Acta Geochim (2018) 37(6):911–921 https://doi.org/10.1007/s11631-018-0267-6

ORIGINAL ARTICLE

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

1 1 1 Abdullah Musa Ali • Eswaran Padmanabhan • Hassan Baioumy

Received: 24 August 2017 / Revised: 28 January 2018 / Accepted: 27 February 2018 / Published online: 3 March 2018 Ó Science Press, Institute of Geochemistry, CAS and Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract Quartz and iron (hydr)oxide are reactive surface Keywords Quartz Goethite Twinned goethite phases that are often associated with one another in soils Microscopic characterization (FESEM and TEM) FT-IR and sediments. Despite the several studies on the coating of spectroscopy 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 synthe- 1 Introduction sized on quartz substrates were characterized using field emission scanning electron microscopy, X-ray diffraction Quartz is the most ubiquitous mineral in the Earth’s crust, (XRD), transmission electron microscopy, and Fourier- and it is a fundamental material in reactions that affect transform infrared (FT-IR) spectroscopy. The SEM char- physiochemical properties of siliciclastics (White et al. acterization revealed that bundles of thin parallel aligned 2001; Lasaga and Luttge 2001; Lu¨ttge 2005; Brantley et al. goethite rods were formed at pH [ 10, while large pseu- 2008; Ali and Padmanabhan 2014; Ali et al. 2017). dohexagonal crystals of twinned goethite needles were Meanwhile, the abundant nature and thermodynamic sta- synthesized at pH B 10 after dehydration and hydration in bility of iron minerals have motivated the study of quartz’s the alkaline media. TEM analysis showed expanded and phase composition, crystallinity and polymorphic varia- distorted lattice spacing of the crystal structure of iron tions (White 1990; Rodgers et al. 1991; Pigna et al. 2006; (hydr)oxide due to silica incorporation. The characteriza- Gotic´ and Music 2007). Quartz and iron (hydr)oxide are tion showed that silica increased the crystallite size of the reactive surface phases often associated with one another in goethite and transformed its acicular texture to a larger, soils and sediments (Davis et al. 1998). Experimental twinned needle structure. FT-IR and XRD analyses studies (Schwertmann and Murad 1977; Hu et al. revealed band shifts in crystal bonds as well as new bond 2012, 2013; Dai and Hu 2015) have shown the nucleation formations, which indicate the presence of changes in the and growth of iron hydroxide on quartz. Related studies chemical environment of Fe–O and Si–O bonds. Thus, the report that iron oxides bind strongly with quartz surfaces presence of sorbed silicates modifies the crystal and lattice (Hendershot and Lavkulich 1983; Scheidegger et al. 1993; structure of goethite. Rusch et al. 2010; Xu and Axe 2005). However, as Rusch et al. (2010) pointed out, little is known about iron-phase reactivity with dissolved silica leached from quartz. In addition, there remains the unresolved issue of whether the interaction is a chemisorption process or simply surface adsorption (Padmanabhan and Mermut 1996). Moreover, & Abdullah Musa Ali the impact of pH on the formation of goethite and hematite [email protected] is not entirely known (Van Ranst et al. 2016). The characterization of the interactions between iron 1 Department of Geosciences, Faculty of Geosciences and Petroleum Engineering, Universiti Teknologi PETRONAS minerals and quartz will be valuable in determining how (UTP), 31750 Tronoh, Perak, Malaysia quartz or dissolved silica may affect the formation of iron 123 912 Acta Geochim (2018) 37(6):911–921

