Biotite Dissolution Research Articles

Simulating fluoride evolution in groundwater using a reactive multicomponent transient transport model: Application to a crystalline aquifer of Southern India

Overexploitation of crystalline aquifers in a semi-arid climate leads to a degradation of water quality, with the main processes responsible for the observed salt loads probably being irrigation return flow (IRF) and a high evaporation rate. The present study has focused on modelling the F− accumulation caused by IRF below rice paddy fields in the small endorheic Maheshwaram watershed (Andhra Pradesh, Southern India). The transient simulation was performed with a 1D reactive transport PHREEQC column and took into account IRF evaporation, kinetically controlled mineral dissolution/precipitation, ion adsorption on Fe hydroxides, and mixing with fresh groundwater. The results revealed the role of cationic exchange capacity (CEC) in Ca/Na exchange and calcite precipitation, both favouring a decrease of the Ca2+ activity that prevents fluorite precipitation. Iron hydroxide precipitation offers a not inconsiderable adsorption capacity for F− immobilization. The principal sources of F− are fluorapatite dissolution and, to a lesser extent, allanite and biotite dissolution. Anthropogenic sources of F−, such as fertilizers, are probably very limited. After simulating an entire dry-season irrigation cycle (120days), the results are in good agreement with the observed overall increase of Cl− in the Maheshwaram groundwater. The model enables one to decipher the processes responsible for water-resource degradation through progressive salinization. It shows that F− enrichment of the groundwater is likely to continue in the future if groundwater overexploitation is not controlled.

Biotite Dissolution in Brine at Varied Temperatures and CO2 Pressures: Its Activation Energy and Potential CO2 Intercalation

For sustainable geologic CO(2) sequestration (GCS), it is important to understand the effects of temperature and CO(2) pressure on mica's dissolution and surface morphological changes under saline hydrothermal conditions. Batch experiments were conducted with biotite (Fe-end member mica) under conditions relevant to GCS sites (35-95 °C and 75-120 atm CO(2)), and 1 M NaCl solution was used to mimic the brine. With increasing temperature, a transition from incongruent to congruent dissolution of biotite was observed. The dissolution activation energy based on Si release was calculated to be 52 ± 5 kJ mol(-1). By comparison with N(2) experiments, we showed that CO(2) injection greatly enhanced biotite's dissolution and its surface morphology evolutions, such as crack formation and detachment of newly formed fibrous illite. For biotite's dissolution and morphological evolutions, the pH effects of CO(2) were differentiated from the effects of bicarbonate complexation and CO(2) intercalation. Bicarbonate complexation effects on ion release from biotite were found to be minor under our experimental conditions. On the other hand, the CO(2) molecules in brine could get into the biotite interlayer and cause enhanced swelling of the biotite interlayer and hence the observed promotion of biotite surface cracking. The cracking created more reactive surface area in contact with brine and thus enhanced the later ion release from biotite. These results provide new information for understanding CO(2)-brine-mica interactions in saline aquifers with varied temperatures and CO(2) pressures, which can be useful for GCS site selection and operations.

Allanite-(Nd) from the Kingman Feldspar Mine, Mojave Pegmatite District, Northwestern Arizona, USA

The Kingman pegmatite, located in the Cerbat Range in northwestern Arizona, is hosted by orogenic 1.7 Ga Paleoproterozoic granitic rocks. This post-orogenic pegmatite was intruded into these granites during the middle Proterozoic ( ca. 1.5 Ga) subsequent to the Yavapai and Mazatzal orogenies. Extreme LREE-enrichment, as well as the HREE-depletion, is exemplified by the presence of abundant large crystals of LREE-enriched allanite and a near-absence of HREE-enriched minerals. Most of the allanite is Nd-rich allanite-(Ce). However, several of the samples have domains that are Nd-dominant, and thus are allanite-(Nd). This represents the fourth world location and the first reported U.S. location for allanite-(Nd). The Nd-dominant domains, which typically occur near the rim of grains and along fractures, may be the result of alteration by highly oxidizing late-stage fluids that replaced allanite with bastnasite-(Ce). Late-stage carbonate fluids, enriched in F from the dissolution of biotite, could have preferentially redistributed Ce into bastnasite-(Ce), resulting in Nd-enriched recrystallized allanite near fractures and along the recrystallized rim. As the Kingman pegmatite is not associated with a parental granite and as it exhibits unusual depletions in F, Nb, Ta, and HREE and an extreme enrichment in the LREE, we favor an anatectic origin.

