Analysis of precipitates from reactions of hyperalkaline solutions with soluble silica

Cancrinite, sodalite, and zeolite A have been found to form upon contacting hyperalkaline simulated tank waste (STW) with vadose zone sediments from the Hanford Reservation. Here, soluble silica and STW are used to study mineral formation and transformation. Two Hanford sediment fractions (diameters <50 and >50 μm instead of soluble silica) are also used as silica sources for comparison. A series of batch experiments at 50 °C and 25 days duration were conducted by reacting 0.026 mol/kg soluble Si with 6 different STW solutions. The STW solutions differed in NaOH and Al concentrations. Cancrinite, sodalite, and zeolite A formed when soluble Si was used as the Si source. The minerals were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and ²⁷Al and ²⁹Si magic-angle spinning nuclear magnetic resonance (MAS-NMR). Larger NaOH and Al concentrations favored formation of the more compact structures of cancrinite and sodalite. At larger NaOH concentration more Al for Si substitution occurred in the tetrahedral sites. A greater Al(4)/Al(6) ratio in the solids was found for the higher Si/Al ratio solutions based on NMR results. Mixtures of cancrinite and sodalite were characterized by particles with lepispheric morphology. At low Al concentration, increasing NaOH resulted in distinct hexagonal, prismatic particles common to crystalline cancrinite. At low Al/Si ratio, the characteristic cubic morphology of zeolite was observed in addition to cancrinite and sodalite.     

Vibrational interactions of tetrahedra in silicate glasses and crystals

Normal coordinate calculations, producing synthetic infrared and Raman spectra, were carried out on melilites, pyroxenes, silica polymorphs and feldspars. Atomic motions are complex in the high-frequency Raman modes of melilites and aluminous pyroxenes. The symmetric T-Onb stretching vibrations of Si and Al tetrahedra with different numbers of bridging oxygens are separate from each other, but may combine individually with oscillation of bridging oxygens between Si and Al tetrahedra. The latter type of vibration tends to dominate as Al/Si increases. The frequencies of these vibrational components and the degree of such intermixing depend on T-O force constants, which vary greatly depending on local bonding configurations; individual bands in the high-frequency Raman cannot in general be assigned to single structural entities or fixed combinations thereof. Calculations confirm that in some Al-Si glasses such as jadeite and spodumene, i.e. those in which all Al can be tetrahedral without non-bridging oxygens, Al-O-Al linkages or linkage of more than two tetrahedra by a single oxygen, aluminum is predominantly in tetrahedral coordination. Other Al-Si glasses which are richer in aluminum or which have non-bridging oxygens may contain Al tetrahedral triclusters, non-tetrahedral Al, or both. On the basis of distinctive 450–750 cm^?1 infrared bands, both silica and feldspar glasses resemble tridymite and related stuffed derivatives, not other crystalline silica polymorphs or feldspars. Either these glasses have a structure like that of tridymite on a local scale, or the disorder of the glasses causes drastic modification to the vibrations in question.     

29Si and 27Al magic-angle-spinning NMR studies of the thermal transformation of kaolinite

Thermal transformations of kaolinite of different degree of crystallinity have been monitored by ²⁷Al and ²⁹Si high-resolution NMR with magic-angle spinning (MAS NMR), X-ray diffraction, Fourier transform infrared, atomic absorption spectrophotometry and thermogravimetric analysis. NMR shows differences in the dehydroxylation process of kaolinites with different degree of crystallinity and reveals the presence of short-range order in metakaolinite. ²⁹Si NMR spectra acquired with a 30 s recycle delay of poorly and highly crystalline samples heated at 480 and 500° C, respectively, contain three distinct signals; we discuss their assignment in the light of experiments involving leaching of the samples with aqueous KOH. Ca. 40% of Si sites retain their original Q ³ symmetry just above the onset of dehydroxylation and the Q ⁴ environment is present showing that a small amount of amorphous silica has already segregated. The spectrum of samples treated at 1000° C contains a signal at -110ppm (from Q ⁴ silicons) and a faint resonance, from mullite, at ca. -87 ppm. ²⁹Si NMR also shows that cristobalite germs are already present at 950–1000° C. The ²⁷Al MAS NMR spectra of metakaolinite reveal the presence of 4-, 5-and 6-coordinated Al. Changes in the three Al populations as a function of temperature have been monitored quantitatively. Below 800° C, 4-and 5-coordinated Al appears at the expense of 6-coordinated Al, but above 800° C the amount of 6-coordinated Al increases again. We suggest a dehydroxylation scheme which accounts for the presence of 4-and 5 coordinated Al. Above 900–950° C the latter signal is no longer present in the ²⁷Al NMR spectra and new 4-and 6-coordinated Al species (mullite and γ-alumina) appear. We propose new ideas for the structure of metakaolinite.     