(hydr)oxides in low-temperature and strongly oxidizing sedimentary environments. It will also ensure reliable prediction of quartz reactivity using laboratory experi- mental data as well as improve the understanding of the actual transformation of iron oxides into more crystalline products. Furthermore, the capacity of iron oxide phases to adsorb cationic Si species from aqueous solutions plays a central role in the mobility and transfer of inorganic species and ionizable organic compounds in environmental con- taminants (Hanna 2007; Rusch et al. 2010). The objectives of this paper are to morphologically characterize the out- come of silicate sorption on quartz surfaces and to describe the surface texture and lattice variations of iron oxide coatings. In essence, this study reports the morphology of the silica-induced transformation of iron oxides. At pH 4–7 and 20 °C, the levels of silicate in solution prevent the transformation of iron (hydr)oxides over a particular length of time (Schwertmann and Murad 1977). Therefore, the Fig. 1 FESEM image of a clean subhedral quartz grain with experiments in this study were carried out at pH C 10 and conchoidal fractures and smoothed edges at 60 °C to reflect natural environments such as oxidized vapor dominated geothermal systems, crater lakes, and dissolved in NaOH solutions that contained varying mine drainage settings as well as geothermal energy and oil amounts of iron. residual phase extractions (White and Brantley 2003). 2.1 Synthesis of iron (hydr)oxides on quartz

2 Materials and methods The iron (hydr)oxides were synthesized on quartz sub- -1 strates by precipitating 0.4 mol L FeCl3 with the addi- To achieve the objectives of this study, two sets of tion of 0.5 mol L-1 NaOH. A series of solutions -1 experiments were conducted: the synthesis of iron comprising 15, 20, and 25 mL of 0.4 mol L of FeCl3 (hydr)oxides on quartz substrates and quartz dissolution in were added to 5 g of quartz substrates (qFe15, qFe20, and the presence of iron oxides. The quartz crystals that were qFe25), and then mLdistilled water was added until sam- used were first crushed in a mortar grinder to increase the ples were 50 mL. The proportions used were roughly exposed surface area and then sieved to obtain [ 50 lm equivalent to the addition of 0.75, 1.0, and 1.25 g of Fe per mesh fraction. After that, about 20 g of the crushed quartz 5 g of quartz substrate. With the exception of qFe20, the crystals were ultrasonified in cold 0.1 mol L-1 HCl solu- initial pH of approximately 2 was then increased to 12 to tion to remove the carbonate coatings and adhering iron ensure iron (hydr)oxide precipitation by adding 100 mL of coating. The washed samples were then soaked in 35% 0.5 mol L-1 NaOH solution. Each suspension was agitated hydrogen peroxide (H2O2) to remove organic debris. The intermittently for 30 min to improve contact between the quartz grains were washed several times in distilled water quartz and the initial products of the Fe hydrolysis. The and subsequently oven-dried at 60 °C for 24 h. Individual samples were aged at 60 °C. The sample aged in 20 mL of quartz grains ranged from 200 to 400 lm in size. This FeCl3 solution (qFe20) was dehydrated after 5 days of pretreatment procedure also dissolved sharp edges, adher- ageing through oven drying at 60 °C for 2 days. After- ing fine particles, heavy minerals, organics, surface arte- wards, fresh NaOH solution was added to the sample, facts and other impurities without introducing metals or which was then left to age for 20 days at a pH of corrosive agents to the system that might affect the sub- approximately 8.5. Silicate was strongly adsorbed on the sequent dissolution rate measurements. The final quartz surfaces of various iron (hydr)oxides, with a maximum in samples were comprised of subhedral to subround grains surface coverage near pH 9 (Taylor 1995). This approach with surface defects that included conchoidal fractures and was also utilized to limit the silica dissolved in the system cleavage planes (Fig. 1). The procedure for synthesizing as well as to synthesize hematite. The experimental con- iron (hydr)oxides on quartz substrates was modified from ditions of the synthesis are outlined in Table 1. The amount earlier studies by Arias et al. (1997) and Rusch et al. of silica in the system was measured using the Silica (2010). In the second experiment, bulk quartz grains were Molybdate spectrophotometry method. The pH of solution

