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1.
Batch and flow-through experiments were performed on quartz–feldspar granular aggregates at hydrothermal conditions (up to ≈150 °C, up to 5 MPa effective pressure, and near-neutral pH) for up to 141 days. The effect of dissolution–precipitation reactions on the surface morphology of the mineral grains was investigated. The starting materials as well as the solids and fluids resulting from the experiments were characterized using BET, energy dispersive X-ray spectroscopy, electron microprobe analysis, inductively coupled plasma-optical emission spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, and X-ray fluorescence spectroscopy. The electrical conductivity of fluid samples was used as a proxy for the evolution of the fluid composition in the experiments. The chemical analyses of the fluids in combination with hydrogeochemical simulations with PHREEQC suggested the precipitation of Al–Si-bearing solid phases. Electron microscopy confirmed the formation of secondary amorphous Al–Si-bearing solid phases. The microscopic observations are consistent with a process of stoichiometric dissolution of the mineral grains, transport of dissolved ions in the fluid phase, and spatially coupled precipitation of sub-μm sized amorphous particles on mineral surfaces. These findings shed light onto early stages of diagenesis of quartz–feldspar sands and indicate that amorphous phases may be precursors for the formation of crystalline phases, for example, clay minerals.  相似文献   

2.
本文对南丹IIICD铁陨石的矿物学特征进行了研究,并与同为铁陨石但化学分类不同的阿根廷IAB铁陨石和西伯利亚IIB铁陨石进行了对比,重点探讨了风化作用对铁陨石矿物学特征的影响.首先用偏光显微镜、静水称重、扫描电镜观察了样品的基本矿物学特征和微形貌特征,然后用振动式样品磁强计、X射线衍射与电子探针能谱半定量测试研究了样品的磁学性质、物相和化学组成.研究结果表明,南丹铁陨石在较强的自然风化作用下,光泽变弱为土状光泽,相对密度降低;风化产生的反铁磁性物质会使陨石的磁性下降;另外,样品表面物相组成也发生较大变化,以针铁矿(FeOOH)和磁铁矿(Fe3O4)等铁的次生矿物为主;但风化壳以下的矿物物相及化学成分均未发生明显变化,以Fe、Ni为主的铁纹石、镍纹石物相存在.  相似文献   

3.
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   

4.
二长花岗岩风化壳自下而上划分为原生带、微风化带、弱风化带和强风化带。风化壳中粘土矿物主要为埃洛石,其次为高岭石及少量伊利石。微风化带下部以高岭石为主,往上埃洛石逐渐占优势,并于弱风化带中、上部和强风化带中局部富集,这与当时地下水活动状态有关。高岭石结晶程度随风化程度的增强而提高。在弱酸性介质条件下及水分丰富、溶出条件强烈时,从长石解理面上和长石的“溶蚀”空隙中可以直接形成高岭石或埃洛石,而过渡阶段的伊利石很不发育或不存在。  相似文献   

5.
Petrographic studies undertaken on samples from outcrops of the Jurassic Pilliga Sandstone intake beds in the extreme southeastern portion of the Great Australian Basin reveal an abundance of low birefringent clay material filling pore spaces and detrital grain fractures. Thin-section petrography and scanning electron microscopy indicate that much of this material is authigenic and X-ray diffraction studies show it to be essentially monomineralic, consisting of well-ordered kaolinite. Although the Pilliga Sandstone is quartz-rich, micrometric analysis indicates that potassium feldspar is present in quantities up to 20% of the total detrital material. Sporadic biotite mica is also present, constituting up to 3% of the detrital volume.Mechanisms for the formation of authigenic kaolinite are discussed in terms of chemical equilibrium and detrital mineral stability in aqueous systems. Chemical data from bore water and surface waters from this stratigraphic unit indicate that kaolinite is the major stable mineral phase in contact with these natural waters and that minerals such as potassium feldspar or mica would chemically alter to kaolinite. Such alteration of detrital mineral grains is supported by thin-section petrography and this mechanism is considered to be the source for the majority of the authigenic kaolinite observed.The hydrogeological characteristics of the Pilliga Sandstone intake beds are related to the extent of development of authigenic kaolinite.  相似文献   