Tree-mycorrhiza symbiosis accelerate mineral weathering: Evidences from nanometer-scale elemental fluxes at the hypha–mineral interface

In soils, mycorrhiza (microscopic fungal hypha) living in symbiosis with plant roots are the biological interface by which plants obtain, from rocks and organic matter, the nutrients necessary for their growth and maintenance. Despite their central role in soils, the mechanism and kinetics of mineral alteration by mycorrhiza are poorly constrained quantitatively. Here, we report in situ quantification of weathering rates from a mineral substrate, (0 0 1) basal plane of biotite, by a surface-bound hypha of Paxillus involutus, grown in association with the root system of a Scots pine, Pinus sylvestris. Four thin-sections were extracted by focused ion beam (FIB) milling along a single hypha grown over the biotite surface. Depth-profile of Si, O, K, Mg, Fe and Al concentrations were performed at the hypha–biotite interface by scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDX). Large removals of K (50–65%), Mg (55–75%), Fe (80–85%) and Al (75–85%) were observed in the topmost 40 nm of biotite underneath the hypha while Si and O are preserved throughout the depth-profile. A quantitative model of alteration at the hypha-scale was developed based on solid-state diffusion fluxes of elements into the hypha and the break-down/mineralogical re-arrangement of biotite. A strong acidification was also observed with hypha bound to the biotite surface reaching pH < 4.6. When consistently compared with the abiotic biotite dissolution, we conclude that the surface-bound mycorrhiza accelerate the biotite alteration kinetics between pH 3.5 and 5.8 to ∼0.04 μmol biotite m −2 h −1. Our current work reaffirms that fungal mineral alteration is a process that combines our previously documented bio-mechanical forcing with the μm-scale acidification mediated by surface-bound hypha and a subsequent chemical element removal due to the fungal action. As such, our study presents a first kinetic framework for mycorrhizal alteration at the hypha-scale under close-to-natural experimental conditions.

The major ion, δ 44/40Ca, δ 44/42Ca, and δ 26/24Mg geochemistry of granite weathering at pH = 1 and T = 25 °C: power-law processes and the relative reactivity of minerals

