Geochemistry and origin of ferruginous nodules in weathered granodioritic gneisses, Mysore Plateau, Southern India

Fe-nodules occur within saprolites formed from weathering of granodioritic gneisses in the rain-shadow region of the Mysore Plateau adjacent to the Sahyadri Mountains in Southern India. These nodules and their host saprolites were studied for their geochemistry, including chemical speciation, to understand nodule formation and chemical redistribution processes during rock weathering. From their mode of occurrence, and mineralogical and geochemical data, we infer that the nodules originated by a two-stage process in which the initial extensive weathering of gneisses likely facilitated subsequent ferrolysis weathering and nodule formation. Nodules originated by precipitation of goethite, hematite and gibbsite along with several amorphous phases within the matrix of weathered gneisses. This is possible only under hydromorphic conditions, suggesting that parts of the plateau must have gone through a humid phase prior to the present aridity. In the saprolites, Al, Fe, and Ti become enriched because of the removal of Si, Ca, Na, and K. However within the nodule, Fe, Ti, Cr, and Ni are deposited after their chemical transport from the saprolite. Titanium, known for its immobile nature, was also mobilized and concentrated under the conditions of nodule formation. The most important elements in the nodule constitution are Fe, Al, Ti, and Mn, each having both crystalline and amorphous phases. Fe-Ti and Mn oxyhydroxides grain coatings in the saprolites and discrete amorphous Mn and Ti phases in the nodules seem to have scavenged trace elements from the weathering profile. REE were mobilized during weathering and nodule genesis in which Ce and Ti show a strong geochemical coherence. The enrichment of only HREE in saprolite, and both HREE and LREE with significant Ce in the nodule, indicate the control of evolving secondary minerals in the REE redistribution during rock weathering. Strong enrichment of Ce in the weathering profile and in nodules has important implications to the REE chemistry of river waters.     

北戴河红色风化壳地球化学特征及气候环境意义

风化壳地球化学特征具有环境指示意义。用X射线荧光光谱法(XRF)和X射线衍射法(XRD)分别测试了秦皇岛北戴河燕山大学北侧红色风化壳(简称燕大风化壳)主量元素和粘粒粘土矿物。结果表明:除Ca外,Si、Al、Fe、Na、K的含量在风化壳上均有不同程度的波动,其中Si、Na、K波动轨迹基本一致,Al、Fe则与其相反,相关性分析显示SiO2与Al2O3、TFe、Fe2O3,Al2O3与TFe、Fe2O3,Na2O与CaO具有较好相关性;粘土矿物组合为1·4nm过渡矿物(25%~45%)+伊利石(10%~20%)+伊蒙混层矿物(20%~35%)+高岭石(15%~30%),矿物演化系列是长石、黑云母→(蛭石→1·4nm过渡矿物)→(伊利石)→高岭石。与粘土矿物以1∶1型高岭石为主的富铝化南方红色风化壳相比,燕大风化壳Si淋失度,Fe、Al富集度,矿物演化程度都较低,属硅铝化风化壳。燕大风化壳是上新世暖温带到北亚热带过渡型气候的风化产物,与现代秦皇岛暖温带半湿润型气候不同,这反映第四纪以来该区气候干旱因子增多。CIA、S/A等指示的风化强度异常表明,燕大风化壳形成后至少遭受过两次构造抬升,为剥蚀型风化壳,反映该区新构造运动间歇式上升的特点。     

Geochemical and mineralogical characteristics of recent clastic sediments from lower Godavari river: Implications of source rock weathering