6.
Batch reactor experiments were conducted to assess perthitic alkali-feldspar dissolution and secondary mineral formation in an initially acidic fluid (pH = 3.1) at 200 °C and 300 bars. Temporal evolution of fluid chemistry was monitored by major element analysis of in situ fluid samples. Solid reaction products were retrieved from two identical experiments terminated after 5 and 78 days. Scanning electron microscopy revealed dissolution features and significant secondary mineral coverage on feldspar surfaces. Boehmite and kaolinite were identified as secondary minerals by X-ray diffraction and transmission electron microscopy. X-ray photoelectron spectroscopy analysis of alkali-feldspar surfaces before and after reaction showed a trend of increasing Al/Si ratios and decreasing K/Al ratios with reaction progress, consistent with the formation of boehmite and kaolinite.Saturation indices of feldspars and secondary minerals suggest that albite dissolution occurred throughout the experiments, while K-feldspar exceeded saturation after 216 h of reaction. Reactions proceeded slowly and full equilibrium was not achieved, the relatively high temperature of the experiments notwithstanding. Thus, time series observations indicate continuous supersaturation with respect to boehmite and kaolinite, although the extent of this decreased with reaction progress as the driving force for albite dissolution decreased. The first experimental evidence of metastable co-existence of boehmite, kaolinite and alkali feldspar in the feldspar hydrolysis system is consistent with theoretical models of mineral dissolution/precipitation kinetics where the ratio of the secondary mineral precipitation rate constant to the rate constant of feldspar dissolution is well below unity. This has important implications for modeling the time-dependent evolution of feldspar dissolution and secondary mineral formation in natural systems.  相似文献   

7.
《Applied Geochemistry》1994,9(5):583-595
A steady-state geochemical model has been developed to study water-rock interactions controlling metal release from waste rock heaps at the Aitik Cu mine in northern Sweden. The Cu release in drainage waters from the site is of environmental concern. The waste rock heaps are treated as single completely mixed flow-through reactors. The geochemical model includes kinetices of sulphide and primary silicate mineral weathering, heterogeneous equilibrium with secondary mineral phases and speciation equilibrium. Field monitoring of drainage water composition provides a basis for evaluation of model performance.The relative rate of oxidative weathering of sulphides and dissolution of primary silicate minerals, using published kinetic data, are consistent with net proton and base cation fluxes at the site. The overall rate of Fe2+ oxidation within the heap is three orders of magnitude faster than that which could be explained by surface-catalysed reaction kinetics. This suggests significant activity of iron-oxidizing bacteria. The absolute weathering rates of sulphides and silicate minerals, normalized to a measured BET surface area, are approximately two orders of magnitude lower at field scale than published rates from laboratory experiments. Because of the relative absence of carbonate minerals, the weathering of biotite and plagioclase feldspar are important sources of alkalinity.  相似文献   

8.
X-ray diffraction (XRD), X-ray Fluorescence (XRF), optical microscopy, Scanning Electron Microscopy coupled with Energy Dispersive Spectrometry (SEM-EDS) and Electron Probe micro-analyser (EPMA) and Wavelength-Dispersive Spectroscopy (WDS) were conducted on charnockite from the Caparaó Suite and its alteration cortex to determine the mineralogical, micromorphological and geochemical transformations resulting from the weathering process. The hydrolysis of the charnockite occurred in different stages, in accordance with the order of stability of the minerals with respect to weathering: andesine/orthopyroxene, pargasite and alkali feldspar. The rock modifications had begun with the formation of a layer of incipient alteration due to the percolation of weathering solutions first in the pressure relief fractures and then in cleavage and mineral edges. The iron exuded from ferromagnesian minerals precipitated in the intermineral and intramineral discontinuities. The layer of incipient alteration evolves into an inner cortex where the plagioclase changes into gibbsite by direct alitisation, the ferromagnesian minerals initiate the formation of goethitic boxworks with kaolinitic cores, and the alkali feldspar initiates indirect transformation into gibbsite, forming an intermediate phase of illite and kaolinite. In the outer cortex, mostly traces of alkali feldspar remain, and they are surrounded by goethite and gibbsite as alteromorphics, characterising the formation of the isalteritic horizon that occurs along the slope and explains the bauxitization process at the Caparaó Range, SE Brazil.  相似文献   