We dissolved Boulder Creek Granodiorite in a plug flow reactor for 5794 h at pH = 1 and T = 25 °C. The primary purpose of the experiment was to identify controls on dissolved δ 44/40Ca, δ 44/42Ca, and δ 26/24Mg values during granite weathering. Herein, we also examine the origin of Ca and Mg isotopic variability among minerals composing the Boulder Creek Granodiorite, and we constrain fundamental characteristics of granite weathering important for quantifying the elemental and isotopic geochemistry of the reactor output. Nine Ca-bearing minerals display an 8.80‰ range of δ 44/40Ca values and a 0.51‰ range of δ 44/42Ca values. Three Mg-bearing minerals display a 1.53‰ range of δ 26/24Mg values. These ranges expressed at the mineralogical scale are higher than the ranges thus far reported for bulk igneous rocks. Most of the δ 44/40Ca variability reflects 40Ca enrichment in K-feldspar, and to a lesser extent, biotite, due to the radioactive decay of 40K over the 1.7 Ga age of the rock, whereas the entire range of δ 44/42Ca values reflects mass-dependent isotope fractionation during igneous differentiation and crystallization. The range of δ 26/24Mg values may represent either fractionation during the chloritization of biotite or interaction of the Boulder Creek Granodiorite with Mg-rich metamorphic fluids having low δ 26/24Mg values. The elemental and isotopic composition of the reactor output varied substantially during the experiment. We synthesize the mineralogical and fluid data using coupled mass-conservation equations solved at non-steady-state. Model calculations reveal an intricate balance between increasing specific surface area and decreasing mineral concentrations. While surface area normalized dissolution rate constants were time-invariant, specific surface area increased as a power-law function of time through positive feedbacks between mechanical disaggregation, chemical dissolution, and mineral depletion. Variations in dissolved δ 44/40Ca, δ 44/42Ca, and δ 26/24Mg values reflect conservative mixing rather than fractionation. Apatite and calcite initially control δ 44/40Ca and δ 44/42Ca values, followed by biotite, titanite, epidote, hornblende, and plagioclase. The release of radiogenic 40Ca clearly defines the period where biotite dissolution dominates. The brucite layer of chlorite initially controls δ 26/24Mg values, followed by biotite, the TOT layer of chlorite, and hornblende. Through direct isotopic tracking, these results demonstrate that trace minerals, such as apatite and calcite in the case of Ca and brucite in the case of Mg, dominate elemental release during the incipient stages of granite weathering. The results further show that biotite dissolution dominates the middle stages of granite weathering and that plagioclase dissolution only becomes important during relatively late stages. The Ca and Mg isotope variations associated with these stages are distinct and potentially resolvable in soil mineral weathering studies.

Biotite–Brine Interactions under Acidic Hydrothermal Conditions: Fibrous Illite, Goethite, and Kaolinite Formation and Biotite Surface Cracking

To ensure safe and efficient geologic CO(2) sequestration (GCS), it is crucial to have a better understanding of CO(2)-brine-rock interactions under GCS conditions. In this work, using biotite (K(Mg,Fe)(3)AlSi(3)O(10)(OH,F)(2)) as a model clay mineral, brine-biotite interactions were studied under conditions relevant to GCS sites (95 °C, 102 atm CO(2), and 1 M NaCl solution). After reaction for 3-17 h, fast growth of fibrous illite on flat basal planes of biotite was observed. After 22-70 h reaction, the biotite basal surface cracked, resulting in illite detaching from the surface. Later on (96-120 h), the cracked surface layer was released into solution, thus the inner layer was exposed as a renewed flat basal surface. The cracking and detachment of the biotite surface layer increased the surface area in contact with solution and accelerated biotite dissolution. On biotite edge surfaces, Al-substituted goethite and kaolinite precipitated. In control experiments with water under the same temperature and pressure, neither macroscopic fibrous illite nor cracks were observed. This work provides unique information on biotite-brine interaction under acidic hydrothermal conditions.

Weathering of Biotite in Acidithiobacillus ferrooxidans Cultures

The purpose of this study was to assess the weathering of biotite under conditions simulating an active bioleaching environment. Finely ground biotite was contacted with Acidithiobacillus ferrooxidans culture solutions for up to 120 days. Biotite was altered under these conditions to interstratified structures comprised of mixed layers of biotite and vermiculite. Interlayer K in biotite was released upon weathering and precipitated with ferric iron and sulfate to form jarosite. Substantial dissolution of biotite was also associated with weathering. The data demonstrate that exposed, biotite-rich rocks in mine tailings and heap and dump leaching systems can undergo acid mediated solubilization and structural changes, which are coupled with the formation of expansive, vermiculite-like minerals and jarosite precipitation.