The major, trace and rare earth elements geochemistry and clay mineral compositions in the river bed sediments from lower reaches of Godavari river suggest that they are derived from weathering of felsic rocks. Trace and rare earth elemental compositions indicate evidence of sedimentary sorting during transportation and deposition. Lower concentrations of transition elements, such as V, Ni and Cr imply enrichment of felsic minerals in these bed sediments. The REE pattern in lower Godavari sediments is influenced by the degree of source rock weathering. The light rare earth elements (LREE) content are indicating greater fractionation compared to the heavy rare earth elements (HREE). A striking relationship is observed between TiO₂ and gZREE content suggesting a strong control by LREE-enriched titaniferous minerals on REE chemistry. Shale-normalized REE pattern demonstrate a positive Eu anomaly, suggesting weathering of feldspar and their secondary products, which are enriched in Eu. Chondrite-normalised REE pattern is characteristic of felsic volcanic, granites and gnessic source rocks. Trace elemental compositions in sediments located near urban areas suggest influence of anthropogenic activity. Chemical Index of Alteration (CIA) is high (avg. 65.76), suggesting a moderate chemical weathering environment. X-ray diffraction analysis of clay fraction shows predominance of clay minerals that are formed because of the chemical weathering of felsic rocks.     

Rare earth distributions in clay minerals and in the clay-sized fraction of the Lower Permian Havensville and Eskridge shales of Kansas and Oklahoma

The REE (rare earth element) content of a wide variety of clay mineral groups have been analyzed using radiochemical neutron activation and have been found to be quite variable in absolute REE content (range of ∑REE = 5.4–1732) and less variable in relative REE content (range of chondritenormalized La/Lu = 0.9–16.5). The variable REE content of the clay mineral groups is probably determined by the REE content of the source rock from which the clay mineral was derived and not from the separate minerals in the rock.The clay-sized fractions of the Havensville and Eskridge shales of Kansas and Oklahoma have similar relative REE distributions and identical negative Eu anomaly size as the composite of NAS (N. American shales), but an absolute REE content (range of ∑REE = 46–348) that may differ significantly from the composite of NAS. The clay-sized fraction of samples from any given outcrop did not vary much in absolute or relative REE content, but samples from northern Oklahoma, probably composed of continental to near-shore marine sediments, have higher absolute REE contents and higher La/Lu ratios than samples of marine deposits in Kansas (e.g. mean ∑REE in Oklahoma = 248; mean ∑REE in Kansas = 69–116). The differencess in the REE content between samples in Oklahoma and Kansas may be caused by chemical weathering processes in the source area, exchange reactions in the environment of deposition, or diagenesis and do not appear to be a result of the different clay minerals.Most samples have Eu anomalies relative to chondrites (range of Eu/Sm ratios of samples = 0.035–1.17; chondrites = 0.35). Some montmorillonites and kaolinites are anomalous in Eu relative to the NAS (range of Eu/Sm ratios of samples = 0.056–0.21; NAS = 0.22). These anomalies may be inherited from source rocks with Eu anomalies originally produced by igneous processes, or they may be produced by chemical weathering processes in the source area.     

川西冕宁-德昌REE矿带风化型矿床的矿石类型及成因

川西冕宁-德昌REE矿带中的风化型矿石研究相对缺乏.根据近几年的野外地质调查,采用电子探针、X光粉晶衍射、全岩分析等对大陆槽、牦牛坪和木落寨矿床中疑似风化矿的样品进行类型和成因分析.结果表明:大陆槽风化型矿石中大量粘土矿物（达40%）和矿石矿物（达60%）发育,风化程度高;相反,牦牛坪和木落寨疑似风化型矿石中几乎没有粘土矿物,风化程度低.通过对牦牛坪和木落寨疑似风化型矿石进行地球化学特征、矿物组成分析并与围岩、矿脉的组成和产状对比,表明疑似风化型矿石主要是由矿脉和围岩在强烈的构造作用下发生机械风化形成.频繁的角砾构造使大陆槽矿石发生强烈的机械风化作用,并加速了矿石发生一定的水岩反应,促进矿石发生中等强度的化学风化作用.目前整个矿带中未发现明显的生物风化作用.整个稀土矿带的风化型矿石比南岭离子吸附型稀土矿石的风化程度低,形成时间较晚（11 Ma）,氟碳铈矿大量保存,还没有形成离子吸附型矿床.      