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Table 1 Experimental condition for the synthesis Sample Volume of solution (distilled Volume of Ageing Initial pH (with the Final pH (after Dissolved water ? NaOH ? FeCl3) (mL) 0.4 M FeCl3 temperature addition of NaOH) 20 days of ageing) Silica (mg/L) (mL) (°C) qFe0 100.0 0.0 60.0 12.0 11.8 36.9 qFe5 100.0 5.0 60.0 12.0 11.5 10.9 qFe10 100.0 10.0 60.0 12.0 11.5 30.5 qFe11 100.0 15.0 60.0 12.0 11.5 114.3 qFe20 100.0 20.0 60.0 10.0a 8.5 26.2 qFe25 100.0 25.0 60.0 12.0 11.5 163.5 aDehydrated after 5 days was monitored in all titration and ageing experiments using mineralogical and textural analysis to differentiate the a pH meter (EUTECH, Model: Cyberscan). quartz from the as-synthesized phases. The mineral com- position of the iron (hydr)oxides precipitated on quartz was 2.2 Quartz dissolution experiment identified using an X-ray diffractometer (brand: Bruker- AXS, model: D8 Advance), with Ni filtered CuKa radia- ´ In a separate experiment, bulk samples of quartz grains tion (wavelength of 1.54056 A˚ ) generated at 40 kV and were placed in the presence of iron oxides using the batch 40 mA. The operating parameters for XRD analysis also bottle system. The batch bottle system comprises sealed included step size and step time of 0.0001° and 1 s, polyethylene bottles (50 mL) floating in a water bath sha- respectively, with a 1 mm divergence slit, a 0.1 mm ker and heater. 5 g of pure quartz grains were dissolved at detector slit and a 1 mm antiscatter slit. Infrared spectra of 60 °C in the separate bottles containing NaOH, and 5 the samples were obtained using Fourier-transform infrared (qFe5), 10 (qFe10), 15 (qFe15), 20 (qFe20) and 25 mL (Shimadzu FT-IR 8400S) to identify the chemical bonds in -1 (qFe25) of 0.4 mol L solutions of FeCl3 fixed at a pH of the synthesized material. 12. Dilute sodium hydroxide (NaOH) and hydrochloric acid were used as neutralizing agents in the titration pro- cess to adjust the pH. 1 mL of NaOH and HCl was added to 3 Results and interpretation the initial solvents to increase and reduce the pH by 0.1, respectively. The bottles were opened periodically to 3.1 Morphological characterization remove solution samples. The solvent was changed every 10 h. The amount of dissolved silica was measured using The iron (hydr)oxides synthesized on quartz substrates the silica molybdate spectrophotometry method (HACH were morphologically described. The stereomicroscopic D2800). Pure quartz substrate was included in the experi- characterization of the samples (Fig. 2) indicated the ment as a control sample (qFe0). The bottles were mildly presence of goethite and hematite as the iron bearing agitated from water circulation in the water bath to ensure phases. The stereomicroscopic photomicrographs showed the solution was always in contact with the quartz surface. the coating of quartz grains with the synthesized hematite Mass transfer limitations were not expected due to the slow and goethite. It was observed that the iron bearing phases dissolution rate conditions. adhere onto the quartz surface, which gives the initial quartz grains a red tint (Fig. 2a). The goethite particles 2.3 Sample characterization (G) were characterized by star-shaped particles dispersed on the quartz surface (Fig. 2b), while the hematite was Stereomicroscopic study of the iron (hydr)oxides synthe- enmeshed with the quartz grains (CQ) to form a distinct sized on quartz was carried out to identify different mineral phase, possibly ferrisilicates. phases. The morphological characterization and surface Goethite crystals were synthesized at pH 12, in solutions composition analysis of the samples was performed using a containing 15 mL (qFe15) and 25 mL (qFe25) of FeCl3. high-resolution field emission scanning electron micro- The FESEM micrographs showed needle/lath shaped goe- scope (FESEM: Carl Zeiss Supra 55VP; operated at thite of acicular texture (Fig. 3). They occurred as 5–20 kV). A transmission electron microscope (TEM: (a) bundles of thin parallel aligned rods ranging from [ 1 Zeiss Libra 200FE) was used to provide high-resolution to 3 lm in size for both samples. The goethite laths existed