Hydrothermal alteration of deep fractured granite: Effects of dissolution and precipitation

This paper investigates the mineralogical effects of hydrothermal alteration at depth in fractures in granite. A fracture accompanied by an alteration halo and filled with clay was found at a depth of 200 m in a drill core through Toki granite, Gifu, central Japan. Microscopic observation, XRD, XRF, EPMA and SXAM investigations revealed that the microcrystalline clays consist of illite, quartz and pyrite and that the halo round the fracture can be subdivided into a phyllic zone adjacent to the fracture, surrounded by a propylitic zone in which Fe-phyllosilicates are present, and a distinctive outer alteration front characterized by plagioclase breakdown. The processes that result in these changes took place in three successive stages: 1) partial dissolution of plagioclase with partial chloritization of biotite; 2) biotite dissolution and precipitation of Fe-phyllosilicate in the dissolution pores; 3) dissolution of K-feldspar and Fe-phyllosilicate, and illite precipitation associated with development of microcracks. These hydrothermal alterations of the granite proceed mainly by a dissolution–precipitation process resulting from the infiltration of hydrothermal fluid along microcracks. Such infiltration causes locally high mobility of Al and increases the ratio of fluid to rock in the alteration halo. These results contribute to an understanding of how granitic rock becomes altered in orogenic fields such as the Japanese island arc.

Facies controls on the distribution of diagenesis and compaction in fluvial-deltaic deposits

The Upper Triassic – Lower Jurassic Åre Formation comprising the deeper reservoir in the Heidrun Field offshore mid-Norway consists of fluvial channel sandstones (FCH), floodplain fines (FF), and sandy and muddy bay-fill sediments (SBF, MBF) deposited in an overall transgressive fluvial to lower delta plain regime. The formation has been investigated to examine possible sedimentary facies controls on the distribution of cementation and compaction based on petrography and SEM/micro probe analyses of core samples related to facies associations and key stratigraphic surfaces. The most significant authigenic minerals are kaolinite, calcite and siderite. Kaolinite and secondary porosity from dissolution of feldspar and biotite are in particular abundant in the fluvial sandstones. The carbonate minerals show complex compositional and micro-structural variation of pure siderite (Sid I), Mg-siderite (Sid II), Fe-dolomite, ankerite and calcite, displaying decreasing Fe from early to late diagenetic carbonate cements. An early diagenetic origin for siderite and kaolinite is inferred from micro-structural relations, whereas pore filling calcite and ankerite formed during later diagenesis. The Fe-dolomite probably related to mixing-zone dolomitization from increasing marine influences, and a regional correlatable calcite cemented layer has been related to a flooding event. Porosity values in non-cemented sandstone samples are generally high in both FCH and SBF facies associations averaging 27%. Differential compaction between sandstone and mudstone has a ratio of up to 1:2 and with lower values for MBF. We emphasize the role of eogenetic siderite cementation in reducing compactability in the fine-grained, coal-bearing sediments most prominent in MBF facies. This has implications for modeling of differential compaction between sandstone and mudstones deposited in fluvial-deltaic environments.

Acidification processes and soil leaching influenced by agricultural practices revealed by strontium isotopic ratios

In natural river systems, the chemical and isotopic composition of stream- and ground waters are mainly controlled by the geology and water–rock interactions. The leaching of major cations from soils has been recognized as a possible consequence of acidic deposition from atmosphere for over 30 years. Moreover, in agricultural areas, the application of physiological acid fertilizers and nitrogen fertilizers in the ammonia form may enhance the cation leaching through the soil profile into ground- and surface waters. This origin of leached cations has been studied on two small and adjacent agricultural catchments in Brittany, western France. The study catchments are drained by two first-order streams, and mainly covered with cambisoils, issued from the alteration and weathering of a granodiorite basement. Precipitations, soil water- and NH 4 acetate-leachates, separated minerals, and stream waters have been investigated. Chemical element ratios, such as Ba/Sr, Na/Sr and Ca/Sr ratios, as well as Sr isotopic ratios are used to constrain the relative contribution from potential sources of stream water elements. Based on Sr isotopic ratio and element concentration, soil water- and NH 4 acetate leaching indicates (1) a dominant manure/slurry contribution in the top soil, representing a cation concentrated pool, with low 87Sr/ 86Sr ratios; (2) in subsoils, mineral dissolution is enhanced by fertilizer application, becoming the unique source of cations in the saprolite. The relatively high weathering rates encountered implies significant sources of cations which are not accessory minerals, but rather plagioclase and biotite dissolution. Stream water has a very different isotopic and chemical composition compared to soil water leaching suggesting that stream water chemistry is dominated by elements issued from mineral and rock weathering. Agriculture, by applications of chemical and organic fertilizers, can influence the export of major base cations, such as Na +. Plagioclase dissolution, rather than anthropogenically controlled soil water, seems to be the dominant source of Na + in streams. However, Ca 2+ in streams is mostly derived from slurries and manures deposited on top soils, and transferred into the soil ion-exchange pool and stream waters. Less than 10% of Na +, 5–40% of Sr 2+ and 20–100% of Ca 2+ found in streams can be directly derived from the application of organic fertilizers.

