首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 25 毫秒
1.
Bangladesh is situated in a subtropical to tropical climatic zone. A recently weathered crust has developed on sedimentary bedrock (sandstone, siltstone, shale and claystones) of Tertiary–Quaternary age. Weathered samples were collected from 16 sections totaling 68 samples and were analyzed mineralogically. The main primary minerals identified in the weathered crust of sedimentary rocks are quartz, plagioclase, K-feldspar, biotite, muscovite, sparse carbonate and epidote. The secondary minerals are kaolinite, illite, chlorite, gibbsite and goethite. Weathering initiated along the grain boundaries and cleavage planes of the minerals, forming small cloudy materials which were very difficult to identify. In the advanced stage of weathering, these cloudy materials have turned into secondary minerals. In region 1, high rain fall (7100 mm/yr) and monsoonic climate resulted in a kaolinite–gibbsite–goethite suite through the weathering of feldspars and biotite. The occurrence of gibbsite in the relatively elevated lands of Sylhet and Fe-kaolinite throughout the study areas is indicative of a humid–tropical climate during formation of the weathered crust.  相似文献   

2.
高玲  闫峻  李全忠  谢建成 《地质论评》2022,68(3):2022062013-2022062013
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。  相似文献   

3.
高玲  闫峻  李全忠  谢建成 《地质论评》2022,68(5):1820-1838
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。  相似文献   

4.
In the mountainous Rio Icacos watershed in northeastern Puerto Rico, quartz diorite bedrock weathers spheroidally, producing a 0.2-2 m thick zone of partially weathered rock layers (∼2.5 cm thickness each) called rindlets, which form concentric layers around corestones. Spheroidal fracturing has been modeled to occur when a weathering reaction with a positive ΔV of reaction builds up elastic strain energy. The rates of spheroidal fracturing and saprolite formation are therefore controlled by the rate of the weathering reaction.Chemical, petrographic, and spectroscopic evidence demonstrates that biotite oxidation is the most likely fracture-inducing reaction. This reaction occurs with an expansion in d (0 0 1) from 10.0 to 10.5 Å, forming “altered biotite”. Progressive biotite oxidation across the rindlet zone was inferred from thin sections and gradients in K and Fe(II). Using the gradient in Fe(II) and constraints based on cosmogenic age dates, we calculated a biotite oxidation reaction rate of 8.2 × 10−14 mol biotite m−2 s−1. Biotite oxidation was documented within the bedrock corestone by synchrotron X-ray microprobe fluorescence imaging and XANES. X-ray microprobe images of Fe(II) and Fe(III) at 2 μm resolution revealed that oxidized zones within individual biotite crystals are the first evidence of alteration of the otherwise unaltered corestone.Fluids entering along fractures lead to the dissolution of plagioclase within the rindlet zone. Within 7 cm surrounding the rindlet-saprolite interface, hornblende dissolves to completion at a rate of 6.3 × 10−13 mol hornblende m−2 s−1: the fastest reported rate of hornblende weathering in the field. This rate is consistent with laboratory-derived hornblende dissolution rates. By revealing the coupling of these mineral weathering reactions to fracturing and porosity formation we are able to describe the process by which the quartz diorite bedrock disaggregates and forms saprolite. In the corestone, biotite oxidation induces spheroidal fracturing, facilitating the influx of fluids that react with other minerals, dissolving plagioclase and chlorite, creating additional porosity, and eventually dissolving hornblende and precipitating secondary minerals. The thickness of the resultant saprolite is maintained at steady state by a positive feedback between the denudation rate and the weathering advance rate driven by the concentration of pore water O2 at the bedrock-saprolite interface.  相似文献   

5.
通过对我国南方许多风化型高岭土矿床研究查明,在表生条件下形成高岭石-多水高岭石矿物的基本因素是决定于风化母岩的岩性和水介质的物化性质。水介质的物化性质又明显地受到风化母岩的岩性类型和结构构造、动力裂隙的发育程度、围岩的稳定性和透水性、气候、地形地貌和植被等因素的综合性制约。  相似文献   

6.
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.  相似文献   

7.
Understanding the relationship between subsurface flow paths on hillslopes and chemical weathering of bedrock is fundamental to understanding the timing and mechanisms that weather bedrock to saprolite. The link between chemical weathering of bedrock and contact time with reactive water along flow paths motivates this study. Water drives the chemical alteration of rock into saprolite, yet connected porosity generally declines with depth into the weathered profile. Saprolite formation, therefore, reflects coupled weathering and permeability development over time. This study uses numerical modeling and soil-moisture monitoring to explore the hydrology of the unsaturated zone and the influence of fracture density, hillslope gradient, and permeability contrasts within the saprolite development horizon on saprolite development.  相似文献   