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Fig. 2 Stereomicroscope photomicrographs showing a iron oxide coated quartz grains and b closer view with arrows denoting star shaped goethite crystals (G), and hematite meshed with finer quartz grains to from a coating phase (CQ)

Fig. 3 FESEM micrographs showing a distinctive needle shaped acicular goethite synthesized on quartz indicated by the inset arrow, b infilling of gouges and notches on the quartz surface with goethite laths as a distinct phase, which was dispersed and admixed over to the interference of OH- species in the growth of goethite the quartz surface (Fig. 3a). The goethite laths were also (Cornell and Giovanoli 1985). observed to be non-uniform and discretely located in the Large pseudohexagonal crystals of twinned goethite stepped and concave areas of the quartz surface, indicating needles (TG) and Hematite (H) (Fig. 4) were synthesized at that etched and pitted areas of quartz ensured higher pH 10 via the dehydration of initially hydrolyzed iron loadings of goethite material. A closer examination also oxides and the subsequent rehydration with the base solu- showed that the goethite laths aggregated preferentially tion (NaOH). The twinned goethite was characterized by a along the edges of the quartz crystal (Fig. 3b), thus the larger crystalline structure compared to the goethite crys- goethite appeared to have been deposited abundantly at tals synthesized at pH 12 (Fig. 4a, b). The twinned goethite specific sites. The very thin and elongated shape indicated crystals occurred as perfect goethite crystal, about 1 mm in an increasingly strong retardation of crystal growth in a- length. Cleavage/twinning planes were evident. The syn- and b-axes and a rapid growth/elongation in the crystallo- thesized twinned goethite and hematite adhered to planar graphic c- axis following rapid crystallization at high pH and pitted surface, infill crevices, notches, and fractures of (Gotic´ and Music 2007). This phenomenon was attributed the quartz grains (Fig. 4a, b). The Fe-(hydr)oxides aggre- gated into isolated patches that partially covered the quartz

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Fig. 4 FESEM micrographs showing a well crystallized goethite needles infilling notches and completely covering the quartz surface, b synthesized twinned goethite needles [TG] and hematite [H] grains (Fig. 4a). EDX analysis of the twinned goethite crystals were characterized by acicular morphology needles showed 80.5 wt% for Fe and 4.0 wt% for Si (Fig. 7a), while the twinned goethite minerals (qFe20) (Fig. 5), thus confirming the incorporation of Si into the were defined by cross hatch twinning planes (Fig. 7b). The goethite crystal. length of the crystal lattice ranged from 100 nm up to 200 nm, with a diameter of approximately 5 nm. 3.2 Lattice structure of the twinned goethite The well faceted crystals of the twinned goethite (Fig. 8a) showed different lengths and curvatures, with High-resolution TEM images were obtained to examine the some of the angles between the lattice fringes deviating lattice structure of the goethite laths dispersed on synthe- significantly from 120°, although some of the hexagonal sized quartz substrates. The parallel goethite needles arrangement showed almost perfect projections (Fig. 8b). ´ occurred as agglomerations of crystal lattices with different The length of the crystal lath ranged from 1000 A˚ up to ´ ´ orientations. The goethite particles observed in the TEM 2000 A˚ , with varying widths of 45–55 A˚ (Fig. 8b). In some images for the sample have dimensions in the order of places, the parallel goethite needles overlapped into bun- 20 9 200 nm. The image depicted in Fig. 6 shows goethite dles with a single orientation. The overlapping bundles laths leaching into the silica lattice structure. The goethite appeared as rafts of crystals, with each single crystal