Kinetics of biotite dissolution and Fe behavior under low O 2 conditions and their implications for Precambrian weathering

Biotite dissolution experiments were carried out to better understand the dissolution kinetics and Fe behavior under low O 2 conditions, and to give an insight into the Precambrian weathering. Mineral dissolution with a continuous flow-through reactor was employed at 25 °C for up to 65 days varying partial pressure of atmospheric oxygen ( PO 2), pH (6.86 and 3.01) and Fe content in mineral (1.06 and 0.11 mol of Fe per O 10(OH,F) 2 for biotite and phlogopite, respectively) independently for the examination of their effects on biotite dissolution. Low PO 2 conditions were achieved in a newly developed glove box ( PO 2 ⩽ 6 × 10 −4 atm; referred to as anoxic conditions), which was compared to the present, ambient air conditions (0.2 atm of PO 2; oxic conditions). The biotite dissolution rate was slightly faster under anoxic conditions at pH 6.86 while it was not affected by PO 2 at pH 3.01. There was no direct effect of Fe content on dissolution rate at pH 6.86 while there was a small difference in dissolution rate between biotite and phlogopite at pH 3.01. The 1.5 order-of-magnitude faster release rate of Fe under anoxic conditions for biotite dissolution at pH 6.86 resulted from the difference in ratio of Fe 3+ precipitates remaining in the reactor to Fe dissolved (about 60% and 100% under anoxic and oxic conditions, respectively), which is caused mainly by the difference in PO 2. The results infer that the Fe 2+ and Fe 3+ contents in the Paleoproterozoic paleosols, fossil weathering profiles, are reflected by atmospheric oxygen levels at the time of weathering.

Influence of microorganisms on biotite dissolution: An experimental approach

Microbially assisted dissolution of biotite was studied using four environmentally important species ( Bacillus subtilis, Shewanella putrefaciens, Streptomyces acisdiscabies, and Schizophyllum commune) incubated for 35 days in batch reactors at slightly alkaline pH conditions (ca. pH 9.5). For comparison we performed a control experiment without biological component. Dissolution was monitored by inductively coupled plasma optical emission spectrometry (ICP-OES) analysis of elements (Al, Fe, K, Mg, Mn, Si, and Ti) released into the leach solution. Structural and chemical alterations of biotite were analyzed by a directly evolved gas analysis system (DEGAS) and transmission electron microscopy (TEM) coupled with electron-energy loss spectroscopy (EELS) and energy-dispersive X-ray (EDX) analysis. Our results show that biotite dissolves incongruently with a preferential release of the interlayer cations followed by the octahedral and subsequently the tetrahedral site. Dissolution rates of biotite calculated on the basis of Si release into the solution are similar for abiotic and biotic experiments, on the order of 10 −13 mol/(m 2 s). However, calculated K release rates as well as Mg/Si and K/Si element ratios show a preferential dissolution of these elements from biotite in the presence of microorganisms. For example, in comparison to the control experiment we observe that B. subtilis enhances the Si-normalized Mg and K solute concentrations by about 50%. DEGAS and EDX analyses reveal that the release of K + from the interlayer is associated with an uptake of Na +, H 3O +, and NH 4 +. These substitutions are strongest for biotite exposed to microorganisms, e.g., in experiments with B. subtilis the uptake of NH 4 + is 6 times higher than in the control experiment. Altogether, these results substantiate the considerable influence of microorganisms on the dissolution of biotite.