Geological and geochemical characteristics in the paleo-weathering crust sedimentary type REE deposits,western Guizhou,China

A supergene REE deposit closely interrelated with the weathering of the Emeishan basalt formation was produced in the Xuanwei formation, the overlying stratum of the late Permian Emeishan basalt formation in West Guizhou, China. The host strata consist primarily of offwhite kaolinite clay rock and/or grayish black carbonaceous shale. Mineralogical analyses reveal that kaolinites are the major minerals in REE ores with small amounts of smectite, illite, boehmite, hornblende, pyrophyllite, calcite, dolomite and/or iron-bearing minerals, with a certain proportion of feldspar, quartz crystal debris and noncrystal debris. Geochemical analyses reveal high enrichment of trace elements like Cu, Nb, Ta, Zr and Hf. The host strata feature considerable lithological variability, close interrelation of the REE grade with the lithology and uneven spatial distribution of the REE ores, which are mostly found in Lufang, Maojiaping and Zhangsigou profiles of Weining County and can be as thick as 20 m. Of the five stratigraphic profiles, 48% have their whole-rock ∑REE higher than 1000 ppm. The REE in this framework consists primarily of ion adsorbed phases and REE-rich residual independent mineral phases. Comprehensive analyses suggest that the source may not only include the Emeishan basalt, but the intermediate acid volcanic rocks evolved from the Emeishan basalt in the later periods; the hydrothermal alteration subsequently imposed on the host strata might have boosted the mineralization of the rare earth. The preliminary genetic model should have been: the denudation product from the weathering of the parent rock was migrated to the sea-continental margin at the continent side carrying huge quantities of REE with it and was preserved by the quick marine transgression. The host strata consist primarily of kaolinite clay rock and/or carbonaceous shale, which are so far believed to be a sedimentary type REE deposit closely interrelated with weathering effect.     

离子吸附型稀土矿床形成的矿物表/界面反应机制

华南离子吸附型稀土矿床提供了全球超过90%的重稀土,是我国优势的战略性关键金属矿产资源。掌握这类矿床的成矿机制和禀赋特征,可为增加稀土资源储量和高效利用稀土资源提供理论支撑。离子吸附型稀土矿床主要发育在富稀土花岗岩、浅变质岩及火山岩的风化壳中。基岩中的(含)稀土矿物是风化壳中离子态稀土的主要来源,其矿物组合很大程度上决定了稀土矿床的禀赋和分异特征。在物理-化学风化和微生物作用下,造岩矿物、含稀土矿物和稀土独立矿物逐渐溶解,使稀土元素活化和再富集。一方面,母岩风化形成的黏土矿物和铁锰氧化物具有较大的比表面积和一定的表面电荷密度,是稀土离子的主要载体;另一方面,稀土离子通过离子交换、表面吸附与络合、共沉淀,以及形成次生稀土矿物等途径富集在次生矿物表面,其富集-分异特征和赋存状态受矿物类型、pH、微生物活动等因素所控制。利用高分辨透射电镜结合选区电子衍射和电子能量损失谱,以及同步辐射X射线吸收精细结构谱,有望在原子级尺度查明稀土的微观赋存状态。未来研究需更多关注基岩中(含)稀土矿物组合及其演化路径的制约因素、微生物风化对离子吸附型稀土矿床成矿作用的约束,以及稀土元素的微观赋存状态等问题。     

