Iron oxide incrustations in wells. Part 2: chemical dissolution and modeling

The formation of Fe oxide and Mn incrustations seriously affects the performance of wells, piezometers and drains. Chemical dissolution can be a valuable tool for their removal. Standardized dissolution experiments were performed to study the efficiency of different chemicals on synthetic Fe oxides. They showed that buffered Na-dithionite and oxalic acid are the most effective agents, followed by ascorbic, malonic and sulfamic acid. Citric acid and especially NaOH proved to be ineffective. As expected, a strong dependency of the dissolution rates on the surface area of the individual Fe oxides was observed, goethite being far less soluble than ferrihydrite. In many cases linear, zeroeth order rate laws were sufficient to model the measured dissolution curves. In other cases more advanced model approaches had to be applied. Sometimes different models were equally well suited to describe a dissolution process.     

Fe(Ⅱ)/铁氧化物表面结合铁系统还原有机污染物的研究进展

在土壤和沉积物的自然厌氧环境中,铁氧化物可被铁还原菌等微生物异化还原产生Fe(Ⅱ),形成的Fe(Ⅱ)/铁氧化物表面结合铁系统具有还原活性,可使有机污染物还原转化。综述了含卤和含硝基有机污染物的非生物还原转化过程和表面结合铁系统与有机污染物之间的界面反应机理,进而揭示了污染物在环境中的赋存状态和迁移转化规律;重点分析了影响该还原过程的因素,如铁氧化物类型、pH值、Fe(Ⅱ)与铁氧化物接触时间,以及过渡金属、腐殖酸等竞争因子对反应过程的影响。强化自然界中天然的Fe(Ⅱ)/铁氧化物表面结合铁系统在有机污染治理中的作用,在受污染环境修复领域具有广阔的应用前景。 [HT5H]关 键 词:[HT5K]     

Sequential extraction of labile elements and chemical characterization of a basaltic soil from Mt. Meru,Tanzania

We conducted a modified Bureau Commun Reference (BCR) sequential extraction on a basaltic soil (phono-tephrite) from Mt. Meru in Northern Tanzania in order to determine the relative contribution of water soluble, carbonate and exchangeable, oxide and organic fractions to the bulk composition of the soil. Elemental compositions were determined by ICP-MS and corrected for loss on ignition. Relatively immobile elements, such as Zr, Hf and Al, are enriched by 10–30% compared to the unweathered protolith, consistent with soil formation being accompanied by mass loss due to chemical weathering. However, superimposed on this mass loss appears to be enrichment of elements such as Fe, Ca and Mg, especially towards the surface. In some cases, the bulk concentrations of these elements at the surface exceed that of the protolith. These data suggest that the surface of the Meru soil columns may have experienced “re-fertilization” by the deposition of volcanic ash. From the carbonate and exchangeable extraction, we found evidence of clay rich horizons which may sequester as much as 5% of the bulk K. The concentration of calcium carbonate appears to decrease with depth, but the largest incorporation of Sr and Ba into carbonates occurs below 114 cm. Fe and Mn oxides scavenge more than 10–20% of total Ti, V, Co, Cu, Zr and Pb below 114 cm. The organic fraction sequestered significant fractions of total Al, Cu, REE’s and Pb throughout the soil column.     

Dependence of microbial magnetite formation on humic substance and ferrihydrite concentrations