Determining Biogenic Silica in Marine Samples by Tracking Silicate and Aluminium Concentrations in Alkaline Leaching Solutions

This study introduces an alkaline leaching technique for the simultaneous analysis of biogenic silica and aluminium in sediments. Measuring aluminium facilitates the discrimination between silica from the biogenic (BSiO₂) and the non-biogenic fraction, because it originates almost solely from the lithogenic phase. The method was tested using fine-grained silicagel, standard clay minerals, artificial sediments, and natural samples ranging from fresh diatoms to aged sediment from different depositional settings. To determine the BSiO₂ content, four different models each describing the dissolution curves, but of increasing complexity, were applied and for each different type of sample the optimum model was selected on the basis of F-test statistics. For mixtures of silicagel and clay minerals, the contribution of Si from the dissolution of clay was negligible compared to Si originating from silicagel. For natural samples with high clay content, complex dissolution curves were observed and single-phase first order dissolution was the exception. This deviation from `ideal' behavior could only be recognized because of high-resolution sampling, especially in the first 20 minutes of the experiment. For most of the samples, the distinction between the biogenic silica fraction and the silica originating from dissolution of clays could be made on the basis of the Si/Al ratios and reactivity constants of the dissolving phases calculated with the models. Clay minerals typically dissolve slowly at a Si/Al ratio close to 1–2, depending on the type of clay mineral. In contrast, biogenic silica displays a wide range of reactivities and Si/Al ratios. Fresh biogenic silica from the water column usually has a high reactivity and a low Al content. Aged biogenic silica from the sediments has a lower reactivity, but Si/Al ratios as low as 5 were found. The method as described here therefore presents an accurate method to analyze biogenic silica in marine sediments with a relatively high clay mineral content.     

A 29Si NMR study of silica species in dilute aqueous solution

The speciation of silica in dilute, nearly neutral, aqueous solution has been determined by ²⁹Si NMR. Our results indicate that tetrahedral dimers in addition to monomers occur in the concentration range of dissolved amorphous silica.     

Formation of Colloidal Silica and Alumina During Experimental Granodiorite Weathering

The role of aluminum and silica in the formation of colloids during granodiorite weathering was studied on the basis of long-term experiments in batch reactors. Rock samples were dissolved in un-buffered solutions of initial pH 3.2, 5.4, and 9.9 at ambient conditions for 500 days. During weathering, extremely high supersaturation with respect to various secondary solids was attained in the solutions. Consequently, new solids, part of which was conserved in solutions as colloids, condensed. The mean concentrations of colloidal Si reached values of 70, 50, and 48 mol 1^–1 in the alkaline, neutral, and acid solutions, respectively. The mean concentrations of colloidal Al, reached values of 34, 22, and 12 mol 1^–1 in the alkaline, neutral, and acid solutions, respectively. The concentration of colloids gradually decreased after 200-400 days of experiment. This phenomenon was interpreted as being due to the competition between homogeneous nucleation and crystal growth. At the initial stages of the experiments, the colloidal species (predominantly colloidal Al) comprised a large proportion of the total amounts of aqueous species. Their share, however, decreased with time. The molar Al/Si-ratios of colloids were as high as 2–2.5 at the early stages of the experiment. After 250–300 days of experiments, on the other hand, these ratios decreased to values of about 0.5 in both the neutral and alkaline solutions and to a value of 0.15 in the acid solution. The evolution of colloids was consistent with the evolution of secondary solids in the sequence Al-hydroxides – clay minerals (illite, chlorite), in both the neutral and alkaline solutions. In acid solutions, the evolution of Al/Si-colloids was influenced by the presence of sulfate ion and Al-sulfate precipitation. Besides Al and Si, other elements, in particular Ca or Mg as a major component and Na, K, P, S, and Cl as minor components, readily participated in the formation of colloids.     