发育完整的灰岩风化壳及其矿物学和地球化学特征

对于碳酸盐岩土覆土壤成因、尽管碳酸盐岩风化残积成土说被多数学者认同,但由于碳酸盐岩中酸不溶物含量极低,在风化成土过程中会伴随着巨大的体积缩小变化,原岩结构和半风化带无法保留,从而缺失了探索上覆土壤物质来源的重要中间环节,使得这种观点缺乏野外宏观证据的支持。最近,我们在贵州、湖南等地发现了数个以泥质灰岩和泥质白云岩为基岩的碳酸盐岩风化壳剖面,尚保留有较好的原岩结构,具有明显的风化壳分带和过渡现象。这些风化壳剖面的发现为深入研究碳酸盐岩风化成土过程提供了良好的研究场所。本文选取了较为典型的吉首泥灰岩风化壳剖面,从矿物学地球化学的角度来探讨碳酸盐岩风化壳的形成过程和发育特征,结果表明该风化壳既遵循非碳酸盐岩（主要是结晶岩类）风化壳的发育特征,也具有自己独特的地球化学演化规律。风化壳总体特点受碳酸盐中的酸不溶物矿物组合及化学成分的影响甚至控制,风化非碳酸盐风壳相似的发育特征。吉首泥灰岩风化壳剖面的发育特征和作者早先提出 的碳酸盐岩风化成土的两阶段模式是一致的,即以碳酸盐矿物大量淋失、酸不溶物逐渐堆积或残积为特征的早期阶段和残积物进一步风化成土的阶段,后一阶段的演化类似非碳酸盐岩类的风化过程。     

江西龙南花岗岩稀土风化壳中粘土矿物的研究

本区燕山早期花岗岩发育的风化壳中的粘土矿物以高岭石和埃洛石(7Å)为主;蒙脱石、三水铝石及其它为新查明矿物。据粘土矿物组合特征,自风化剖面深部到地表分为三个带:含蒙脱石带,高岭石和埃洛石(7Å)带,含三水铝石带。本文探讨了矿物在风化过程中的生成演化顺序,并进行了热力学的解释。据各带粘土物质的阳离子交换量与稀土含量变化的不一致关系认为,稀土在C带中的富集是化学风化的结果,与粘土矿物组合无关。     

Climatic control on clay mineral formation: Evidence from weathering profiles developed on either side of the Western Ghats

Many physico-chemical variables like rock-type, climate, topography and exposure age affect weathering environments. In the present study, an attempt is made to understand how the nature of clay minerals formed due to weathering differs in tropical regions receiving high and low rainfall. Clay mineralogy of weathering profiles in west coast of India, which receives about 3 m rainfall through two monsoons and those from the inland rain-shadow zones (<200 cm rainfall) are studied using X-ray diffraction technique. In the west coast, 1:1 clays (kaolinite) and Fe—Al oxides (gibbsite/goethite) are dominant clay minerals in the weathering profiles while 2:1 clay minerals are absent or found only in trace amounts. Weathering profiles in the rain shadow region have more complex clay mineralogy and are dominated by 2:1 clays and kaolinite. Fe—Al oxides are either less or absent in clay fraction. The kaolinite—smectite interstratified mineral in Banasandra profiles are formed due to transformation of smectites to kaolinite, which is indicative of a humid paleoclimate. In tropical regions receiving high rainfall the clay mineral assemblage remains the same irrespective of the parent rock type. Rainfall and availability of water apart from temperature, are the most important factors that determine kinetics of chemical weathering. Mineral alteration reactions proceed through different pathways in water rich and water poor environments.     

Chemical weathering and related controls on surface water chemistry in the Absaroka Mountains,Wyoming

Chemical relationships among surface waters, soils and rocks were investigated in the drainage basin of the North Fork of the Shoshone River in northwestern Wyoming. The area is underlain entirely by andesitic volcanic rocks. Smectite is the only clay mineral forming in soils over much of the area, although minor kaolinite occurs in a few areas of higher-than-average rainfall.Mass-balance calculations relating stream water chemistry to rock alteration indicate that controls on the chemistry of surface waters take place not in the soil zone but in the altered rock zone. The dominant weathering process which controls the water chemistry is slight alteration of large volumes of rock, rather than development of chemical equilibria involving secondary phases in the soil zone. The altered rock is enriched in feldspars and depleted in ferromagnesian minerals compared to fresh rock. The high rate of physical erosion of the area is enough to remove the residue, reexpose the bedrock, and continue the weathering process.     