Iron mineral (trans)formation during microbial Fe(III) reduction is of environmental relevance as it can influence the fate of pollutants such as toxic metal ions or hydrocarbons. Magnetite is an important biomineralization product of microbial iron reduction and influences soil magnetic properties that are used for paleoclimate reconstruction and were suggested to assist in the localization of organic and inorganic pollutants. However, it is not well understood how different concentrations of Fe(III) minerals and humic substances (HS) affect magnetite formation during microbial Fe(III) reduction. We therefore used wet-chemical extractions, magnetic susceptibility measurements and X-ray diffraction analyses to determine systematically how (i) different initial ferrihydrite (FH) concentrations and (ii) different concentrations of HS (i.e. the presence of either only adsorbed HS or adsorbed and dissolved HS) affect magnetite formation during FH reduction by Shewanella oneidensis MR-1. In our experiments magnetite formation did not occur at FH concentrations lower than 5 mM, even though rapid iron reduction took place. At higher FH concentrations a minimum fraction of Fe(II) of 25-30% of the total iron present was necessary to initiate magnetite formation. The Fe(II) fraction at which magnetite formation started decreased with increasing FH concentration, which might be due to aggregation of the FH particles reducing the FH surface area at higher FH concentrations. HS concentrations of 215-393 mg HS/g FH slowed down (at partial FH surface coverage with sorbed HS) or even completely inhibited (at complete FH surface coverage with sorbed HS) magnetite formation due to blocking of surface sites by adsorbed HS. These results indicate the requirement of Fe(II) adsorption to, and subsequent interaction with, the FH surface for the transformation of FH into magnetite. Additionally, we found that the microbially formed magnetite was further reduced by strain MR-1 leading to the formation of either dissolved Fe(II), i.e. Fe²⁺, in HEPES buffered medium or Fe(II) carbonate (siderite) in bicarbonate buffered medium. Besides the different identity of the Fe(II) compound formed at the end of Fe(III) reduction, there was no difference in the maximum rate and extent of microbial iron reduction and magnetite formation during FH reduction in the two buffer systems used. Our findings indicate that microbial magnetite formation during iron reduction depends on the geochemical conditions and can be of minor importance at low FH concentrations or be inhibited by adsorption of HS to the FH surface. Such scenarios could occur in soils with low iron mineral or high organic matter content.     

陕南凤凰岭南缘变质镁铁质—超镁铁质侵入岩矿物学特征——对Fe-Ti 成矿作用的指示

陕南凤凰岭南缘变质镁铁质—超镁铁质侵入岩为富含Fe、Ti等有用元素的含矿岩体。利用岩石地球化学方法、偏光显微镜和电子探针技术对该区镁铁质—超镁铁质侵入岩的岩石学、地球化学和矿物学特征进行了研究,并对Fe-Ti氧化物的形成、富集进行了初步探讨,对成岩、成矿作用具有一定的参考意义。结果表明,成矿镁铁质—超镁铁质侵入岩与非成矿镁铁质—超镁铁质侵入岩为不同岩浆源区、同期岩浆作用的产物,镁铁质—超镁铁质侵入岩类的Fe-Ti矿化作用与初始岩浆的源岩之间具有密切关系。Fe-Ti氧化物主要以钛铁矿和榍石的形式存在,局部见少量金红石。化学成分上,钛铁矿具富锰贫镁特征,本区Fe-Ti氧化物的形成和富集过程经历了岩浆结晶分异阶段和热液蚀变阶段。     

Interpreting nanoscale size-effects in aggregated Fe-oxide suspensions: Reaction of Fe(II) with Goethite

The Fe(II)/Fe(III) redox couple plays an important role in both the subsurface fate and transport of groundwater pollutants and the global cycling of carbon and nitrogen in iron-limited marine environments. Iron oxide particles involved in these redox processes exhibit broad size distributions, and the recent demonstrations of dramatic nanoscale size-effects with various metal oxides has compelled us, as well as many others, to consider whether the rate and extent of Fe(II)/Fe(III) cycling depends upon oxide particle size in natural systems. Here, we investigated the reaction of Fe(II) with three different goethite particle sizes in pH 7.5 suspensions. Acicular goethite rods with primary particle dimensions ranging from 7 by 80 nm to 25 by 670 nm were studied. Similar behavior with respect to Fe(II) sorption, electron transfer and nitrobenzene reduction was observed on a mass-normalized basis despite almost a threefold difference in goethite specific surface areas. Scanning electron microscopy (SEM) images, dynamic light scattering (DLS) and sedimentation measurements all indicated that, at pH 7.5, significant aggregation occurred with all three sizes of goethite particles. SEM images further revealed that nanoscale particles formed dense aggregates on the order of several microns in diameter. The clear formation of particle aggregates in solution raises questions regarding the use of primary particle surface area as a basis for assessing nanoscale size-effects in iron oxide suspensions at circum-neutral pH values. In our case, normalizing the Fe(II) sorption densities and rate constants for nitrobenzene reduction by BET surface area implies that goethite nanoparticles are less reactive than larger particles. We suspect, however, that aggregation is responsible for this observed size-dependence, and argue that BET values should not be used to assess differences in surface site density or intrinsic surface reactivity in aggregated particle suspensions. In order to realistically assess nanoscale size-effects in environmentally relevant systems that are likely to aggregate, new methods are needed to quantify the amount of surface area accessible for sorption and reaction in wet nanoparticle suspensions, rather than assuming that this value is equivalent to the surface area determined from the characterization of dry nanoparticles.     