Coexisting plagioclases from basic charnockites of India

Plagioclases of widely different compositions are found to coexist in basic charnockites of India. This may be due to structural discontinuities or difficult and slow substitution of Si/Al in tetrahedral sites. The presence of hydrous environment in making plagioclases homogeneous is discussed.     

Mineralogy of poorly crystalline aluminium phases in the B horizon of Podzols in southern Sweden

Poorly crystalline Al components of the clay fraction are often neglected in soil mineralogical studies. In this study 7 B horizons from podzolised soils in Sweden were analysed using a combination of infrared (IR) spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD) and selective extractions. It was found that most Bhs and Bs horizons contained allophane, imogolite and more or less hydroxy-interlayered vermiculite. Some Bhs and Bs horizons also contained small amounts of kaolinite and/or gibbsite. In one acid Bh horizon organically complexed Al was the only reactive Al fraction of importance. The vertical patterns of vermiculite and allophane/imogolite suggested that both had formed during the podzolisation process, but due to different mechanisms. The pattern of kaolinite and gibbsite occurrences indicated that these minerals were mostly inherited from the parent material. Oxalate and pyrophosphate extractions suggested that allophane and imogolite constituted the most important reactive inorganic Al fraction in the soils. This shows that allophane and imogolite seem to be the typical, rather than the occasional, reactive inorganic Al phases that form in the B horizon as a result of podzolisation.     

An experimental study on the process of zeolite formation

Results are reported of an experimental study which examined the effect of solution composition on the composition, structure, and crystallization path of phillipsite and merlinoite in the system Na₂OK₂O-Al₂O₃-SiO₂H₂O at 80°C and $\text{pH = 13.34–13.71}$.At a fixed 3.5 M total dissolved silica concentration, zeolite Si/Al ratio was found to be a linear function of pH within the pH range of the experiments.²⁹Si NMR spectra of the initial solutions show that pH determines the distribution of aqueous aluminosilicate species and, as a result, the precipitated zeolite Si/Al ratio. SEM observations reveal that zeolites may precipitate with or without the presence of an intermediary gel phase, depending on solution composition. The growth rate of the zeolites was found to be dependent upon solution pH and total dissolved aluminum concentration. These observations are discussed in terms of their possible applications to natural zeolite paragenesis and serve to delineate the framework of a comprehensive theory for the mechanism of zeolite crystallization from highly alkaline solutions.     

Mechanisms and geochemical significance of Si–Al substitution in zeolite solid solutions

Rock-forming zeolites often exhibit complex solid solutions reflecting isomorphous substitutions between Si and Al in tetrahedral framework sites, between charge-balancing extraframework cations, and between water molecules and vacancies. Although the number of moles of charge on extraframework cations in a zeolite must equal the moles of Al in order to maintain charge balance, the relationships between Si–Al and extraframework substitutions vary considerably across this mineral group. Review of available compositional data suggests that there are three main modes of Si–Al substitution in zeolites: 1) coupled CaAl–NaSi substitution; 2) coupled substitution of a single extraframework cation plus Al for Si; and 3) completely uncoupled substitution among extraframework cations and Si and Al on tetrahedral sites. Among zeolites that exhibit the latter two modes of solid solution, Si–Al substitution can be described by an SiO₂ (± H₂O) compositional exchange vector from a hypothetical, pure-silica endmember composition. Recent calorimetric, structural, and theoretical investigations suggest that Si–Al substitution follows a non-ideal, athermal solution model characterized by no excess enthalpies of mixing and negative excess entropies of mixing. Because Si–Al exchange in these minerals can be explicitly or implicitly described by exchange of an SiO₂ component, the Si/Al ratio in their framework can be predicted solely as a function of temperature, pressure, and the chemical potential of SiO₂. Application of this model leads to calculated Si/Al ratios in stilbite (coexisting with albite), analcime, and chabazite consistent with observed mineral compositions and parageneses in very low-grade metamorphic environments. Coexistence of silica polymorphs with zeolites containing SiO₂·nH₂O exchange vectors potentially provides a means of performing thermobarometric calculations in very low-grade metamorphic and diagenetic environments.     