粘土矿物对形成过渡带气的催化作用研究

本文分析了有机质在粘土矿物中的赋存状态，测定了粘土矿物的表面酸、实验模拟了低演化程度的烃源岩及其抽提物干酪根＋不同粘土矿物的催化机制、并通过粘土催化醇脱水反应对粘土过渡带有机质成气机理进行了探讨。研究结果表明，过渡带气的形成主要是受蒙脱石粘土矿物的催化所致，其原因是成岩过程中蒙脱石向混层矿物转化在蒙脱石晶间发生大量的铝代硅，因此在粘土表面产生电荷不平衡而形成酸性，井以路易斯酸和布郎酸作用于有机质，使碳－碳键发生断裂以形成气态烃。     

皖南姚村岩体花岗岩风化壳稀土元素赋存特征

皖南地区花岗岩风化壳中稀土元素普遍富集，局部已成为矿床，其中，郎溪县姚村岩体风化壳富集程度较高。LA- ICP- MS锆石U- Pb定年表明，姚村花岗岩体的形成年龄为127. 9±1. 4 Ma，属于皖南地区燕山期晚期岩浆作用的产物。风化壳可细分为残坡积层（A）、强半风化层（C1）、过渡层（C2）、弱半风化层（C3）和基岩（D） 5层。稀土总量在纵向剖面上呈“波浪式”分布，各层稀土分布型式表现出对原岩的继承性。风化壳稀土配分型式与基岩一致， 富集LREE，轻重稀土分馏明显\[(La/Yb)N=15. 6\]，但总含量明显更高。基岩∑REE为338×10-6，半风化层∑REE最高达642×10-6，富集约两倍。风化壳物质由风化残余主矿物（石英、钾长石、斜长石、黑云母）、黏土矿物（高岭石、埃洛石、伊利石、三水铝石等）和副矿物（锆石、磷灰石、榍石等）等组成。黏土矿物以伊利石含量最高，指示风化壳发育不成熟。REE与埃洛石含量明显正相关，与其他黏土矿物关系不明显。（含）稀土矿物（尤其是榍石）对风化壳中稀土元素的贡献量超过 50%，其次为斜长石，是风化壳中REE的重要来源。     

豫西偃师龙门地区上石炭统本溪组含铝岩系矿物学特征及其原岩分析

以华北陆块南部豫西偃师县龙门镇地区全取芯铝土矿钻孔ZK4704为主要研究对象,通过岩芯观察和垂向上连续取样,运用扫描电镜、能谱分析、X射线衍射分析、差热分析、红外光谱分析等手段对本溪组含铝岩系的矿物学特征及其垂向变化规律进行了分析。研究表明,偃龙地区本溪组含铝岩系的矿物均为自生矿物,除后期重结晶作用的影响外,主要以隐晶质或微晶存在,矿物成分在垂向上具有明显的变化规律:下部和上部泥岩以粘土矿物为主,但下部泥岩以伊利石为主,上部泥岩以高岭石为主。中部铝矿物含量较高,粘土矿物含量较少。在上述分析基础上,以化学风化作用的基本原理和产生的条件为联结本溪组含铝岩系矿物学特征和原岩的纽带,分析了铝土矿的原岩。认为,可以迅速水解的火山灰是最有可能的含铝岩系的原岩,而由弱碱性的海相环境逐渐转变为弱酸性沼泽环境的海退序列可以很好地解释水解过程中化学环境的变化,含铝岩系基底强烈的古岩溶作用,可以为硬水铝石的产生提供良好的泄水条件。偃龙地区本溪组铝土矿的火山灰物源可由华北陆块北缘和我国西部地区同时期强烈活动的火山作用提供。     