Fig. 5 EDX spectrum showing the elemental concentrations of the twinned goethite

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discontinuous lattice rows in the crystal (Fig. 8a). The laths ´ showed a prominent 40 A˚ lattice repeat, separated by four ´ ´ 23.5 A˚ fringes. The crystals also showed 25 A˚ lath fringes ´ perpendicular to a 30 A˚ structure (Fig. 8a). In some areas, ´ ´ the 3 A˚ regularity was interrupted by shorter (25 A˚ )or ´ ´ longer (50 A˚ ) repeats (Fig. 7b). The lattice spacings [d(A˚ )] expanded significantly from the characteristic d-spacing ´ values of goethite (4.18, 2.69, 2.452, 2.490, 2.192 A˚ )to ´ 4.839 and 3.225 A˚ . The contrasting lattice arrangement/ alignment and changes in the stacking sequence of the goethite crystals arose from positional, or electron density variation.

3.3 X-ray diffraction (XRD) spectra analysis

XRD spectra were obtained to determine the mineralogy of iron (hydr)oxide synthesized on quartz substrates. Some of the samples (qFe15, qFe20 and qFe25) were selected for the analysis. Goethite and hematite were found to be the dominant phases of the precipitates, as shown in Fig. 9. Ferrihydrite was the precursor mineral from which goethite Fig. 6 TEM micrograph of acicular goethite crystals (G) complexed and hematite were synthesized, the former from dissolution with quartz (Q) showing goethite crystals intruding into the lattice and re-precipitation, while the latter from thermal dehy- structure of quartz dration and aggregation (Cornell and Schwertmann 2003). The peaks at d-spacing = 4.26, 2.46, 2.28, 1.817 and ´ projected at the edges of the rafts. However, two or more 1.541 A˚ were assigned to quartz. The peak at d-spac- crystals were superimposed to form moire´ fringes, as ing = 2.83 was typical for trioctahedral chlorites with Fe- shown in Fig. 7a. The depicted imperfections showed dominant octahedral cations (Bailey 1988). The chlorite

Fig. 7 TEM micrographs showing a laths of goethite crystals, b twinned goethite 123 Acta Geochim (2018) 37(6):911–921 917