Unraveling the atomic structure of biogenic silica: evidence of the structural association of Al and Si in diatom frustules

We used X-ray absorption spectroscopy at the Al K-edge to investigate the atomic structure of biogenic silica and to assess the effect of Al on its crystal chemistry. Our study provides the first direct evidence for a structural association of Al and Si in biogenic silica. In samples of cultured diatoms, Al is present exclusively in fourfold coordination. The location and relative intensity of X-ray absorption near-edge structure (XANES) features suggests the structural insertion of tetrahedral Al inside the silica framework synthesized by the organism. In diatom samples collected in the marine environment, Al is present in mixed six- and fourfold coordination. The relative intensity of XANES structures indicates the coexistence of structural Al with a clay component, which most likely reflects sample contamination by adhering mineral particles. Extended X-ray absorption fine structure spectroscopy has been used to get Al-O distances in biogenic silica of cultured diatoms, confirming a tetrahedral coordination. Because of its effect on solubility and reaction kinetics of biogenic silica, the structural association between Al and biogenic silica at the stage of biosynthesis has consequences for the use of sedimentary biogenic silica as an indicator of past environmental conditions.     

伊毛缟石在我国的首次发现及其分析电子显微术研究

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。     

Thermodynamic and kinetic aspects of formation of bauxites

Experiments in which cleavage nepheline samples were reacted with aqueous solutions at fixed pH's and temperature were carried out in the laboratory. The chemistry of the solution as a function of time was monitored, as well as the chemistry of the nepheline surfaces.

At 25°C, Al derived from the nepheline stays in solution due to slow precipitation kinetics of Al(OH)₃. At 60° and 80°C, precipitation of Al(OH)₃ is so rapid that Al concentration in solution is below 0.05 ppm. This indicates that precipitation kinetics favour the formation of bauxite deposits in tropical regions (i.e. T25°C), but not in temperate regions.

Precipitation products on the surface of the nepheline fragments at 60° and 80°C depend on the pH. At pH 3.0, an amorphous aluminium silicate (proto-kaolinite?) is formed. At pH>7.0, the precipitated phase contains, in addition to Al and Si, high amounts of Na and K (proto-muscovite?). The optimum pH for the formation of bauxite is in the range 5–7. These results are in agreement with thermodynamic calculations.     

Stoichiometry of smectite dissolution reaction

The dissolution stoichiometry of smectite-rich bentonites SAz-1, STx-1 and SWy-1 was studied at 50°C and pH 2 and 3 using flow-through reactors. In addition to smectite, these samples contain considerable amounts of silica phases (quartz, cristobalite and/or amorphous silica). As a result, the molar Al/Si ratios of the bulk samples are significantly lower than those of the pure smectite.Smectite dissolution was highly incongruent during the first few hundred to few thousand hours of the experiments. Release rates of Si, Mg, Ca and Na underwent a distinct transition from an initial period of rapid release to significantly lower release rate at steady state. A reversed trend was observed for release of Al, which gradually increased from very low starting release rate to higher release rate at steady state. At steady state the ratio of released Al to released Si was found to be constant and independent of the experimental conditions. We suggest that this ratio represents the Al/Si ratio of the smectite itself, and it is not influenced by the presence of accessory phases in the sample.The rapid release of calcium, sodium and magnesium from the interlayer sites is explained by ion-exchange reactions, whereas the fast release of silicon is explained by dissolution of amorphous silica. We interpret the initial slow release of Al as the result of inhibition of smectite dissolution due to coating or cementation of the smectite aggregates by amorphous silica. As the silica is dissolved, the aggregates fall apart and more smectite surfaces are exposed, resulting in an increase in the smectite dissolution rate. Thereafter, the system approaches steady state, in which the major tetrahedral and octahedral cations of smectite are released congruently.     

