The products of the interaction of feldspars with aqueous solutions at 25°C

Chemical analyses of 300 solutions produced by the artificial weathering of eight different feldspars in fourteen experiments of up to 1200hr duration were used to study the evolution of water during weathering. The range of pH was between 4 and 5·5. Within 4 hr of dissolution, the activity of Al was controlled by the pH and the solubility of microcrystalline gibbsite. After 100 hr of dissolution, the pH and microcrystalline halloysite controlled the activities of Al and silicic acid in all of the solutions.Microcrystalline halloysite was the only phase identified in the weathering of plagioclases in distilled water and 1 atm CO₂ partial pressure. Montmorillonites, halloysite and other clay minerals were produced from oligoclase in aqueous solutions containing high initial concentrations of Ca. Mg, K and SiO₂.The experimentally determined log solubility product of microcrystalline gibbsite was ?32·78 ± 0·04 and log $\text{K}$ for the hydrolysis of microcrystalline halloysite was 11.58 ± 0·05. The results suggest that very poorly crystalline metastable phases may control the initial compositions of some waters in contact with rocks containing feldspar minerals.     

A mass transfer model of bauxite formation

The formation of bauxite due to weathering of a granitic protolith has been simulated by means of a one-dimensional flow and reaction model based on the mass transfer principle. The model couples mineral dissolution and precipitation reactions, speciation in solution, and advective solute transport in a porous medium. A very important aspect of the modeling study is the use of mineral reaction rates determined experimentally in the laboratory. The important effects of solution saturation state and pH have been incorporated into the kinetic rate laws governing the heterogeneous reactions. The values of these parameters have been obtained from the scientific literature to guarantee that realistic reaction rates are used in the simulations. Albite and quartz are the minerals that make up the parent rock in the model. Gibbsite, kaolinite, and a Na-mica (as a surrogate for smectite) are the secondary minerals that have been taken into account. Long-term simulations (>1 Ma) have been run, and the formation of a bauxitic profile, with an upper gibbsite-rich and a lower kaolinite-rich zone, is predicted. In early stages of the process (up to a few hundreds of thousands of years), both gibbsite and kaolinite precipitate directly from solution as a consequence of albite dissolution. In later stages, the bulk of gibbsite precipitation derives from the incongruent dissolution of kaolinite, while kaolinite precipitation is still caused by the dissolution of albite. This is also reflected by the formation of two reaction fronts in the profile. These results are compared with weathering sequences from the Los Pijiguaos bauxite deposit, Venezuela. The overlap between the gibbsite and kaolinite zones and the replacement of kaolinite by gibbsite are consistent with model calculations. Mechanical denudation has to be called upon to explain the limited thicknesses of the weathering profiles in the field. The role of mechanical erosion is supported by the presence of microsedimentary structures in the bauxite and the balance between dissolved and suspended loads in the streams draining the area.     

Apport de l'analyse d'images dans l'étude de l'altération d'un basalte alcalin (Moyen Atlas, Maroc)Contribution of image analysis to the study of weathering process in alkaline basalt (Middle Atlas, Morocco)

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.     

高岭石-多水高岭石矿物晶体形态与演化

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。     

The role of reaction affinity and secondary minerals in regulating chemical weathering rates at the Santa Cruz Soil Chronosequence, California

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.     

苏州阳西风化型高岭土矿物的形成与演化

苏州高岭土矿床可划分为两种成因类型^[1,2],即风化型¹⁾和热液蚀变型。高岭土集中分布在阳山东、西、北三个矿区。本文仅就阳西风化型高岭土中产出的不同种类高岭土矿物的形成及其互相演变的关系作初步探讨。     

Minéralogie et bilan de matière de l''altération supergène d''un basalte alcalin du Moyen Atlas Marocain

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.     

福建郭山高岭土矿床中高岭矿物的研究

本文运用X射线衍射、红外吸收光谱、分析电子显微术和化学分析等多种方法对福建郭山高岭土矿床中的高岭矿物(高岭石和埃洛石)的矿物学性质、分布及矿物形成的阶段性变化进行了系统研究。结果表明,根据高岭石和埃洛石相对含量的多少,花岗岩风化剖面全风化带划分的4个矿物段可以反映这两种矿物的分布特点。埃洛石为具有管状和多面体球状的7?型以及10?—7?、7?-高岭石过渡型。高岭石晶体形态和结晶有序度在风化剖面垂直方向上的规律性变化与高岭石的形成经历了初始结晶——强烈高岭土化——风化作用后期的阶段性变化有关。     