8.
Garnet-biotite (-cordierite) phase relations in high-grade gneisses of the south coast of Western Australia reflect at least two metamorphic episodes. Chemical uniformity of the interiors of garnet and cordierite grains suggest thorough equilibration during a major phase of metamorphism. Narrow Mg-depleted rims on garnet grain boundaries in contact with biotite or cordierite, and complementary Mg-enriched rims on contiguous cordierites are the result of subsequent retrograde re-equilibration. The absence of reaction zoning in biotites suggests more complete retrograde modification of this mineral.Comparison between granulite and amphibolite facies garnet-biotite pairs shows that Mn contents of both minerals are higher, and Ti contents of the biotites are lower, in the lower-grade rocks. These differences, although not entirely unrelated to grade, are more directly controlled by variations in host rock chemistry and modal amounts of garnet and biotite.Partitioning of Mg, Fe2+ and Mn between garnet and biotite is fairly uniform, with no clear differences between granulite and amphibolite facies pairs. Application of the Mg-Fe2+ distributions to the geothermometers devised by Perchuk, Thompson, and Goldman & Albee yields variable T estimates of 600–680°C, 580–780°C, and 475–715°C respectively, for the main metamorphism. These estimates are low compared with the T indicated for the granulite facies rocks by other evidence (i.e. > 750°C at 5 kb PT). The Mg-Fe2+ distributions between contiguous garnet-biotite rims suggest that retrograde re-equilibration occurred at least 20–140°C below the T of the main metamorphism.  相似文献   

9.
Germanium-silicon (Ge/Si) ratios were determined on quartz diorite bedrock, saprolite, soil, primary and secondary minerals, phytolith, soil and saprolite pore waters, and spring water and stream waters in an effort to understand Ge/Si fractionation during weathering of quartz diorite in the Rio Icacos watershed, Puerto Rico. The Ge/Si ratio of the bedrock is 2 μmol/mol, with individual primary mineral phases ranging between 0.5 and 7 μmol/mol. The ratios in the bulk saprolite are higher (∼3 μmol/mol) than values measured in the bedrock. The major saprolite secondary mineral, kaolinite, has Ge/Si ratios ranging between 4.8 and 6.1 μmol/mol. The high Ge/Si ratios in the saprolite are consistent with preferential incorporation of Ge during the precipitation of kaolinite. Bulk shallow soils have lower ratios (1.1-1.6 μmol/mol) primarily due to the residual accumulation of Ge-poor quartz.Ge/Si ratios measured on saprolite and soil pore waters reflect reactions that take place during mineral transformations at discrete depths. Spring water and baseflow stream waters have the lowest Ge/Si ratios (0.27-0.47 μmol/mol), reflecting deep initial weathering reactions resulting in the precipitation of Ge-enriched kaolinite at the saprolite-bedrock interface. Mass-balance calculations on saprolite require significant loss of Si and Al even within 1 m above the saprolite-bedrock interface. Higher pore water Ge/Si ratios (∼1.2 μmol/mol) are consistent with partial dissolution of this Ge-enriched kaolinite. Pore water Ge/Si ratios increase up through the saprolite and into the overlying soil, but never reach the high values predicted by mass balance, perhaps reflecting the influence of phytolith recycling in the shallow soil.  相似文献   

10.
综合应用Ｘ射线衍射、扫描电镜、透射电镜及偏光显微镜等技术手段研究了我国热带、亚热带部分地区花岗岩和片麻岩中黑云母的风化现象。结果表明，化学风化产物主要有黑云母／蛭石不规则间层矿物、水黑云母、蛭石、０．７ｎｍ埃洛石及铁的氢氧化物（氧化物）等。影响黑云母风化及次生产物形成的主要因素包括黑云母的晶体结构和化学成分、气候、风化壳母岩的矿物成分等。  相似文献   

11.
In the Middle Atlas of Morocco, alkali basaltic flows record successive weathering phases during the Quaternary. In fresh basalt interior and intermediate external zones, the first weathering stage is characterised by glass dissolution and the formation of a Si-Al poorly-crystallised product. Advanced weathering phases are characterised by 10 Å halloysite, kaolinite and goethite, located within the primary minerals or as secondary products in fissures. Olivine and iddingsite are transformed into Si-rich goethite, plagioclase into halloysite and pyroxene into a mixture of halloysite + geothite. Dissolution of Ti-magnetite and ilmenite yielded Ti-rich products. In these conditions, the weathering of basalts and development of a soil matrix are accompanied by the elimination of certain chemical elements, such as Si, Ca, Na and K, and the concentration of Fe and Al. In the soil, clay minerals such as illite and vermiculite, do not have any genetic relationship with weathered basalt and were probably introduced externally.  相似文献   

