英石中铁赋存对其可见-近红外光谱特征的影响，将采用典型天然水铝英石和不同铁价态、含量的合成水铝英石开展研究，采用同步辐射X射线吸收精细结构谱、穆斯堡尔谱和冷冻透射电镜等手段，查明铁赋存的微观机制，揭示铁赋存所致结构和表面基团变化及其对可见-近红外光谱特征的影响和机理。预期成果将为火星表面低结晶度含水矿物的鉴定及古环境反演提供矿物学依据，也为深入理解天然含铁水铝英石的表-界面反应性及其环境效应奠定基础。

水铝英石是地球和火星表面的一种常见矿物组分，其被发现常与以结构铁或铁（氧氢）氧化物形式存在的铁紧密结合。铁的赋存，不仅可能改变水铝英石的结构和表面基团性质，进而影响其在表生风化环境中参与地球物质迁移、循环过程中的作用；还可能改变水铝英石的光谱学特征，进而影响其在火星表面的分析鉴定和与之相关的古环境反演。然而，到目前为止，水铝英石在不同环境铁浓度下的形成和演化过程尚不清楚，铁在水铝英石中的赋存状态及铁赋存对水铝英石结构-性质的影响更是鲜有报道。为此，本研究采用X射线衍射分析（XRD）、傅里叶变换红外光谱（FTIR）、透射电子显微镜（TEM）、穆斯堡尔谱（Mössbauer）和可见-近红外光谱（VNIR）等手段，研究了不同浓度Fe(III)存在的条件下，水铝英石水热合成反应所生成产物的物相、结构和光谱特征变化，探讨了实验结果对地球和火星表面相关环境的指示意义。该研究发现：（1）环境铁浓度控制着铝-硅-铁水热体系中水铝英石的形成和转化：随着铁浓度升高，反应体系内的物相组成经历了（含铁）水铝英石——含铁（原）水铝英石+（原）四方纤铁矿——（原）四方纤铁矿+非晶质二氧化硅等转化过程。（2）常用于火星表面矿物鉴定的近红外光谱波段（1.2~2.6 μm）在鉴定该体系的非晶质产物时不敏感，而归属于铁原子激发的特征吸收带（0.4~1.2 μm）对产物的含铁量敏感；例如，0.97 μm处的吸收带的强度随铁含量呈线性变化，可用于定量评价产物中的总含铁量（包括铁氧化物中的铁和其它矿物的结构铁）。（3）结合水铝英石和四方纤铁矿在地球上的独特产出环境可推测，火星表面同时产出水铝英石和四方纤铁矿的地区可能存在一个富氯的铝-硅-铁体系（铁/(铁+铝)适中），并经历了弱酸性到中性的环境。上述认识为理解地球表生风化铝-硅-铁体系中水铝英石的形成、演化提供了新依据，并有助于提高基于轨道光谱数据鉴定火星表面的非晶质矿物、反演其古环境的准确度。