赣南花岗岩风化壳型稀土矿床中纳米级稀土矿物的研究

为揭示次生风化成因稀土矿物的赋存形态和分布特征,对赣南安远岗下稀土矿区花岗岩风化壳剖面进行了研究。采集风化壳不同层位的样品进行分选、提纯,场发射扫描电镜和透射电镜研究结果表明,与稀土矿物紧密相关的粘土矿物是高岭石和埃洛石等,风化壳中发现的纳米级稀土矿物既有附着于其他矿物表面的微细颗粒,也有呈集合体存在的稀土矿物(可能为方铈石),这一现象有助于进一步认识风化壳稀土矿床的形成过程。     

Geochemistry and mineralogy of saprolite in Finnish Lapland

An ancient saprolite has developed on the Palaeoproterozoic granulite, granite gneiss and amphibolite bedrock of the Vuotso–Tankavaara area of central Finnish Lapland. The present day climatic regime in Finnish Lapland lies within the northern boreal zone and so the saprolite there can be regarded as fossil. Cores of saprolite were collected from 4 sections (42 samples) and analyzed chemically and mineralogically. In the study area, progressive weathering of the rocks has been marked by gradual enrichment in Al, Fe and Ti; and depletion of Na, K and Ca. The higher concentration of Fe(III) and water and reduced Na and Ca in weathered bedrock in the 4 sections are indicative of oxidation, hydration and leaching processes involved during weathering. The primary minerals in the saprolite are plagioclase feldspar, K-feldspar, quartz, garnet (almandine) and hornblende; the common secondary minerals are kaolinite, halloysite, and vermiculite in addition to minor amounts of sericite. Intense weathering is indicated by: (1) the presence of kaolinite and halloysite in 4 sections of different bedrock types, and (2) the comparatively lower SiO₂/Al₂O₃ (wt.%) ratio (2.30) of weathered granulites (3 sections) as compared to fresh granulite (4.33) and that of weathered amphibolite (2.68) as compared to fresh amphibolite (3.56). In general, kaolinite and halloysite have formed through the weathering of feldspars, garnet, and biotite. Vermiculite is the most probable alteration product of biotite. The formation of kaolinite and halloysite in Finnish Lapland indicates wetter and warmer climatic conditions during the time of their formation than at present. The possible time for formation of the saprolite is early Cretaceous–early Tertiary into Middle Miocene.     

Alkali feldspar dissolution and secondary mineral precipitation in batch systems: 3. Saturation states of product minerals and reaction paths

In order to evaluate the complex interplay between dissolution and precipitation reaction kinetics, we examined the hypothesis of partial equilibria between secondary mineral products and aqueous solutions in feldspar-water systems. Speciation and solubility geochemical modeling was used to compute the saturation indices (SI) for product minerals in batch feldspar dissolution experiments at elevated temperatures and pressures and to trace the reaction paths on activity-activity diagrams. The modeling results demonstrated: (1) the experimental aqueous solutions were supersaturated with respect to product minerals for almost the entire duration of the experiments; (2) the aqueous solution chemistry did not evolve along the phase boundaries but crossed the phase boundaries at oblique angles; and (3) the earlier precipitated product minerals did not dissolve but continued to precipitate even after the solution chemistry had evolved into the stability fields of minerals lower in the paragenesis sequence. These three lines of evidence signify that product mineral precipitation is a slow kinetic process and partial equilibria between aqueous solution and product minerals were not held. In contrast, the experimental evidences are consistent with the hypothesis of strong coupling of mineral dissolution/precipitation kinetics [e.g., Zhu C., Blum A. E. and Veblen D. R. (2004a) Feldspar dissolution rates and clay precipitation in the Navajo aquifer at Black Mesa, Arizona, USA. In Water-Rock Interaction (eds. R. B. Wanty and R. R. I. Seal). A.A. Balkema, Saratoga Springs, New York. pp. 895-899]. In all batch experiments examined, the time of congruent feldspar dissolution was short and supersaturation with respect to the product minerals was reached within a short period of time. The experimental system progressed from a dissolution driven regime to a precipitation limited regime in a short order. The results of this study suggest a complex feedback between dissolution and precipitation reaction kinetics, which needs to be considered in the interpretation of field based dissolution rates.     