Manifestation of nitrogen interstitials in synthetic diamonds obtained using a temperature gradient technique (Fe–Ni–C system)

The IR-peak 1450 cm^–1 (H1a-center) associated with nitrogen interstitials have been studied in nitrogen-bearing diamonds synthesized at high P-T parameters in the Fe–Ni–C system. FTIR study shows that manifestation of this nitrogen form is restricted to the regions of active transformation of C-defects into A-defects, which confirms the connection of its formation with C => A aggregation process. An examination of the dependence of the 1450 cm^–1 peak on the degree of nitrogen aggregation indicates that H1a-centers are not only formed during C/A aggregation but also disappear simultaneously with the end of C => A transformation. Established facts suggest direct involving of nitrogen as interstitials in the C => A aggregation and serve as strong experimental argument in support of the “interstitial” mechanism of nitrogen migration during aggregation in diamonds containing transition metals.     

A surface-chemical study of adsorption of flotation collectors on a Swedish hematite

Heats of adsorption of sodium dodecyl sulphate and sodium dodecanoate on to finely-divided natural hematite have been determined by calorimetry, and the results interpreted in relation to the adsorption isotherm, the titration curve and zeta-potentials of the hematite. In general, the data are consistent with the surface association (“hemi-micelle”) theory.As well as showing surface “ageing”, the ground hematite is chemically heterogeneous, its adsorptive activity depending considerably on the method of dry grinding employed in preparing the powder. Reactivity is depressed by pre-heating to 300°C, suggesting that hydrated sites are the most reactive towards anionic surfactants, although the whole surface is covered on completion of the hemimicelle layer.     

Managanese and iron in the White Sea: Sedimentation and diagenesis

Iron and manganese in bottom sediments studied along the sublatitudinal transect from Kandalaksha to Arkhangelsk are characterized by various contents and forms depending on sedimentation environments, grain size of sediments, and diagenetic processes. The latter include redistribution of reactive forms leading to enrichment in Fe and Mn of the surface sediments, formation of films, incrustations, and ferromanganese nodules. Variations in the total Fe content (2–8%) are accompanied by changes in the concentration of its reactive forms (acid extraction) and the concentration of dissolved Fe in the interstitial water (1–14 μM). Variations in the Mn content in sediments (0.03–3.7%) and the interstitial water (up to 500 μM) correspond to a high diagenetic mobility of this element. Changes in the valence of chemical elements results in the redox stratification of sediment strata with maximum concentrations of Fe, Mn, and sulfides. Organic matter of sediments with a considerable terrestrial constituent is oxidized by bottom water oxygen mainly at the sediment surface or in anaerobic conditions within the sediment strata. The role of inorganic components in organic matter oxidation changes from surface sediments, where manganese oxyhydroxide dominates among oxidants, to deeper layers, where sulfate of interstitial water serves as the main oxidant. Differences in river runoff and hydrodynamics are responsible for geochemical asymmetry of the transect. The deep Kandalaksha Bay serves as a sediment trap for manganese (Mn content in sediments varies within 0.5–0.7%), whereas the sedimentary environment in the Dvina Bay promotes its removal from sediments (Mn 0.05%).     

Monohydrocalcite,hydromagnesite, nesquehonite,dolomite, aragonite,and calcite in speleothems of the Fränkische Schweiz,Western Germany

The formation of calcite, aragonite, dolomite, hydromagnesite, and nesquehonite in speleothems of a small cave within dolostone may be compared with the formation of these carbonate minerals in much larger environments in which evaporation exceeds precipitation (salt lakes, tropical lagoons, sebkhas etc...). In all cases the formation and diagenetic alteration is mainly governed by the Mg/Ca-ratio of the solutions, from which precipitation (or transformation) takes place.The certain occurrence of monohydrocalcite in the speleothems and in technical incrustations lead to the conclusion, that this very rare mineral precipitates from solutions which are transported as aerosols.     

Lateritization cycles and their relation to the formation and quality of kaolin deposits in south Kerala, India

Laterite occurs extensively over the crystalline and sedimentary rocks in the midland and lowland areas of south Kerala, India. Two lateritization cycles are identified in this area. Large, good-quality kaolin deposits, composed mostly of kaolinite, are characteristic of the sedimentary sequence in south Kerala. These deposits were formed on deposition of the weathering materials of the khondalites towards the first cycle of lateritization. After deposition and uplift of the sedimentary rocks, another lateritization cycle affected these, as well as the khondalites during pre-Quaternary times with the formation of a planation surface at 25–125 m above sea level having thick laterite profiles. The laterite profiles over the kaolin deposits show higher concentration of Fe-oxides (mostly in the form of hematite) and titania, compared to their concentration in the kaolins. Higher contents of Cr and Ni are also characteristic of the laterite over kaolin deposits. Recrystallization of the kaolinite, appearance of Al, Fe and Si amorphous phases in the kaolin clays and partial removal of Fe and Ti from them are attributed to the second lateritization cycle.     