Fig. 8 a FESEM image of the twinned goethite needles, b TEM micrograph of goethite lath showing lattice distortion and disparity in crystallite width (as indicated by the inset arrows) due to silica absorption was also denoted by weak reflections at 3.53. Goethite was assigned to Fe–O as Si cage. In addition, the IR band at ´ identified with d-spacing of 2.69 A˚ in substrates synthe- 1084 cm-1 in the spectra of pure quartz (qFe0) suggested sized with 20 and 25 mL of FeCl solution [qFe15 and Si–O stretching. ´3 qFe25]. A d-spacing value of 2.52 A˚ indicated the presence The appearance of the peak at 1080 cm-1 may also of hematite in qFe20. Similar to the TEM results, shifts in indicate enhanced asymmetric stretching of SiO4 (Xu and basal d-spacings for goethite were identified. The charac- Axe 2005). The symmetrical peak at 1635 cm-1 for the Fe- teristic peaks for goethite shifted from 4.18, 2.69 and 2.49 oxide coated quartz grain was assigned to H–O–H bending ´ to 4.238/4.20, 2.70/2.71/2.73 and 2.50/2.52/2.53 A˚ , of water (OH deformation of water). The considerable shift respectively. from 1080 cm-1 to asymmetric Si–O–Si stretching band at 1100 cm-1 and symmetric peak at Si–O–Si band at 3.4 FT-IR spectra for the identification of chemical 1635 cm-1 indicated increasing lattice substitution of bonds trivalent Fe3?. This is consistent with similar research that reported that the substitution of Fe3? for Si4? caused a The FT-IR spectra of Fe (hydr)oxides synthesized on substantial shift of the Si–O–Si stretching band quartz grains is shown in Fig. 10. A spectrum of pure (1080 cm-1) to lower frequencies (Nowok et al. 2001; quartz substrate was included as a means to identify any Schwertmann and Thalmann 1976). H–O–H stretching variance resulting from the coating by Fe (hydr)oxide. The bonds and H–O–H bending bonds of absorbed water pre- IR bands at 789 and 780 cm-1 present in both the pure sented at approximately 3377 cm-1 of the synthesized Fe- quartz and Fe-oxide coated quartz grains denoted Si–O–Si oxides, which was absent in the pure quartz sample. The -1 inter tetrahedral bridging bonds in SiO2. The IR band at peak at 1437 cm for qFe20 sample (Fig. 10), which 889 cm-1, which is conspicuously absent in the pure quartz denoted Fe–O as Si cages, further supported the can be assigned to OH- deformation linked to Fe3?, sug- chemisorption of iron oxides on quartz. This peak was gesting unambiguously that goethite was coated on the absent in the other series. quartz surface. This also suggested the formation of Si–O– Fe bond, which is characteristic of Fe–Si hydrous oxides 3.5 Quartz dissolution in the presence of iron oxides and silicates (Gallup and Reiff 1991). Si–O-Fe bonds in Fe–Si hydrous oxides and silicates indicated the spatial The change in pH with time as Fe-(hydr)oxides were absorption and bonding of the Fe cations to the quartz synthesized on quartz is observed in Table 1. The pH was siloxanes to form ferrisilcates. The broadening peak at monitored daily for 60 days to detect any deviations. After 1347 cm-1 for the Fe-oxide coated quartz grains was 20 days of ageing, the silica concentration of the solvent

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was measured. The pH set at 12 remained relatively steady for the ageing duration, whereas the solution with pH set at 10 declined to 8.5. The parameters of the ageing experi- ment is outlined in Table 1. The silica concentrations for qFe0, qFe5, qFe10, qFe15, qFe20 and qFe25 were 36.9, 10.9, 30.5, 114.3, 26.2 and 163.5 mg/L, respectively. The amounts of dissolved silica was observed to increase with increasing iron content. The dissolution or chemical disintegration of quartz is mostly pH dependent. To fully elucidate quartz—Fe- (hydr)oxide interactions, quartz dissolution experiments were conducted in the presence of iron. Silica concentra- tion was measured after the agitation of quartz grains for 60 h in NaOH solutions containing Fe ions. The silica dissolution rates in the presence variable iron concentra- tions were directly calculated from the experimental data and plotted against time, as shown in Fig. 11. The silica concentration for qFe5–qFe25 varied from 6.7 to 4.9 mg/L in the first 10 h and steadily increased for the next 20 h, and then gradually declined for the last 20 h (Fig. 11). Equilibrium (steady state) was attained after 60 h of dis- solution. In contrast, high silica value (36.5 mg/L) was recorded for NaOH solution containing no Fe ions for the initial 20 h, but gradually decreased to 4.7 mg/L after 30 h of dissolution. Table 1 and Fig. 11 indicates quartz dissolution varies in tandem with pH and Fe content, although in a non- sequential pattern. The variation in pH with time can be explained by the following mechanism. The trivalent Fig. 9 X-ray diffraction spectra of Fe (hydr)oxides synthesized on compounds (ferric chloride) basically hydrolyze in water quartz substrates at 60 °C, indicating the presence of goethite and solution and ionize to iron (ferric) and chloride ions; the hematite (Chl chloride, Qtz quartz, Hem hematite, Goe goethite) water ionizes to hydrogen and hydroxide ions, making the solution strongly acidic. Afterwards, the ferric ions par- tially combine with the hydroxide ions released from the dissociation of water to form ferric hydroxide, a compound