皖南姚村岩体花岗岩风化壳稀土元素赋存特征

皖南地区花岗岩风化壳中稀土元素普遍富集，局部已成为矿床，其中，郎溪县姚村岩体风化壳富集程度较高。LA- ICP- MS锆石U- Pb定年表明，姚村花岗岩体的形成年龄为127.9±1.4 Ma，属于皖南地区燕山期晚期岩浆作用的产物。风化壳可细分为残坡积层（A）、强半风化层（C1）、过渡层（C2）、弱风化层（C3）和基岩（D）五层。稀土总量在纵向剖面上呈“波浪式”分布，各层稀土分布型式表现出对原岩的继承性。风化壳稀土配分型式与基岩一致， 富集LREE，轻重稀土分馏明显(La/Yb)N=15.6），但总含量明显更高。基岩∑REE为338×10-6，半风化层∑REE最高达642×10-6，富集约两倍。风化壳物质由风化残余主矿物（石英、钾长石、斜长石、黑云母）、黏土矿物（高岭石、埃洛石、伊利石、三水铝石等）和副矿物（锆石、磷灰石、榍石等）等组成。黏土矿物以伊利石含量最高，指示风化壳发育不成熟。REE与埃洛石含量明显正相关，与其他黏土矿物关系不明显。（含）稀土矿物（尤其是榍石）对风化壳中稀土元素的贡献量超过百分之五十，其次为斜长石，是风化壳中REE的重要来源。     

山东蓬莱史家沟西山火山碎屑岩风化壳元素地球化学特征

以山东蓬莱城西史家沟的西山火山碎屑岩风化壳为研究对象,系统采集样品,并测试了微量元素、主量元素、稀土元素以及黏土矿物等指标,揭示了西山火山碎屑岩风化壳的元素地球化学特征,讨论了风化壳的形成年代及其气候条件。史家沟火山碎屑岩风化壳源自基性-超基性玻屑凝灰岩母岩,CIA平均值为92.3,总体上处于为强度风化水平,K、Na、Ca和Mg等元素淋失殆尽,出现了脱硅富铝铁现象,为轻稀土富集型风化壳。西山火山碎屑岩风化壳可分为两个喷发旋回,大致以200 cm为界限,在此以下第一旋回,以上为第二旋回。第一旋回的CIA介于94.0~97.2之间,V、Rb、Sb、Mo、Li和Ga等微量元素出现淋失,在213 cm处并出现了?REE、?LREE和?HREE的峰值、Ce的负异常,蒙脱石化彻底,风化壳呈现灰白色。第二旋回的CIA在86.3~89.2之间,风化程度较第一旋回稍低,微量元素和稀土元素垂直变化较小,蒙脱石化并不完全彻底,仍残留长石、赤铁矿和石英等原生矿物,因而仍然呈现红色或淡红色。西山火山碎屑岩风化壳形成于中新世末到上新世,为温暖湿润的亚热带环境;中新世后的气候趋冷造成了两个旋回风化程度和元素地球化学特征的差异。     

The weathering of norite at Black Hill,South Australia

Black Hill is a boulder‐strewn residual of norite standing 45 m above the Murray Plains about 80 km northeast of Adelaide. Between the boulders, the crystalline rock has weathered to a dark‐brown terra rossa‐rendzina soil with calcite, illite, kaolin, and hematite as the principal secondary minerals. At one site on the smooth lower slopes of the outcrop, the material above the norite consists of partially weathered granular fragments with considerable dolomite and some calcite in nodular form. Below the surface, the rock has been weathered along joints to produce the clay mineral nontronite, and between each corestone and this plastic clay there is a zone of laminated but essentially unaltered rock 10–25 cm thick. Pieces of amorphous silica occur sporadically in some joints and on the surface.

The dark‐brown soil appears to be related to the present environment but the nontronite in the joint weathering, the dolomite, and the amorphous silica are all consistent with the norite having been inundated, possibly during the Miocene marine transgression.