Genesis of variegated redbeds in the fluvial Aztec Siltstone (late devonian), southern victoria land,Antarctica

The Aztec Siltstone (Late Devonian) formed in a meandering fluviatile environment and is a variegated redbed sequence consisting of red or drab overbank mudstones and drab channel sandstones, commonly arranged in fining-upwards cycles. The pigmentation in the red mudstones is caused by euhedral cryptocrystalline haematite which is dispersed throughout the matrix, masking the green colour of the underlying illitic and chloritic clays. Textural, mineralogical and chemical analyses suggest that the pigment formed in situ by dehydration and crystallization of a detrital, amorphous or poorly crystalline, yellow-brown ferric oxide precursor. The hydrated precursor apparently was derived from the weathering of iron silicates in a source area regolith, and transported in suspension with the Aztec alluvium, commonly by attachment onto surfaces of clay platelets. The association of the precursor with the clays is a contributing reason for the absence of red sandstones in the formation.Dehydration and crystallization of the yellow-brown ferric oxide to form the red haematite pigment occurred prior to deep burial of the host sediment. At least part of it occurred before burial, in an oxidising environment in well-drained and well-aerated overbank soils which were exposed for prolonged periods of time (4,000–10,000 years) under a hot and at least seasonally arid climate. Crystallization of the red pigment may have been completed during early diagenesis.Reducing conditions, induced by the presence of organic debris in the sediment and a relatively high water table, affected much of the deposit by removing the “free” hydrated ferric oxide, either partially or completely, from the in-channel sands, the low-lying and poorly drained overbank sediments, and the lacustrine sediments. This reduction was both a syndepositional and an early post-depositional process.Iron analyses indicate that, on average, the red mudstones are enriched in total iron (Fe) and Fe³⁺ by 2%, all of which is attributable to the inherited “free” ferric oxide. There is no other significant difference in whole-rock mineralogy or major-element chemistry between the red and non-red mudstones.     

Metacolloid pyrite-pitchblende veinlets of high-grade hydrothermal ores at the Dalmatovskoe uranium deposit,Transural Region,Russia: New data on the mineralogy,geochemistry, age,and uranium sources

Micro to nano-scale structures of pockets of solidified uranium oxide gel were found for the first time and deciphered ontogenetically during electron-microscopic mineralogical study of four samples of high-grade uranium ores from the Dalmatovskoe deposit. Mineralogical-geochemical features of paragenesis and structural features are reported for unique hydrothermal veinlets consisting of variably crystallized gelpyrite and subveinlets of gel-pitchblende of several generations. Mineralogical features indicate repeated opening of veinlets during hydrothermal ore formation and provide insight into physical and chemical features of non-solidified gel, its mobility, and aggressive behavior during metasomatism. It was proposed that the veinlets are genetically related to the main hydrothermal sources and possibly to the endogenous uranium occurrence, which was formed owing to opening of main fault marked by paleoriver bed. The find of such veinlets is considered as evidence for repeated influx of hydrothermal uranium in the uranium ores of the sedimentary-epigenetic deposit simultaneously with accumulation and transformation of sediments, which made it possible to ascribe this deposit to the hydrothermal-sedimentary type. The depth of core sampling of high-grade ores corresponds to the “ore zone” of the Dalmatovskoe Deposit, near the Paleozoic basement. For this reason, the entire “zone” was ascribed to the specific endogenous metasomatic epihydrothermal occurrences, possibly, of elision nature, with approximate temperature of ore formation of 150–250°C.     