Silicate mineral dissolution in the presence of acidophilic microorganisms: Implications for heap bioleaching

Silicate minerals are found with sulfide minerals and therefore, can be present during heap bioleaching for metal extraction. The weathering of silicate minerals by chemical and biological means is variable depending on the conditions and microorganisms tested. In low pH metal rich environments their dissolution can influence the solution chemistry by increasing pH, releasing toxic trace elements, and thickening of the leach liquor. The amenity of five silicate minerals to chemical and biological dissolution was tested in the presence of either ‘ Ferroplasma acidarmanus’ Fer1 or Acidithiobacillus ferrooxidans with olivine and hornblende being the most and least amenable, respectively. A number of the silicates caused the pH of the leach liquor to increase including augite, biotite, hornblende, and olivine. For the silicate mineral olivine, the factors affecting magnesium dissolution included addition of microorganisms and Fe 2+. XRD analysis identified secondary minerals in several of the experiments including jarosite from augite and hornblende when the medium contained Fe 2+. Despite acidophiles preferentially attaching to sulfide minerals, the increase in iron coupled with very low Fe 2+ concentrations present at the end of leaching during dissolution of biotite, olivine, hornblende, and microcline suggested that these minerals supported growth. Weathering of the tested silicates would affect heap bioleaching by increasing the pH with olivine, fluoride release from biotite, and production of jarosite during augite and hornblende dissolution that may have caused passivation. These data have increased knowledge of silicate weathering under bioleaching conditions and provided insights into the effects on solution chemistry during heap bioleaching.

Assimilation of Plutonic Roots, Formation of High-K ‘Exotic’ Melt Inclusions and Genesis of Andesitic Magmas at Volcán De Colima, Mexico

Phenocryst-hosted melt inclusions from the 1998^2005 andesite eruptions of Volcan de Colima (Mexico) show broad ranges of major and trace element contents that do not overlap with the bulk- rock compositions and indicate that melt inclusions can be formed by and record a range of processes involved in the genesis of andesites. The melt inclusions that demonstrably record the evolution of the melt feeding the eruption indicate low-pressure (130^10 MPa) crys- tallization of a dacitic melt despite the monotonous bulk andesitic composition of historical magmas at Volcan de Colima. Mingling of dacite melt with gabbroic fragments in the shallow sub-volcanic system is the process responsible for generating the bulk andesitic composition of the magmas. A significant proportion of the melt inclusions have distinctive high large ion lithophile element (LILE) signatures. These 'exotic' high-K melt inclusions in pyrox- enes are thought to result from incongruent melting of interstitial bio- tite during assimilation of gabbroic fragments in the dacitic melt. A second group of exotic high-K melt inclusions found in plagioclase are likely to result from dissolution of higher-pressure (4200 MPa) amphibole, plagioclase, magnetite and biotite cumulates during assimilation in the ascending dacitic melt. Although they are not volumetrically abundant, high-K melts formed during assimilation of plutonic fragments and crystal cumulates made a significant con- tribution to the LILE contents of the magmas and represent a poten- tial source for this group of elements. The range of melt inclusion compositions in Volcan de Colima magmas emphasizes the impor- tance of mixing between ascending evolved melts and crystal popula- tions formed during previous episodes of magmatism over a range of pressures, temperatures and volatile contents. Cannibalization of plutonic roots appears to be a fundamental process in the genesis of andesite magmas and melt inclusions at continental arc volcanoes.