9.
This study investigates the retention of heavy metals in secondary precipitates from a sulfidic mine rock dump and underlying podzolic soils by means of mineralogical and chemical extraction methods. The rock dump, which is at least 50 years old, consists of a 5–10-cm-thick leached zone and an underlying 110–115-cm-thick accumulation zone. Optical microscopy and electron microprobe analyses confirm that pyrrhotite weathering has proceeded much further in the leached horizon relative to the accumulation horizon. The weathering of sulfides in the leached zone has resulted in the migration of most heavy metals to the accumulation zone or underlying soils, where they are retained in more stable phases such as secondary ferric minerals, including goethite and jarosite. Some metals are temporarily retained in hydrated ferrous sulfates (e.g., melanterite, rozenite). Received: 28 October 1996 · Accepted: 24 February 1997  相似文献   

10.
伊毛缟石在我国的首次发现及其分析电子显微术研究   总被引：1，自引：0，他引：1  
伊毛缟石是一种具有纤维状形态的类晶质水化铝硅酸盐矿物。本文对我国首次发现于福建郭山风化型高岭土矿床中的伊毛缟石用分析电子显微术进行了矿物学研究。研究结果表明,在透射电镜下,伊毛缟石呈长数μm,宽60—250的似线状或纤维状,这是它最重要的矿物学特征之一。选区电子衍射分析给出d_(42)=3.30、d_(04)=2.10和d_(06)=1.40三个比较(氵弥)散的衍射环所表征的典型伊毛缟石衍射花样。其化学成分主要由SiO_2、Al_2O_3和H_2O组成。  相似文献   

11.
In order to explore the reasons for the apparent discrepancy between laboratory and field weathering rates and to determine the extent to which weathering rates are controlled by the approach to thermodynamic equilibrium, secondary mineral precipitation, and flow rates, a multicomponent reactive transport model (CrunchFlow) was used to interpret soil profile development and mineral precipitation and dissolution rates at the 226 ka Marine Terrace Chronosequence near Santa Cruz, CA. Aqueous compositions, fluid chemistry, transport, and mineral abundances are well characterized [White A. F., Schulz M. S., Vivit D. V., Blum A., Stonestrom D. A. and Anderson S. P. (2008) Chemical weathering of a Marine Terrace Chronosequence, Santa Cruz, California. I: interpreting the long-term controls on chemical weathering based on spatial and temporal element and mineral distributions. Geochim. Cosmochim. Acta72 (1), 36-68] and were used to constrain the reaction rates for the weathering and precipitating minerals in the reactive transport modeling. When primary mineral weathering rates are calculated with either of two experimentally determined rate constants, the nonlinear, parallel rate law formulation of Hellmann and Tisserand [Hellmann R. and Tisserand D. (2006) Dissolution kinetics as a function of the Gibbs free energy of reaction: An experimental study based on albite feldspar. Geochim. Cosmochim. Acta70 (2), 364-383] or the aluminum inhibition model proposed by Oelkers et al. [Oelkers E. H., Schott J. and Devidal J. L. (1994) The effect of aluminum, pH, and chemical affinity on the rates of aluminosilicate dissolution reactions. Geochim. Cosmochim. Acta58 (9), 2011-2024], modeling results are consistent with field-scale observations when independently constrained clay precipitation rates are accounted for. Experimental and field rates, therefore, can be reconciled at the Santa Cruz site.Additionally, observed maximum clay abundances in the argillic horizons occur at the depth and time where the reaction fronts of the primary minerals overlap. The modeling indicates that the argillic horizon at Santa Cruz can be explained almost entirely by weathering of primary minerals and in situ clay precipitation accompanied by undersaturation of kaolinite at the top of the profile. The rate constant for kaolinite precipitation was also determined based on model simulations of mineral abundances and dissolved Al, SiO2(aq) and pH in pore waters. Changes in the rate of kaolinite precipitation or the flow rate do not affect the gradient of the primary mineral weathering profiles, but instead control the rate of propagation of the primary mineral weathering fronts and thus total mass removed from the weathering profile. Our analysis suggests that secondary clay precipitation is as important as aqueous transport in governing the amount of dissolution that occurs within a profile because clay minerals exert a strong control over the reaction affinity of the dissolving primary minerals. The modeling also indicates that the weathering advance rate and the total mass of mineral dissolved is controlled by the thermodynamic saturation of the primary dissolving phases plagioclase and K-feldspar, as is evident from the difference in propagation rates of the reaction fronts for the two minerals despite their very similar kinetic rate laws.  相似文献   

12.
Mineral composition was analysed within a saprolite profile from NE Nigeria. The determination of identifiable feldspar phases is only based on X-ray powder diffractograms. Representative K-feldspar fragments were selected for detailed SEM (scanning electron microscope) studies and EDX (energy-dispersive X-ray analysis) from the profile.