The Association of Cobalt with Iron and Manganese (Oxyhydr)oxides in Marine Sediment

Formation and dissolution of authigenic Fe and Mn (oxyhydr)oxides influence cycling of trace metals in oxic/suboxic surface sediments. We used the diffusive gradients in thin films technique (DGT) to estimate the association of cobalt with iron and manganese oxides. We compared Co, Fe and Mn maxima measured by DGT in the pore waters of fresh and aged marine sediment cores and estimated the Co/Fe and Co/Mn ratios in the metal oxides. A Mn maximum was not visible in DGT concentration profiles of freshly collected sediment cores, but after ageing the sediment, we observed a distinct Mn peak, presumably due to broadening of the depth range over which the various electron acceptors occur. Estimated Co/Mn ratios from both experiments are within the range of literature values for marine sediments, but the value from the aged experiment is at the lower end of the range. This is attributed to stimulation of sulphate reduction and precipitation of cobalt sulphides. The good correlation between Co and Fe maxima in the fresh sediments is attributed to the similarity of their reactions with sulphide rather than Co being released during authigenic Fe oxide reduction.     

THE PRESENT STATUS OF ZIRCON

Saxena (1966a) proposed that zircon could be authigenic and low-grade metamorphic, as well as magmatic in origin. He questioned the mechanical and chemical stability of zircon, the use of crystal morphology in correlation, and the use of roundness as a criterion of detrital origin. In so doing, he challenged the role of zircon in problems of stratigraphy, pedogenesis, and sedimentary, igneous, and metamorphic petrology. SAXENA'S(1966a, b) evidence for authigenic and low-grade metamorphic zircon is unconvincing, since conventional explanations adequately encompass his obser- vations, and do not conflict with established knowledge of zircon behaviour. During weathering, authigenesis, and metamorphism, zircon is shown to conform with the concepts that: (1) its removal and transport in solution is minimal, (2) zircon formation during authigenesis and low-grade metamorphism is insignificant. Rare occurrences of hydrozircon confirm this paucity of authigenic zirconium minerals. At high metamorphic grades zircon-transformation is favoured by the “wet” metasomatic processes of the amphibolite facies and impaired by the “dryness” of the granulite facies. The occurrence of zircon transformation in the granulite facies may also be prevented by incorporation of zirconium in the lattice of newly-formed pyroxene. Release of this zirconium during diaphthoresis of pyroxene-bearing granulites could give rise to new zircon. The formation of zircon during authigenesis, and its formation and transfor- mation during low-grade metamorphism, would diversify initially homogeneous assemblages; but Saxena has failed to prove that either process does in fact occur. It is contended that, since the positive correlation of zircon assemblages from co-eval sedimentary and metamorphic rocks of the Kingsbridge area is opposed to Saxena's views regarding the origin and behaviour of zircon, the roles of authigenesis in the formation of zircon, and of low-grade metamorphism in its formation and transformation, are insignificant or non-existent.     

Metallogenic Model of Bauxite in Central Guizhou Province: an Example of Lindai Deposit

The bauxites in central Guizhou are hosted by the Lower Carboniferous Jiujialu Formation.Geochemistrial characteristics of the Lindai bauxite deposit indicate that the underlying Shilengshui Formation dolomite is the precursor rock of mineral resources.Weathering simulation experiments show that Si is most likely to migrate with groundwater,the migration rate of which is several magnitude higher than Al and Fe under nature conditions （pH=3-9）.The neutral and acid nonreducing condition is the most conducive to the Al rich and Si removal,while the acid reducing conditions is the most conducive to the Al rich and Fe removal.In the process of bauxite formation,coal beds overlying the Al-bearing rock series or other rock formation rich in organic materials can produce acid reducing groundwater,which are important for the bauxite formation.Finally,propose the metallogenic model of the bauxite in central Guizhou Province and put forward three new words which are ＂original bauxite material＂,＂bauxite material＂ and ＂original bauxite＂.     

Mössbauer hyperfine parameters of iron species in the course of <Emphasis Type="Italic">Geobacter</Emphasis>-mediated magnetite mineralization

