摘 要
铁锰结核是土壤演化的中间产物，其内部处于一个相对封闭的环境。因此其矿物类型、含量、氧化物活性与元素富集特点包含了土壤演化过程中的某些重要信息。本课题运用化学选择性溶提技术、全量分析、X射线衍射（XRD）及示差XRD分析，研究比较了我国几种地带性土壤（红壤、黄棕壤、黄褐土和棕壤）及铁锰结核胶体中，层状硅酸盐矿物、氧化铁矿物及氧化锰矿物在类型、含量、结晶程度上的异同；分析了不同金属元素在铁锰结核中的富集情况；比较了不同地域间、不同层次间以及土壤和铁锰结核间铁、锰氧化物的活性特点；结合化学分析，探讨了土壤矿物的演化过程及环境对铁锰结核生长的物理化学性质影响。主要结果有： 1、铁锰结核中，不同元素的富集程度迥异。与土壤相比，铁锰结核中Mn呈极显著富集，为土壤含量的99-137倍；Fe、Ca、Li、Pb显著富集，为土壤中含量的1.81、1.51、2.04、6.31倍；K在铁锰结核结核中的含量低于其相应土壤，为土壤中含量的63.6%。不同地带性土壤及铁锰结核间元素含量的比较中，Fe含量差异不显著；红壤中Mn含量约为其他地区2倍；各地区Li含量高且分布均匀；Pb含量差异极显著。
2、土壤层状硅酸盐矿物从北到南逐渐由2：1型矿物向1：1型矿物过渡，红壤和黄棕壤中均含有1.4nm矿物，但其铁锰结核中无1.4nm矿物；黄褐土和棕壤中除含少量的1.4nm矿物外，均含有较多的蒙脱石，但
其铁锰结核中，1.4nm矿物与蒙脱石含量之和相对减少。铁锰结核中水云母含量高于其相应土壤，枣阳黄褐土和铁锰结核中的1.4nm矿物为蛭石。在土壤成土过程的铁锰结核形成阶段及其以前，土壤中的层状硅酸盐矿物主要为水云母，矿物组合相对简单，1.4nm矿物以及部分蒙脱石等主要是其土壤成土过程的后期产物，是水云母进一步演化的结果。铁锰结核作为土壤风化成土过程中间阶段的产物，在形成后逐渐成为封闭环境，其中的层状硅酸盐矿物进一步分解转化延缓。 3、铁锰结核中，铁的活化度远高于土壤组分.，说明铁锰结核处于相对潮湿的环境。由南至北土壤及铁锰结核中氧化铁矿物类型从针、赤混合型过渡到针铁矿型，相应氧化铁游离度也逐渐降低。
4、各地区土壤铁锰结核中锰氧化物组成和结晶程度不同。红壤地区锰矿物结晶度高、矿物种类较单一，主要为锂硬锰矿；棕壤地区高低结晶度锰矿物相互交杂，矿物种类较丰富，主要有钙锰矿、锂硬锰矿、黑锌锰矿等；黄棕壤和黄褐土地区锰矿物的结晶度和矿物类型介于以上两地之间。由北向南，湿热和酸性环境增强有利于单一锰矿物的形成。
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5、锂硬锰矿普遍存在于由南至北的供试土壤铁锰结核中，其形成和结晶程度受Al含量以及pH值的制约。低pH值更有利于锂硬锰矿的形成；钙锰矿产生于较高pH值、高钙环境下；铅锰矿的形成与土壤中Pb的积累相关。
6、国内首次应用的示差X射线衍射技术（DXRD）能够对供试样品中含量较低，结晶度较弱的铁、锰矿物有较好的鉴定效果。
关键词：铁锰结核氧化铁矿物氧化锰矿物XRD示差分析
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ABSTRACT
Soil Fe-Mn nodules, one of weathering products in soils, have a static and closed environment. Therefore, mineral contents and types, activities of oxides and characteristics of enriched elements within Fe-Mn nodules can provide significant information for the history of soil weathering. In this study, sequence of selective dissolution, chemical analysis, and differential X ray diffraction was used to investigate the differences in the types, contents and crystallinities of phyllosilicates, Fe oxides and Mn oxides between Fe-Mn nodules and their parent soils. The Fe-Mn nodules were collected in the typical zonal soils in China, such as red soil, yellow brown soil, yellow cinnamon soil and brown soil. The results in this study were listed as follows:
1. Compared with soils, Fe-Mn nodules strongly accumulate Mn, significantly accumulate Fe, Ca, Li
and Pb, while K were negatively accumulated in nodules. The distribution of Fe and Li in Fe-Mn nodules is even while the distribution of Mn and Pb is greatly heterogeneous.