Extent of diagenetic transformations in severely altered biogenic silica deposits from Tunisia: new insights from mineralogy and geochemistry

Sedimentary biogenic silica from Redeyef in Gafsa basin (southern Tunisia) was analysed for its ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra and complemented by X-ray diffraction and SEM observations. The ²⁹Si MAS NMR spectrum is characterized by the abundance of hydroxylated silicon, displayed in resonance intensities and reflects a clear tendency towards dissolution of diatomaceous amorphous silica and the occurrence of the hydrated silica, which is the main component that ensures the diagenetic transition via the mechanism of dissolution–precipitation to other more crystalline silica phases, after the lost of its hydroxyls groups (water) by heating (burial). ²⁷Al MAS NMR reveals two coordinations of Al; the octahedrally coordinated Al suggests the presence of clay relics trapped during crystal growth or a microcrystalline zeolite (clinoptilolite detected by SEM observations), while the tetrahedrally coordinated Al suggests the presence of minor quantities of minerals with tetrahedral Al, such as an Al-rich fluid and/or minerals such as feldspars.     

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.     

An X-ray absorption fine structure and nuclear magnetic resonance spectroscopy study of gallium-silica complexes in aqueous solution

The influence of aqueous silica on gallium(III) hydrolysis in dilute (2 × 10⁻⁴ ≤ m_Ga ≤ 5 × 10⁻³) and moderately concentrated (0.02 ≤ m_Ga ≤ 0.3) aqueous solutions was studied at ambient temperature, using high resolution X-ray absorption fine structure (XAFS) and nuclear magnetic resonance (NMR) spectroscopies, respectively. Results show that, in Si-free acidic solutions (pH < 3), Ga is hexa-coordinated with oxygens of H₂O molecules and/or OH groups in the first coordination sphere of the metal. With increasing pH, these hydroxyl groups are progressively replaced by bridging oxygens (-O-), and polymerized Ga-hydroxide complexes form via Ga-O-Ga chemical bonds. In the 2.5-3.5 pH range, both XAFS and NMR spectra are consistent with the dominant presence of the Ga₁₃ Keggin polycation, which has the same local structure as A1₁₃. Under basic pH (pH > 8), Ga exhibits a tetrahedral coordination, corresponding to Ga(OH)₄⁻ species, in agreement with previous NMR and potentiometric studies. Major changes in Ga hydrolysis have been detected in the presence of aqueous silica. Ga is tetra-coordinated, both in basic and acid (i.e., at pH > 2.7) Si-bearing solutions (0.01 ≤ m_Si ≤ 0.2), and forms stable gallium-silicate complexes. In these species, Ga binds via bridging oxygen to 2 ± 1 silicons, with an average Ga-Si distance of 3.16 ± 0.05 Å, and to 2 ± 1 silicons, with an average Ga-Si distance of 3.39 ± 0.03 Å. These two sets of Ga-Si distances imply the formation of two types of Ga-silicate aqueous complex, cyclic Ga-Si_2-3 species (formed by the substitution of Si in its tri-, tetra- or hexa-cyclic polymers by Ga atoms), and chainlike GaSi_2-4 species (similar to those found for A1), respectively. The increase in the number of Si neighbors (a measure of the complex concentration and stability), in alkaline media, with increasing SiO₂(aq) content and decreasing pH is similar to that for A1-Si complexes found in neutral to basic solutions. At very acid pH and moderate silica concentrations, the presence of another type of Ga-Si complex, in which Ga remains hexa-coordinated and binds to the silicon tetrahedra via the GaO₆ octahedron corners, has also been detected. These species are similar to those found for Al³⁺ in acid solutions. Thus, as for aluminum, silicic acid greatly hampers Ga hydrolysis and enhances Ga mobility in natural waters via the formation of gallium-silicate complexes.     