Amorphous ferric iron species (ferrihydrite or akaganeite of <5 nm in size) is the only known solid ferric iron oxide that can be reductively transformed by dissimilatory iron-reducing bacteria to magnetite completely. The lepidocrocite crystallite can be transformed into magnetite in the presence of abiotic Fe(II) at elevated pH or biogenic Fe(II) with particular growth conditions. The reduction of lepidocrocite by dissimilatory iron-reducing bacteria has been widely investigated showing varying results. Vali et al. (Proc Natl Acad Sci USA 101:16121–16126, 2004) captured a unique biologically mediated mineralization pathway where the amorphous hydrous ferric oxide transformed to lepidocrocite was followed by the complete reduction of lepidocrocite to single-domain magnetite. Here, we report the ⁵⁷Fe Mössbauer hyperfine parameters of the time-course samples reported in Vali et al. (Proc Natl Acad Sci USA 101:16121–16126, 2004). Both the quadrupole splittings and linewidths of Fe(III) ions decrease consistently with the change of aqueous Fe(II) and transformations of mineral phases, showing the Fe(II)-mediated gradual regulation of the distorted coordination polyhedrons of Fe³⁺ during the biomineralization process. The aqueous Fe(II) catalyzes the transformations of Fe(III) minerals but does not enter the mineral structures until the mineralization of magnetite. The simulated abiotic reaction between Fe(II) and lepidocrocite in pH-buffered, anaerobic media shows the simultaneous formation of green rust and its gradual transformation to magnetite plus a small fraction of goethite. We suggested that the dynamics of Fe(II) supply is a critical factor for the mineral transformation in the dissimilatory iron-reducing cultures.     

Impact of chemical clogging on de-watering well productivity: numerical assessment

Among the processes leading to a decrease in productivity, chemical clogging is often mentioned as one of the major features. De-watering of a confined aquifer caused by an unsuitable pumping scheme produces a phenomenon involving the diffusion of oxygen in the aquifer which disturbs the geochemical conditions in the initial system. Coupled chemical and transport processes are proposed in an assessment of the impact of de-watering on the precipitation of carbonate and iron oxide. The reactions are studied for waters showing low dissolved iron concentrations such as commonly observed in drinking water supplies. The quantity and distribution of precipitated iron oxide and calcium carbonate are used in a permeability model to calculate the productivity loss. For the conditions used in the simulations, the carbonate precipitate can be neglected compared to iron deposits which remain weak. The spatial distribution is heterogeneous and quite similar to the patterns observed in the field. This shape is mainly caused by a competition between the diffusion of oxygen due to the de-watering process and the rate of precipitation of iron oxide. However, the loss of well productivity remains moderate. It is clearly shown that de-watering of the well and the associated chemical incrustations that this induces cannot alone explain field data. More complex processes involving biological clogging and accurate hydrodynamic behaviour in the closest part of the well remain to be included in the modelling approach in order to provide valuable insights into the problem of well ageing.     

鄂尔多斯盆地东缘煤层气勘探开发历程与启示

鄂尔多斯盆地东缘煤层埋深变化较大,不同埋深的煤层气成藏特征及储层改造方式差别较大.目前煤层气勘探开发深度逐渐从1000 m以浅延伸到2000 m以深,为了研究不同埋深条件下煤层气资源高效勘探开发理论技术,系统梳理回顾了鄂尔多斯盆地东缘近30年的勘探开发实践,按照地质认识转变、技术发展、勘探工作量、勘探成果和产气量变化,...     

The effect of Fe on Si adsorption by Bacillus subtilis cell walls: insights into non-metabolic bacterial precipitation of silicate minerals

Si adsorption onto Bacillus subtilis and Fe and Al oxide coated cells of B. subtilis was measured both as a function of pH and of bacterial concentration in suspension in order to gain insight into the mechanism of association between silica and silicate precipitates and bacterial cell walls. All experiments were conducted in undersaturated solutions with respect to silicate mineral phases in order to isolate the important adsorption reactions from precipitation kinetics effects of bacterial surfaces. The experimental results indicate that there is little association between aqueous Si and the bacterial surface, even under low pH conditions where most of the organic acid functional groups that are present on the bacterial surface are fully protonated and neutrally charged. Conversely, Fe and Al oxide coated bacteria, and Fe oxide precipitates only, all bind significant concentrations of aqueous Si over a wide range of pH conditions. Our results are consistent with those of Konhauser et al. [Geology 21 (1993) 1103; Environ. Microbiol. 60 (1994) 49] and Konhauser and Urrutia [Chem. Geol. 161 (1999) 399] in that they suggest that the association between silicate minerals and bacterial surfaces is not caused by direct Si–bacteria interactions. Rather, the association is most likely caused by the adsorption of Si onto Fe and Al oxides which are electrostatically bound to the bacterial surface. Therefore, the role of bacteria in silica and silicate mineralization is to concentrate Fe and Al through adsorption and/or precipitation reactions. Bacteria serve as bases, or perhaps templates, for Fe and Al oxide precipitation, and it is these oxide mineral surfaces (and perhaps other metal oxide surfaces as well) that are reactive with aqueous Si, forming surface complexes that are the precursors to the formation of silica and silicate minerals.