2. From south to north, the types of phyllosilicates in soils changed from 1:1 kaolinite to 2:1 mineral. 1.4nm mineral existed in red soil and yellow brown soil, while was not found in the corresponding nodules. Abundant of montmorillorite and 1.4nm mineral was found in yellow cinnamon soil and brown soil while obviously less of those minerals existed in nodules. On the contrary, the contents of hydromica were higher in nodules than in soils. 1.4nm mineral was intergrade mineral in yellow cinnamon soil while vermiculite in the corresponding nodules. These showed that the main types of phyllosilicates in soils before the formation of nodules are hydromica, while 1.4nm mineral and part of montmorillorite formed later which is the product of hydromica by weathering. Nodules became a closed environment gradually and the weathering rate of the minerals in it slowed down after formation.
3. Compared with soils, Feo/Fed is higher in the corresponding Fe-Mn nodules. It shows the Fe-Mn nodule was formed in wet environment. From south to north，the composition of Fe oxide minerals changed from goethite-hematite mixture type to goethite type. Fed/Fet also decreased..
过氧化锰
4. The crystallniity of Mn minerals is high in red soil while low in brown soil. Types of Mn minerals increased from south to north. Lthiophorite was found in red soil and Todorokite, Lthiophorite, Chalcophanite et. al. were found in brown soil. From north to south, wet and acidic environment is suit for the formation of single type Mn mineral.
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5. Lthiophorite distributed widely in Fe-Mn nodules from south to north. Its crystallization is influenced by pH and the content of Al. Todorokite was formed in soils with high pH and high content of Ca. Formation of coronadite requires the accumulation of Pb.
6. The first time application of differential X-ray diffraction (DXRD) is effective to evaluate Fe oxide and Mn oxide with low content and low degree of crystallinity in examinated Fe-Mn nodules
Key words: Fe-Mn nodule Mn oxide mineral Fe oxide mineral DXRD
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1前言
土壤作为人类赖以生存和发展最重要的环境与自然资源，在农业发展、经济建设以及环境保护等方面发挥着重要作用（Sposito，Reginato, 1992）。铁锰结核是土壤重要的新生体，也是土壤中重要的物质组成。它既是动植物锰素的重要来源，又是土壤中的重要吸附载体、化学反应主体和催化剂。在反映土壤成土过程和环境条件的特点，调节土壤中元素化学反应的平衡，控制土壤中某些营养元素和污染元素的有效性和毒性方面起着重要作用（刘良梧和张民1995；谭文峰，1997; 熊毅，1990)。
铁锰结核的形成是地表环境的物质循环和土壤演化过程中的重要环节，对环境系统的演变有着潜在的指示作用。由于铁锰结核特殊的物质组成、具有很强的吸附能力与很高的氧化还原活性，能吸附多种重金属和微量元素，决定这些元素在陆地表层系统中的形态分布及生物有效性。因此，铁锰结核在土壤演化过程中的形成，对土壤中元素的地球化学循环有着特别重要的意义（Webb et. al., 2005; Toner et. al., 2005; Villalobos et. al., 2005; Toner et. al., 2006; Feng, 2004a, 2004b; 谭文峰，2000）。
1.1 铁锰结核的形态与构造特征
土壤铁锰结核的结构通常为：由一个核心及核外一层或数层铁、锰氧化物与粘土的环带构成。铁锰结核构造分为斑杂状、层纹状、同心柱状、似同心柱状和破碎构造；而其结构分为胶状、波浪形环带、同心环带、球粒状、纤维状、蜂窝状、树枝状和生物结核（单连芳，1988）。潘根兴等(1991)用偏光显微镜观察了淮北白浆土中铁锰质物相，并将其分为瘤状物、帽状物、亚包膜、准包膜等微形态类型；
在电镜观察下，光滑的结核内部呈现明显的铁、锰淀积带，而不规则结核颗粒内部，铁、锰分布是随机而散乱的(张民等，1997)。电子探针分析表明，结核内部铁占优势，边缘主要是锰；凝团内的铁、锰分布均匀。说明前者在缓慢氧化条件下形成的，后者在快速氧化条件下形成(Ceseas et. al.，1973)。
铁锰结核的形态与构造受土壤地下水位的强烈影响。排水良好的土壤中形成同心圆微构造结核，排水不良的土壤中则形成扩散状凝团(Phiilippe, 1972)。Zhang(1997)研究美国肯塔基州弱发育半干润淋溶土，发现粘质障碍层(Bt2)和石质界面(BC层)的铁锰结核内部构造不规则、环带不明显、边界模糊，而悬着水区域AB层中的棕和黑铁锰结核呈现紧密、边界清晰的环带状构造；并推测这与土壤大孔隙比率及粘粒含量有关。同心圆微构造是季节性干湿交替的结果，环带特性记录着铁锰结核的氧化还原信息，可用来推断土壤氧化还原的历史(White, 1996; Tan, 2000)。
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