The analytical data indicate a significant contrast in weathering states between the lower and the upper profile section. This contrast reflects a different weathering history: the feldspars from the lower section exhibit weathering features that are characteristic for decomposition below the water table. When raised above the water table, another type of decomposition attacked only the feldspars within the upper section while those in the lower section still preserve the older weathering state.  相似文献   


13.
《Applied Geochemistry》2001,16(7-8):931-937
Weathering in an upland catchment on granitic parent material has been studied by chemical and mineralogical analyses of soils. Long-term weathering rates for base cations, calculated from chemical analyses of the mineral horizons from soil profiles using Zr as an internal, immobile, index element, are among the smallest recorded for Scottish soils (1.7–3.1 meq m−2 a−1), indicating that these soils are susceptible to acid deposition. Sodium is the base cation lost to the greatest extent from the soils, due to weathering of plagioclase feldspar, mainly in the coarse size-fractions. Calcium is lost not only from plagioclase feldspar, but also from hornblende, grains of which show dissolution etch pits and denticulate surface features when examined by scanning electron microscopy. Weathering of hornblende, present in basic inclusions in the granite, is a significant weathering process in these soils. A range of values for 87Sr/86Sr ratios in stream-waters confirms the spatial variability of the material supplying Ca to the streams. The current weathering rate, calculated from input–output budgets to be 28.9 meq m−2 a−1, is much greater than the long-term weathering rate, but small compared to other catchments on similar parent material.  相似文献   

14.
贵州碳酸盐岩红土中的粘土矿物及其形成机理   总被引：13，自引：2，他引：13  
朱立军  傅平秋 《矿物学报》1996,16(3):290-297,T001
本文运用Ｘ射线衍射、红外光谱、差热、透射电镜和扫描电镜等方法对贵州碳酸盐岩红土中的粘土矿物进行了系统研究。高岭石和０．７ｎｍ埃洛石是碳酸盐岩红土中的主要粘土矿物，其次为伊利石、蛭石、绿泥石、绿泥石／蛭石混层矿物，水铝英石和三水铝石等。  相似文献   

15.
To analyze the composition and structure of the pale blue outcrop from the UBC Cliffs and research the environmental significance of aerinite recorded in the sediments, mineral composition, bulk chemical composition and structural analyses were carried out by means of X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), electron microprobe analysis (EMPA) and X-ray fluorescence spectra (XRF). Quartz, feldspar, mica, chlorite and aerinite as well as a small amount of amphibole, calcite, clinopyroxene, rutile, perovskite and apatite are recorded in the sediments. It is reported for the first time that the aerinite occurs in a high latitude area and in sediments paragenetic with mica and chlorite. The large span in grain size shows a typical characteristic of fluvioglacial sediments. Geochemistry characteristics (CIA, CIW, PIA, WIP and ICV) verify that there is virtually no chemical weathering. Mineralogy and geochemistry features of the UBC Cliffs sediments indicate that aerinite is accommodate to the high latitude and cold, dry climate. As the sediments show high similarity to those of their provenance, they have great significance in the identification of the forming condition and origin of the aerinite.  相似文献   

16.
The thorium and rare-earth element (Th-REE) deposit at Morro do Ferro formed under supergene lateritic weathering conditions. The ore body consists of shallow NW-SE elongated argillaceous lenses that extend from the top of the hill downwards along its south-eastern slope. The deposit is capped by a network of magnetite layers which protected the underlying highly weathered, argillaceous host rock from excessive erosion. The surrounding country rocks comprise a sequence of subvolcanic phonolite intrusions that have been strongly altered by hydrothermal and supergene processes.From petrological, mineralogical and geochemical studies, and mass balance calculations, it is inferred that the highly weathered host rock was originally carbonatitic in composition, initially enriched in Th and REEs compared to the surrounding silicate rocks. The intrusion of the carbonatite caused fenitic alteration in the surrounding phonolites, consisting of early potassic alteration followed by a vein-type Th-REE mineralization with associated fluorite, carbonate, pyrite and zircon. Subsequent weathering has completely decomposed the carbonatite forming a residual supergene enrichment of Th and REEs.Initial weathering of the carbonatite has created a chemical environment that might have been conductive to carbonate and phosphate complexing of the REEs in groundwaters. This may have appreciably restricted the dissolution of primary REE phases. Strongly oxidic weathering has resulted in a fractionation between Ce and the other light rare earth elements (LREEs). Ce3+ is oxidized to Ce4+ and retained together with Th by secondary mineral formation (cerianite, thorianite), and by adsorption on poorly crystalline iron- and aluminium-hydroxides. In contrast, the trivalent LREEs are retained to a lesser degree and are thus more available for secondary mineral formation (Nd-lanthanite) and adsorption at greater depths down the weathering column. Seasonally controlled fluctuations of recharge waters into the weathering column may help to explain the observed repetition of Th-Ce enriched zones underlain by trivalent LREE enriched zones.  相似文献   