Fig. 10 Stacked FT-IR spectra of pure quartz (qFe0) and Fe-oxide coated quartz grains (qfe) showing disparity in characteristic peaks Fig. 11 Plot of log rate of quartz dissolution (mg/L/s) with time 123 Acta Geochim (2018) 37(6):911–921 919 that is only slightly soluble and precipitates from the adsorption of silica is slow in comparison with many ionic solution as a brown solid. This process is catalyzed by adsorption processes, and it may take several weeks to heating up the solution and adding a base. As a base was approach equilibrium at ambient temperature. At pH below - added to the FeCl3 solution, the OH ions depleted and 10, there is a relatively lower level of silica in the system, neutralized the available H? ions to form water, causing which can strongly influence the course of iron oxide inter- the pH value to rise until the pH of the base being added conversion reactions, by inhibiting the nucleation of pro- was attained. The further addition of a base had little effect, duct phases. with the pH remaining relatively constant based on the The role of silica in limiting the growth of Fe- excess amount of OH- in the system. Within this range, (hydr)oxides extends beyond a surface phenomenon as OH- became constantly absorbed by Fe cations overtime indicated by Eggleton and Fitzpatrick (1988). The substi- to form oxyhydroxide species. Thus, the OH- ions were tution of Fe3? with tetrahedral Si introduces lattice dis- neutralized by the trivalent and H? ions, leading to the tortions and crystal expansion due to variable Fe:O:H ratio decline in pH. However, the pH of the null sample of only [considering the composition of goethite crystals having an pure quartz remained constant at 12.5, possibly due to the internal stoichiometry FeO(OH), with an OH surface]. The absence of neutralizing agents. interaction between the dissolved silica and goethite nanocrystallites alters the intrinsic crystal structure of goethite, thus inducing morphological changes. The lea- 4 Discussion ched silica does not bridge related silanol groups in the presence of Fe-(hydr)oxide but instead differentially The formation of Fe-(hydr)oxides is generally dependent on absorbs into the lattice structure of the Fe (hydr)oxides, precursor materials and conditions of synthesis (Schwert- thus spatially modifying the goethite minerals (Fig. 8). mann and Taylor 1977; Lewis and Schwertmann 1979). In Therefore, in a way, the Fe-(hydr)oxides do affect quartz this study, goethite crystals were formed in solution from by creating a micro environment for the incorporation of dissolved Fe3? ions, while hematite was synthesized by silica. The mobilization of Fe generally occurs via reduc- means of internal dehydration and reordering within the tive reactions. As the reduction condition transforms to goethite aggregates. Thermodynamics may indicate that oxidation with a drop in pH, the Fe may become immo- goethite and hematite are the most stable Fe oxides, but bilized (Krauskopf and Bird 1995). The few nuclei of chemical equilibrium is frequently impeded by slow kinetics goethite that form grow gradually to large crystalline (Van Ranst et al. 2016). The synthesis of hematite via structures. The silicate ions are taken up by the iron oxides dehydration of goethite can be attributed to its relatively via a ligand-exchange mechanism between silicate coor- higher magnetic ordering temperature and magnetic hyper- dinates and iron oxide precursor-ferrihydrite (Cornell and fine field (Murad 2010). Although both goethite and silica are Schwertmann 2003), while Dzombak and Morel (1990) negatively charged, suggesting electrostatic repulsion, the earlier asserted that the mechanism most likely involves plastering of goethite on quartz surface indicate electrostatic, ligand exchange in which hydroxyl radical surface groups van der Waals, and/or chemical bonding. At low silica are replaced by silicates as illustrated in Eqs. (1) (2) and concentrations and pH 10, the initial