Coupled alkali feldspar dissolution and secondary mineral precipitation in batch systems: 4. Numerical modeling of kinetic reaction paths

This paper explores how dissolution and precipitation reactions are coupled in batch reactor experimental systems at elevated temperatures. This is the fourth paper in our series of “Coupled Alkali Feldspar Dissolution and Secondary Mineral Precipitation in Batch Systems”. In our third paper, we demonstrated via speciation-solubility modeling that partial equilibrium between secondary minerals and aqueous solutions was not attained in feldspar hydrolysis batch reactors at 90-300 °C and that a strong coupling between dissolution and precipitation reactions follows as a consequence of the slower precipitation of secondary minerals (Zhu and Lu, 2009). Here, we develop this concept further by using numerical reaction path models to elucidate how the dissolution and precipitation reactions are coupled. Modeling results show that a quasi-steady state was reached. At the quasi-steady state, dissolution reactions proceeded at rates that are orders of magnitude slower than the rates measured at far from equilibrium. The quasi-steady state is determined by the relative rate constants, and strongly influenced by the function of Gibbs free energy of reaction (ΔG_r) in the rate laws.To explore the potential effects of fluid flow rates on the coupling of reactions, we extrapolate a batch system (Ganor et al., 2007) to open systems and simulated one-dimensional reactive mass transport for oligoclase dissolution and kaolinite precipitation in homogeneous porous media. Different steady states were achieved at different locations along the one-dimensional domain. The time-space distribution and saturation indices (SI) at the steady states were a function of flow rates for a given kinetic model. Regardless of the differences in SI, the ratio between oligoclase dissolution rates and kaolinite precipitation rates remained 1.626, as in the batch system case (Ganor et al., 2007). Therefore, our simulation results demonstrated coupling among dissolution, precipitation, and flow rates.Results reported in this communication lend support to our hypothesis that slow secondary mineral precipitation explains part of the well-known apparent discrepancy between lab measured and field estimated feldspar dissolution rates (Zhu et al., 2004). Here we show how the slow secondary mineral precipitation provides a regulator to explain why the systems are held close to equilibrium and show how the most often-quoted “near equilibrium” explanation for an apparent field-lab discrepancy can work quantitatively. The substantiated hypothesis now offers the promise of reconciling part of the apparent field-lab discrepancy.     

苏州高岭土矿成矿期后的矿物演化及其与矿体改造的关系

苏州高岭土矿主要由高岭石、7埃洛石、10埃洛石及少量绢云母、蒙脱石、明矾石、三水铝石组成。高岭土矿物形成后因外界地化条件改变发生了以下转变:(1)埃洛石脱水向高岭石转化;(2)次生淋滤埃洛石形成;(3)埃洛石和三水铝石之间的互相转化;(4)Ca型蒙脱石形成;(5)高岭土的磷酸盐化作用;(6)次生淋滤明矾石的形成。矿物生成的先后顺序和共生关系可将矿物形成分为主要成矿期和成矿期后演化两个阶段。矿物的后期演化使优质高岭土进一步富集,改造,形成量大质优的高岭土矿。同时,非高岭土矿物的生成又使部分矿石质量变差,降低了矿石的工业价值。     

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。     

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。     

Simulation of the nucleation and growth of binary solid solutions in aqueous solutions

We present a model which, for the first time, accounts for nucleation, growth and/or resorption of particles of variable composition in aqueous solutions (AS). Devised for describing the precipitation of binary solid solutions, it yields the time evolution of all ion activities in the AS, together with the particle population characteristics: number, size and composition profile of particles as a function of time and of their time of nucleation. We apply this numerical approach to the prototypical case of (Ba,Sr)CO₃ solid solution precipitation. We demonstrate the great sensitivity of the composition profiles and particle sizes to the initial conditions under which the AS is prepared, and thus illustrate the possibility of engineering the particle characteristics into a chosen state. Finally, by comparing the precipitation of two solid solutions (Ba,Sr)CO₃ and (Ba,Sr)SO₄, we evidence the sensitivity of the particle composition profiles to the ratio of the end-member solubility products, which leads to the formation of core-shell particles in the case of (Ba,Sr)SO₄.     

贵州碳酸盐岩红土中的粘土矿物及其形成机理

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。     

高岭石-多水高岭石演化系列的热谱特征

高岭石-多水高岭石演化系列共包括四种矿物:结晶良好的高岭石、结晶差的或b轴无序的高岭石、7Å多水高岭石和10Å多水高岭石。我们将结晶良好的高岭石和10Å多水高岭石分别视为这一演化系列的两个端元矿物,其余两种矿物则是演化系列的中间矿物。该系列中的矿物同属于1:1型含水的层状铝硅酸盐矿物,因而它们的化学成分基本相同,主要差别是在晶体结构上由于单位结构层沿c轴彼此堆叠的方式而引起从有序向无序变化,同时层间键力的减弱引起水分子进入,促使层间水的含量逐渐增大。基于以上特征,这一系列中的四种类型矿物受热以后的热效应很灵敏,详细研究它们在加热脱水过程中的变化规律,可以加深对矿物特性的认识和鉴别。