12.
Sangrumba landslide is one of the largest and the most active landslides in Nepal Himalaya. Geologically the landslide belongs to the Higher Himalaya and consists of Pre-Cambrian biotite–garnet and sillimanite gneiss with some quartzite. The present paper aims at describing various degrees of rock weathering and their effect in Sangrumba landslide. Field study followed by mineralogical, geochemical and geotechnical analyses of the collected rock and soil samples from the landslide zone were used in characterizing weathering degree. The gneisses are intensely weathered while quartzite is unweathered. Petrographical and X-ray diffraction analyses showed that the rocks in the landslide zone had undergone weathering process with the formation of different types of clay minerals as kaolinite, vermiculite, smectite and chlorite. This was further confirmed by the Scan Electron Microscope and Energy Dispersive X-ray analyses. These clay minerals drastically reduced the rock strength facilitating the extensive failure of the Sangrumba landslide. The major and trace element composition of the rock and soil samples was calculated from the XRF analyses. The geochemical analyses and weathering indexes of rocks showed that they are significantly weathered and had a major influence in the formation of the Sangrumba landslide. In addition, mechanical strength measurement of rock/soil showed that the strength drastically decreases as the weathering intensity increases. Rainfall followed by the rock type are the most dominant parameters influencing the weathering process which leads to the formation of large landslide as the present one. These findings can be used in other areas with similar geological and topographical conditions.  相似文献   

13.
发育完整的灰岩风化壳及其矿物学和地球化学特征   总被引：25，自引：5，他引：20  
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。  相似文献   

14.
Ge/Si and 87Sr/86Sr data from primary and secondary minerals, soil waters, and stream waters in a tropical granitoid catchment quantitatively reflect mineral alteration reactions that occur at different levels within the bedrock–saprolite–soil zone. Near the bedrock–saprolite interface, plagioclase to kaolinite reaction yields low Ge/Si and 87Sr/86Sr. Higher in the regolith column, biotite weathering and kaolinite dissolution drive Ge/Si and 87Sr/86Sr to high values. Data from streams at base flow sample the bedrock–saprolite interface zone, while at high discharge solutes are derived from upper saprolite–soil zone. Coupled Ge/Si and 87Sr/86Sr can be effective tools for quantifying the importance of specific weathering reactions, and for geochemical hydrograph separation.  相似文献   

15.
Rapid weathering and erosion rates in mountainous tropical watersheds lead to highly variable soil and saprolite thicknesses which in turn impact nutrient fluxes and biological populations. In the Luquillo Mountains of Puerto Rico, a 5-m thick saprolite contains high microorganism densities at the surface and at depth overlying bedrock. We test the hypotheses that the organisms at depth are limited by the availability of two nutrients, P and Fe. Many tropical soils are P-limited, rather than N-limited, and dissolution of apatite is the dominant source of P. We document patterns of apatite weathering and of bioavailable Fe derived from the weathering of primary minerals hornblende and biotite in cores augered to 7.5 m on a ridgetop as compared to spheroidally weathering bedrock sampled in a nearby roadcut.Iron isotopic compositions of 0.5 N HCl extracts of soil and saprolite range from about δ56Fe = 0 to ? 0.1‰ throughout the saprolite except at the surface and at 5 m depth where δ56Fe = ? 0.26 to ? 0.64‰. The enrichment of light isotopes in HCl-extractable Fe in the soil and at the saprolite–bedrock interface is consistent with active Fe cycling and consistent with the locations of high cell densities and Fe(II)-oxidizing bacteria, identified previously. To evaluate the potential P-limitation of Fe-cycling bacteria in the profile, solid-state concentrations of P were measured as a function of depth in the soil, saprolite, and weathering bedrock. Weathering apatite crystals were examined in thin sections and an apatite dissolution rate of 6.8 × 10? 14 mol m? 2 s? 1 was calculated. While surface communities depend on recycled nutrients and atmospheric inputs, deep communities survive primarily on nutrients released by the weathering bedrock and thus are tightly coupled to processes related to saprolite formation including mineral weathering. While low available P may limit microbial activity within the middle saprolite, fluxes of P from apatite weathering should be sufficient to support robust growth of microorganisms in the deep saprolite.  相似文献   

