Quantification of mineral assemblages in near-surface Earth materials is a challenge because of the often abundant and highly variable crystalline and chemical nature of discrete clay minerals. Further adding to this challenge is the occurrence of mixed-layer clay minerals, which is complicated because of the numerous possible combinations of clay layer types, as defined by their relative proportions and the ordering schemes. The problem of ensuring accurate quantification is important to understanding landscape evolution because mineral abundances have a large influence on ecosystem function. X-ray diffraction analysis of the variable cation-saturated clay fraction in soil and regolith from the Calhoun Critical Zone observatory near Clinton, South Carolina, USA, was coupled with modeling using NEWMOD2 to show that mixed-layer clays are often dominant components in the mineral assemblages. Deep samples in the profile (>6.5 m) contain mixed-layer kaolinite/smectite, kaolinite/illite-like, kaolinite-vermiculite, illite-like/biotite, and illite-like/vermiculite species (with ‘illite-like’ defined herein as Fe-oxidized 2:1 layer structure with a negative layer charge of ~0.75 per unit formula, i.e. weathered biotite). The 2:1 layers in the mixed-layer structures are proposed to serve as exchange sites for K+, which is known to cycle seasonally between plant biomass and subsurface weathering horizons. Forested landscapes have a greater number of 2:1 layer types than cultivated landscapes. Of two nearby cultivated sites, the one higher in landscape position has fewer 2:1 layer types. Bulk potassium concentrations for the forested and two cultivated sites show the greatest abundances in the surface forested site and lowest abundance in the surface upland cultivated site. These observations suggest that landscape use and landscape position are factors controlling the mixed-layer mineral assemblages in Kanhapludults typical of the S.E. United States Piedmont. These mixed-layer clays are key components of the proposed mechanism for K+ uplift concepts, whereby subsurface cation storage may occur in the interlayer sites (with increased negative 2:1 layer charge) during wetter reduced conditions of the winter season and as biomass decay releases cation nutrients. Cation release from the mixed-layer clays (by decreased 2:1 layer charge) occurs under drier oxidized conditions during the growing seasons as biota utilize cation nutrients. The types and abundances of mixed layers also reflect long-term geologic factors including dissolution/alteration of primary feldspar and biotite and the subsequent transformation and dissolution/precipitation reactions that operate within the soil horizons. Thus, the resulting mixed-layer clay mineral assemblages are often complex and heterogeneous at every depth within a profile and across landscapes. X-ray diffraction (XRD) assessment, using multiple cation saturation state and modeling, is essential for quantifying the clay mineral assemblage and pools for cation nutrients, such as potassium, in the critical zone.

中文翻译：

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使用 NEWMOD2 对多饱和状态下的混合层粘土进行量化：对美国东南部钾上升假说的影响

近地表地球材料中矿物组合的量化是一项挑战，因为离散粘土矿物的结晶和化学性质通常丰富且高度可变。进一步增加了这一挑战的是混合层粘土矿物的出现，由于粘土层类型的多种可能组合，如它们的相对比例和排序方案所定义，混合层粘土矿物的出现很复杂。确保准确量化的问题对于理解景观演变很重要，因为矿物丰度对生态系统功能有很大影响。来自美国南卡罗来纳州克林顿附近的卡尔霍恩临界区天文台的土壤和风化层中可变阳离子饱和粘土部分的 X 射线衍射分析，结合使用 NEWMOD2 进行建模，以表明混合层粘土通常是矿物组合中的主要成分。剖面中的深层样品 (>6.5 m) 包含混合层高岭石/蒙脱石、高岭石/类伊利石、高岭石-蛭石、类伊利石/黑云母和类伊利石/蛭石物种（此处定义为“类伊利石”作为铁氧化的 2:1 层结构，每单位配方具有 ~0.75 的负层电荷，即风化黑云母）。混合层结构中的 2:1 层被提议用作 K+ 的交换位点，已知 K+ 在植物生物量和地下风化层之间季节性循环。森林景观比栽培景观具有更多数量的 2:1 层类型。在附近的两个耕地中，景观位置较高的一个2：1层类型较少。林地和两个耕地的体钾浓度显示，地表林地丰度最高，地表高地耕地丰度最低。这些观察结果表明，景观利用和景观位置是控制美国东南山麓典型的 Kanhapludults 混合层矿物组合的因素。这些混合层粘土是 K+ 抬升概念的拟议机制的关键组成部分，因此在冬季潮湿减少的条件下以及随着生物质衰变释放，地下阳离子存储可能发生在层间位置（具有增加的 2:1 层负电荷）阳离子养分。混合层粘土中的阳离子释放（减少 2：1 层电荷）发生在生长季节的干燥氧化条件下，因为生物群利用阳离子养分。混合层的类型和丰度也反映了长期的地质因素，包括原生长石和黑云母的溶解/蚀变，以及随后在土壤层内发生的转化和溶解/沉淀反应。因此，由此产生的混合层粘土矿物组合通常在剖面内和跨景观的每个深度都是复杂和异质的。X 射线衍射 (XRD) 评估使用多阳离子饱和状态和建模，对于量化粘土矿物组合和关键区域中阳离子营养物（例如钾）的池至关重要。混合层的类型和丰度也反映了长期的地质因素，包括原生长石和黑云母的溶解/蚀变，以及随后在土壤层内发生的转化和溶解/沉淀反应。因此，由此产生的混合层粘土矿物组合通常在剖面内和跨景观的每个深度都是复杂和异质的。X 射线衍射 (XRD) 评估使用多阳离子饱和状态和建模，对于量化粘土矿物组合和关键区域中阳离子营养物（如钾）的池至关重要。混合层的类型和丰度也反映了长期的地质因素，包括原生长石和黑云母的溶解/蚀变，以及随后在土壤层内发生的转化和溶解/沉淀反应。因此，由此产生的混合层粘土矿物组合通常在剖面内和跨景观的每个深度都是复杂和异质的。X 射线衍射 (XRD) 评估使用多阳离子饱和状态和建模，对于量化粘土矿物组合和关键区域中阳离子营养物（如钾）的池至关重要。由此产生的混合层粘土矿物组合在剖面内和跨景观的每个深度通常是复杂和异质的。X 射线衍射 (XRD) 评估使用多阳离子饱和状态和建模，对于量化粘土矿物组合和关键区域中阳离子营养物（如钾）的池至关重要。由此产生的混合层粘土矿物组合在剖面内和跨景观的每个深度通常是复杂和异质的。X 射线衍射 (XRD) 评估使用多阳离子饱和状态和建模，对于量化粘土矿物组合和关键区域中阳离子营养物（例如钾）的池至关重要。