Calculations based on the retention of elements such as aluminium, potassium, titanium, and iron have been used to predict quantitatively the amount of certain minerals such as quartz, calcite, and dolomite introduced to the weathering profiles.     

Chemical weathering in Luzon, Philippines from clay mineralogy and major-element geochemistry of river sediments

Clay mineralogy and major-element geochemistry of 35 surface sediment samples collected in 21 major to moderate rivers of Luzon, Philippines are used to evaluate the present chemical weathering process. The clay mineral assemblage consists mainly of smectite (average 86%) with minor kaolinite (9%) and chlorite (5%) and very scarce illite (1%), and does not show strong island-wide differences. The major element results of both bulk and clay-fraction sediments indicate that the formation of clay minerals is accompanied by leaching of Ca and Na first and of Fe and Mn thereafter during the chemical weathering process. A low-moderate chemical weathering degree of bulk sediments and a moderate-intensive degree of clay-fraction sediments are obtained in Luzon rivers based on proxies of chemical index of alteration (CIA) and smectite crystallinity. It is suggested that the majority of andesitic–basaltic volcanic and sedimentary rocks along with the tectonically active geological setting and sub-tropical East Asian monsoon climate are responsible for the predominance of smectite in the clay mineral assemblage.     

REE Mineralization of Weathered Crust and Clay Sediment on Granitic Rocks in the Sanyo Belt,SW Japan and the Southern Jiangxi Province,China

Rare earth element (REE) geochemistry and mineralogy have been studied in the weathered crusts derived from the Early Yanshanian (Jurassic) biotite granites of Dabu and Dingnan, as well as in the Indosinian (Permian) muscovite–biotite granite of Aigao in southern Jiangxi province, China, and the weathered crusts and clay sediments on biotite granites in the Sanyo belt, SW Japan, that is, Okayama, Tanakami, and Naegi areas. In all of the weathered crusts, biotite and plagioclase commonly tend to decrease toward the upper part of the profile, whereas kaolinite and residual quartz and K‐feldspar increase. The weathered crusts of the Dingnan granites and some Naegi granites, which are characterized by the enrichment in light REE (LREE) in C horizons, have higher total REE (ΣREE) content than the parent REE‐enriched granites. Weathering of LREE‐bearing apatite and fluorocarbonates in the Dingnan granites and allanite and apatite in some Naegi granites may account for the leaching of LREE at the B horizons. The leached LREE must result in subsequent enrichment of LREE in the C horizons. The enrichment is probably associated with mainly adsorption onto kaolinite and partly formation of possible secondary LREE‐bearing minerals. In Japan it was found that REE mineralization occurs not in the weathered granitic crusts but in reworked clay sediments, especially kaolinite‐rich layers, derived mainly from the weathering materials of REE‐enriched granitic rocks. The clay sediments are more enriched in LREE, which likely adsorbed onto kaolinite. Concentration of heavy REE within almost all the weathered crusts and clay sediments, however, may reflect mainly residual REE‐bearing minerals such as zircon, which originated in the parent granitic rocks. The findings of the present study support the three processes for fractionation of the REE during weathering: (i) selective leaching of rocks containing both stable and unstable REE‐bearing minerals; (ii) adsorption onto clay minerals; and (iii) presence of possible secondary LREE‐bearing minerals.     

广西桂林石灰土元素迁移特征及影响因素 ——以会仙峰丛谷地石灰土为例

碳酸盐岩的化学风化是岩溶关键带各圈层相互作用的主要形式,风化壳中蕴含重要气候环境和物质循环信息.通过对广西桂林会仙峰丛谷地石灰土的化学风化强度及元素迁移特征的研究,并与滇黔湘和青藏高原的岩溶风化壳的对比分析,结果表明:(1)会仙石灰土化学蚀变指数(CIA)均值为92.14,与贵州兴义岩溶风化壳相当,反映炎热潮湿气候下的...