16.
The Pinxiang weathering profile is well developed on Early Triassic dacite lavas of the Baisi Formation. At the top of the profile is developed a red clay zone which is characterized mineralogically by kaolinite, iron oxide minerals, quartz, and a small amount of illite, montmorillonite and vermiculite. In going downwards the red clay zone gives way to a saprolite zone in which plagioclase pseudomorphs have been well preserved although replaced by kaolinite. Beneath the saprolite zone is the saprock zone characterized by less weathering for dacite. At the bottom of the weathering profile is the parent material, dacite, which is composed mainly of plagioclase, quartz, K-feldspar and biotite which have been largely altered into chlorite owing to submarine extrusion of dacite lavas. Some layers in the weathering profile show obvious sodium enrichment and potassium depletion relative to others. In the Al2O3-(CaO* Na2O)-K2O triangular diagram, the weathering trends of these layers in the middle stage are remarkably deviated from normal ones. Both mineralogy and micro-morphology of these layers indicate such deviation resulted from sodic metasomatism of orthoclase.  相似文献   

17.
江西龙南花岗岩稀土风化壳中粘土矿物的研究   总被引：8，自引：0，他引：8       下载免费PDF全文
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。  相似文献   

18.
The high-temperature and high-pressure experiment on natural block rock indicates that dehydration-melting of hydrous biotite (Bi) and partial melting of felsic minerals in garnet-biotite-plagioclase gneiss are mainly controlled by temperature, while mineral phase transformation is not only controlled by temperature-pressure conditions but also genetically associated with hydrous mineral dehydration-melting and partial melting of felsic minerals. According to the characteristics of biotite dehydration-melting and garnet transformation reaction, three stages may be distinguished: (1) when the experimental temperature is 700℃, biotite transforms to ilmenite (Ilm) + magnetite (Mt) + H2O and garnet to magnetite (Mt); (2) when the temperature is 730-760℃, biotite is dehydrated and melted and transformed into K2O-rich melt + Ilm + Mt, and garnet, into hypersthene (Hy) + cordierite (Crd); (3) when the temperature is up to or higher than 790℃, biotite is dehydrated and melted and transformed into melt + Hy +  相似文献   

19.
Biotite is a common constituent of silicate bedrock. Its weathering releases plant nutrients and consumes atmospheric CO2. Because of its stoichiometric relationship with its transformational weathering product and sensitivity to botanical activity, calculating biotite weathering rates using watershed mass-balance methods has proven challenging. At Coweeta Hydrologic Laboratory the coupling of biotite to its transformational weathering product is only valid if the stoichiometric relationship for the two phases is known; this relationship is unlikely layer-for-layer. Rates of biotite weathering and transformation of its secondary weathering product at the Coweeta Hydrological Laboratory are comparable with other Appalachian watersheds. The magnitude and sign of the difference between field- and laboratory-determined biotite weathering rates are similar to those of other silicate minerals. The influence of major-cation proportions in biomass on the rates of biotite weathering and transformational weathering product is greatest for watersheds with high biomass aggradation rates. The watershed with the lowest bedrock reactivity and highest flushing rate yielded the highest gibbsite formation rate of ~500 mol ha?1 year?1 and lowest kaolin-group mineral formation rates of 4–78 mol ha?1 year?1. The kaolin-group mineral formation rate increases as bedrock reactivity increases and flushing rate decreases to a maximum of ~300 mol ha?1 year?1, with a similar minimum gibbsite formation rate. The relative differences in bedrock reactivity and flux of water through Coweeta Hydrological Laboratory watersheds studied appear to be invariant over geologic timescales.  相似文献   

20.
Quaternary alkaline basalts of Middle Atlas, are weathered into spheroidal volumes organized into weathering cover. In the profile studied, the study of transformations from a fresh core basalt to the most weathered rinds, has been analysed using BESI images (backscattred electron image analysis). A part from the microscopic analysis of each weatherted basalt, proportions of primary minerals, clays and pore space has been quantified in different stages of weathering. Indeed, in fresh core basalt, weathering is characterized by a first dissolution of the glass, followed by feldspar transformations in twinning plans, cleavages and in micropores inherited from the magmatic and cristallographic history of the rock.

In most basaltic weathered rinds, pore spaces given by the dissolution of the glass, were filled by a mixture of clays and iron products. In these samples, feldspars are completely dissolved giving important porosity occupied by small quantity of halloysite and kaolinite. The olivine and pyroxene transformations are accompanied by weak pore space formation. At the bottom of the soil, this porosity decreases in response to filling by later secondary products which come from the upper part of the soil by weathering solutions.  相似文献   


设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号