17.
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.  相似文献   

18.
岩矿鉴定是各类地质工作开展的基础,其鉴定水平和质量直接影响着工作的深入程度和研究程度。传统鉴定方法受人员自身经验水平、光学显微镜分辨率等因素的影响较大,对于现今需要研究的细微稀有矿物、细粒沉积岩矿物等很难准确地识别鉴定。而依托高精密大型仪器的技术方法多数对样品制备有特殊要求,不利于样品的再利用,诸如扫描电镜、电子探针等在高倍数反射光下探寻、观测特定的细微透明矿物也存在一定的不足。本文将激光拉曼高分辨大面积快速成像方法(StreamLineHR)运用于两块标准岩石光薄片的全区域大面积扫谱,准确识别出其中透明矿物有碱性长石、斜长石、石英、普通角闪石、黑云母、方解石、榍石、磷灰石、锆石和绿帘石,不透明矿物有磁铁矿,部分矿物间存在紧密伴生的情况(如石英与长石、榍石与角闪石)和次生蚀变的情况(如长石碳酸盐化蚀变为方解石)。并以此为基础进行了含量统计,将其分别定名为细粒角闪石英二长闪长岩与细粒黑云母斜长角闪岩。实验过程中,荧光效应,类质同象类矿物(长石、角闪石)峰位相似性和蚀变矿物峰位偏移会对矿物识别、谱图解析造成干扰,可结合矿物镜下光性特征来解决。另外,面扫步长设置越小,分析精确度越高,时间成本也会相应增加,应用时需兼顾考虑。该方法实现了对细微矿物便捷、直观、准确的大范围快速识别鉴定,可弥补传统岩矿鉴定和其他技术方法的不足,拓展了拉曼光谱法在地质工作中的应用范围。  相似文献   

19.
Quantitative mineralogy correlates with major-, minor- and trace-element chemistry for 387 samples of A-horizon and deeper soils collected from east–west and north–south transects across the USA and Canada, where the deeper soils were collected beneath the A-horizon samples. Concentrations of the major elements correlate with specific mineral phases. Minor- and trace-element concentrations correlate with the same phases as the major elements with which they share similar geochemical behavior. Concentrations of quartz and feldspar correlate with precipitation trends east of the Rocky Mountains, and are independent of the underlying rock type and age, indicating that the weathering of soils in this region may have reached a steady-state mineralogy. Other trends in mineralogy relate to physiographic province. The combination of quantitative mineralogy and chemical analysis yields a much richer portrait of soils than can be gained from chemistry alone, because the origins of chemical trends and the chemical availability of specific elements are related to mineralogy.  相似文献   

20.
It has been widely accepted that a chemically altered, protective surface layer regulates the dissolution, and hence the weathering, of plagioclase feldspars under Earth's surface conditions. In this study, we examine this hypothesis in detail with the aid of scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Using these techniques, we have been unable to find any direct evidence suggesting the presence of a chemically altered coating on feldspar surfaces which have been weathered in the lab. Instead, our results suggest that the mechanism controlling feldspar weathering is a surface controlled reaction.Based on SEM observations and measurements of rate of release of silica, we postulate that the process of feldspar dissolution proceeds in two stages in the lab. Initially, one observes the dissolution of ultrafine (? 1 μm diameter) particles which are produced during grinding of the sample, and which adhere tenaciously to the surfaces of larger grains. This is the stage which results in the non-linear rates of dissolution which are commonly observed in the lab (parabolic kinetics). Secondly, the dissolution occurs at sites of excess surface energy such as at dislocations or similar crystal defects. This process yields linear rates of dissolution, and it dominates the weathering of feldspars in the field.  相似文献   

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