Modeling the dissolution behavior of standard clays in seawater

The present investigation tests a model to explain the behavior of dissolved Si during early diagenesis in sediments. The model assumes that low-Fe clays and other minerals can be treated simply as Al hydroxides, having attached silica. When the minerals are placed in seawater solutions, Si is released, causing exposure of fresh Al-octahedra. which are reactive toward Si and other elements in solution. Standard clays (kaolinite, montmorillonite) and solid silicic acid were suspended in seawater solutions in various combinations and dissolved Al, Si, Ca, pH and alkalinity were determined as a function of time. The theoretical model correctly predicts the behavior of Si in mixtures of the different minerals, based upon the dissolution behavior of the minerals in suspension alone. Further, the decrease in the alkalinity of kaolinitecontaining seawater solutions, where carbonate dissolution, organic matter decomposition and reduced sulfur oxidation are apparently unimportant, can be predicted from a simple extrapolation of the silica model results. The alkalinity changes observed in this study, as well as the pH changes observed in standard clay suspensions by other researchers, can be explained simply by equilibration of the charge on exposed Al-octahedra with the pH of the surrounding waters. The results of this study indicate that theoretical models of Si diagenesis in sediments should have general usefulness for predicting the effects of clay dissolution on sediment properties.     

云南个旧似长石正长岩类及其岩石成因

云南个旧碱性岩体主要的岩石类型有碱性正长岩和似长石正长岩,其中,似长石正长岩中出现大量似长石矿物霞石、方钠石和碱性暗色矿物。本文根据矿物成分及特征,将这些似长石正长岩进一步划分为黑榴霞石方钠正长岩、霞石方钠正长岩、霞石正长岩及方钠霞石正长岩4类。岩石地球化学结果表明,4类岩石的地球化学行为整体表现出过碱质岩的特征,K2O+Na2O含量很高,为钾玄岩系列,同时表现出钾质的特点。分异指数高,呈现高度分异演化特点。稀土元素变化大,轻重稀土元素分异明显,富集轻稀土元素,亏损重稀土元素。微量元素富集大离子亲石元素Th、U及Zr、Hf等高场强元素,亏损Ba、Sr大离子亲石元素,而P和高场强元素Ta、Ti亏损,同时Cr、Co、Ni含量非常低,具有中等的负Eu异常和微弱的负Ce异常。研究表明,似长石正长岩在岩浆演化过程中表现出明显的分离结晶作用特征,且岩浆起源温度较高,约为835℃,起源较深。个旧似长石正长岩为A型岩套A1亚型,结合构造判别图解,认为其可能来自角闪石或者金云母相矿物存在的富集地幔,形成于燕山晚期伸展的构造背景,岩浆在较高温度下高度结晶分异,并在侵位过程中伴随陆壳成分的混染。     

Incorporation of silica into the goethite structure: a microscopic and spectroscopic study

Quartz and iron (hydr)oxide are reactive surface phases that are often associated with one another in soils and sediments. Despite the several studies on the coating of quartz with iron oxides, the reactivity of dissolved species (Si) leached from quartz with iron (hydr)oxides has received limited attention. In this study, goethite synthesized on quartz substrates were characterized using field emission scanning electron microscopy, X-ray diffraction (XRD), transmission electron microscopy, and Fourier-transform infrared (FT-IR) spectroscopy. The SEM characterization revealed that bundles of thin parallel aligned goethite rods were formed at pH?>?10, while large pseudohexagonal crystals of twinned goethite needles were synthesized at pH?≤?10 after dehydration and hydration in the alkaline media. TEM analysis showed expanded and distorted lattice spacing of the crystal structure of iron (hydr)oxide due to silica incorporation. The characterization showed that silica increased the crystallite size of the goethite and transformed its acicular texture to a larger, twinned needle structure. FT-IR and XRD analyses revealed band shifts in crystal bonds as well as new bond formations, which indicate the presence of changes in the chemical environment of Fe–O and Si–O bonds. Thus, the presence of sorbed silicates modifies the crystal and lattice structure of goethite.