Micromorfologia e gênese de luvissolos e planossolos desenvolvidos de rochas metamórficas no semi-árido brasileiro

Apesar da ocorrência comum de Luvissolos e Planossolos nas áreas de rochas do Pré-Cambriano na região semi-árida do Brasil, há poucas informações disponíveis sobre a micromorfologia e a gênese desses solos. Objetivou-se com este trabalho realizar a caracterização micromorfológica de perfis de solos dispostos em toposseqüências nas quais os Luvissolos são os principais componentes e, com isso, gerar informações para subsidiar a interpretação da sua gênese. Doze perfis de solos, sendo 10 de Luvissolos e dois de Planossolos, distribuídos em quatro toposseqüências, foram selecionados em áreas semi-áridas dos Estados de Pernambuco e da Paraíba. Amostras indeformadas foram coletadas de horizontes selecionados, impregnadas, cortadas e polidas para confecção das seções delgadas, que foram analisadas sob microscópio petrográfico. A descrição das amostras indicou que não há evidências micromorfológicas que sustentem que a argiluviação tenha sido um processo efetivo na formação do gradiente textural dos solos estudados. A remoção preferencial (erosão diferencial) da argila dos horizontes superficiais é apontada como o principal processo atuante na formação do gradiente textural encontrado na maioria dos solos estudados. As principais pedofeições observadas estão relacionadas ao intemperismo dos minerais primários, notadamente da biotita, à dinâmica de formação e dissolução dos compostos de Fe e à reorganização da massa do solo em função das mudanças de umidade do solo decorrentes das alternâncias entre períodos secos e chuvosos.

Characterization and appraisal of facets influencing geochemistry of groundwater in the Kulpawn sub-basin of the White Volta Basin, Ghana

Groundwater composition in the Kulpawn basin is largely controlled by aluminosilicates dissolution and cation exchange resulting in mainly Ca-Mg-HCO3 and NaHCO3 water types. Principal component analysis, Piper graphical classification, and stable isotope (18O and 2H) of groundwater and surface-water samples were used to delineate geochemical processes and groundwater facies. The groundwater is mildly acid to neutral and low in conductivity. Chemical constituents except HCO3− and SiO2 have low concentration. No cation shows clear majority, however, the order of relative abundance is Na+ > Ca2+ > Mg2+ > K+. HCO3− is the predominant anion and the order of abundance is HCO3− > NO3− > SO42− > Cl−. SiO2 concentration is high compared with the major cations. Dissolution of plagioclase, pyroxene and biotite and cation exchange are responsible for groundwater composition. Isotopic data suggest integrative, smooth and rapid recharge from meteoric origin. The groundwater quality is generally good for domestic usage; however, 18 and 47% of boreholes respectively have NO3− and F− levels outside WHO recommended limits suggesting potential physiological problems in some localities. The groundwater has low sodium absorption ratio and low to moderate salinity hazard but significant magnesium hazard partially limiting its use for irrigation.

Chlorite and biotite weathering, Fe2+-rich corrensite formation, and Fe behavior under low PO2 conditions and their implication for Precambrian weathering

Fresh and weathered granite from drill cores in Tono, Gifu, Japan, was examined to understand weathering products and the mechanisms of chlorite and biotite weathering under low P O 2 conditions. A fresh sample from 365 m depth, a slightly weathered light-green sample from 367 m depth, and a nearly fresh sample from 369 m depth (but with brown stains on fractures), were investigated. The XRD, SEM, EMPA, and TEM analysis of green grains present within chlorite, biotite, and plagioclase grains and in veins was found to be Fe 2+ -rich corrensite [about 40 wt% FeO with Fe/(Fe + Mg) = 0.94]. The corrensite is interpreted to have formed from chlorite and biotite via a dissolution-precipitation mechanism. The 2+ /∑Fe value of 0.69, which, when combined with the presence of amorphous Fe 3+ (hydr)oxides confirmed by TEM, indicates that the Fe 2+ /∑Fe value of corrensite is >0.69. These results indicate that on dissolution of chlorite and biotite, Fe 2+ was transported as Fe 2+ and precipitated as Fe 2+ -rich corrensite and a part of the dissolved Fe 2+ was oxidized to amorphous Fe 3+ (hydr)oxides under low P O 2 conditions. The formation of Fe 2+ -rich corrensite and that of Fe 2+ -rich smectite or vermiculite in the laboratory at 1 atm of P CO 2 and ≤3 × 10 −5 atm of P O 2 (Murakami et al. 2004) suggest that a possible Fe 2+ -bearing product during Precambrian weathering is Fe 2+ -rich sheet silicates but not siderite.

Biotite weathering and nutrient uptake by ectomycorrhizal fungus, Suillus tomentosus, in liquid-culture experiments

Ectomycorrhiza-forming fungi (EMF) alter the nutrient-acquisition capabilities of vascular plants, and may play an important role in mineral weathering and the partitioning of products of weathering in soils under nutrient-limited conditions. In this study, we isolated the weathering function of Suillus tomentosus in liquid-cultures with biotite micas incubated at room temperature. We hypothesized that the fungus would accelerate weathering by hyphal attachment to biotite surfaces and transmission of nutrient cations via direct exchange into the fungal biomass. We combined a mass-balance approach with scanning electron microscopy (SEM) and atomic force microscopy (AFM) to estimate weathering rates and study dissolution features on biotite surfaces. Weathering of biotite flakes was about 2–3 orders of magnitude faster in shaken liquid-cultures with fungus compared to shaken controls without fungus, but with added inorganic acids. Adding fungus in nonshaken cultures caused a higher dissolution rate than in inorganic pH controls without fungus, but it was not significantly faster than organic pH controls without fungus. The K +, Mg 2+ and Fe 2+ from biotite were preferentially partitioned into fungal biomass in the shaken cultures, while in the nonshaken cultures, K + and Mg 2+ was lost from biomass and Fe 2+ bioaccumulated much less. Fungal hyphae attached to biotite surfaces, but no significant surface changes were detected by SEM. When cultures were shaken, the AFM images of basal planes appeared to be rougher and had abundant dissolution channels, but such channel development was minor in nonshaken conditions. Even under shaken conditions the channels only accounted for only 1/100 of the total dissolution rate of 2.7 × 10 −10 mol of biotite m −2 s −1. The results suggest that fungal weathering predominantly occurred not by attachment and direct transfer of nutrients via hyphae, but because of the acidification of the bulk liquid by organic acids, fungal respiration (CO 2), and complexation of cations which accelerated dissolution of biotite. Results further suggest that both carbohydrate source (abundant here) and a host with which nutrients are exchanged (missing here) may be required for EMF to exert an important weathering effect in soils. Unsaturated conditions and physical dispersal of nutrient-rich minerals in soils may also confer a benefit for hyphal growth and attachment, and promote the attachment-mediated weathering which has been observed elsewhere on soil mineral surfaces.

High-resolution imaging of biotite dissolution and measurement of activation energy

AbstractWe have used a direct imaging technique, in situ atomic force microscopy(AFM) to observe the earliest stages of the dissolution of a biotite surface byoxalic acid at temperatures close to ambient conditions, using a speciallydesigned AFM liquid cell and non-invasive intermittent contact mode of operation. From the nm-resolution data sets in x, yand z dimensions, we have measured dissolution rates and determined activation energies for the process as a function of temperature, via a mass-loss calculation. The value of Ea obtained, 49±2 kJ mol-1, appears to be too high to indicate a diffusion-controlled process and is more in line with expectations based on a process limited by the rate of ligand-induced metal cation detachment from the (001) surface. This is consistent with visual observations of the relative rates of etch-pit formation and growth, and accepted knowledge of the biotite crystal structure. Separate calculations based on planar area etch-pit growth, and measurements of etch-pit perimeters confirm this result, and also indicate substantiallyhigher activation energy, up to 80 kJ mol-1, when the edge pits are in an incipient stage.

The effects of dissolved oxygen on Fe behavior during dissolution of biotite

The effects of dissolved oxygen on Fe behavior during dissolution of biotite