goethite is transformed (3) shown below: by dissolution and re-precipitation to hematite and large FeOH2 ! FeOH þ H ð1Þ pseudohexagonal crystals of twinned goethite by internal rearrangement and crystal lattice displacement, Thus, the FeOH þ H4SiO4 ! FeH3SiO4 þ H2O ð2Þ morphology of hematite synthesized from ferrihydrite is þ FeOH þ H4SiO4 ! FeH2SiO4 þ H þ H2O ð3Þ influenced by the presence of SiO 2-. In contrast, the higher 3 4? silica concentrations at pH 12 and 60 °C suppressed the On the other hand, for steric reasons, Si should not 3? transformation of ferrihydrite to hematite. replace structural Fe in the Fe-(hydr)oxides. However, in 4? Therefore, the pH and presence of silica in solution a reverse mechanism, tetravalent Si can be replaced by 3? influence the synthesis of goethite since silicate anions the trivalent Fe at specific locations, leaving uncom- typically coexist with Fe-(hydr)oxides and retard or inhibit pensated charges in the structure (Kronenberg 1994). To the transformation of these minerals to more crystalline maintain electrical balance, monovalent cations such as ? ? ? products (Schwertmann 1985; Schwertmann and Taylor H ,Na , and/or K enter the quartz structure in interstitial 1989; Vempati et al. 1990). The silicate adsorption appears spaces, not at original Si-locations. The differential lattice to have a distinct effect on the adsorption of other anions, spacing suggests expansion of the lattice structure due to or, the displacement of the limiting pH for anion adsorption the incorporation of silica. The different crystallite spacing to lower values, but it has little effect on cation adsorption values indicate the preferential sorption of silica, which in (Taylor 1995; Pham et al. 2012). At pH above 10, the turn spatially modifies the goethite minerals. Silica are 123 920 Acta Geochim (2018) 37(6):911–921 adsorbed by ligand exchange onto the goethite surfaces, as Davis JA, Coston JA, Kent DB, Fuller CC (1998) Application of the evidenced by the infrared spectroscopy for the formation of surface complexation concept to complex mineral assemblages. Environ Sci Technol 32:2820–2828 unidentate surface complexes of the = Fe-OSi(OH)3 type Dzombak DA, Morel FMM (1990) Surface complexation modeling. -1 at 889 cm . Hydrous ferric oxide. Wiley, New York, p 393 Eggleton RA, Fitzpatrick RW (1988) New data and a revised structural model for ferrihydrite. Clays Clay Miner 36(2):111–124 5 Conclusion Gallup DL, Reiff WM (1991) Characterization of geothermal scale deposits by Fe-57 Mossbauer spectroscopy and complementary The pH parameter was shown to be the key component in X-ray diffraction and infra-red studies. Geothermics 20:207–224 the formation of Fe-(hydr)oxides. Quartz also acted as a Gotic´ M, Music S (2007) Mossbauer, FT-IR and FESEM investiga- tion of iron oxides precipitated from FeSO4 solutions. J Mol media for the precipitation of Fe-(hydr)oxides. Goethite Struct 834–836:445–453 was the only Fe-(hydr)oxide formed at pH 12, while Hanna K (2007) Sorption of two aromatic acids onto iron oxides: hematite and twinned goethite were synthesized at pH 10. experimental study and modeling. J Colloid Interface Sci The dissolved silica favored the formation of hematite and 309:419–428 Hendershot WH, Lavkulich LM (1983) Effect of sequioxide coatings twinned goethite. The Fe-(hydr)oxides formed discrete on surface charge of standard mineral and soil samples. Soil Sci mineral coatings on the quartz surface and aggregated in Soc Am J 47:1252–1260 surface defects such as edges, pits, and fracture sites. Hu Y, Lee B, Bell C, Jun YS (2012) Environmentally abundant Vibrational characteristics indicated bonding between anions influence the nucleation, growth, Ostwald ripening, and aggregation of hydrous Fe(III) oxides. Langmiur synthesized Fe-(hydr)oxides and dissolved silica as well as 28(20):7737–7746 expansion in the lattice spaces of goethite due to incorpo- Hu Y, Neil CW, Lee B, Jun YS (2013) Control of heterogeneous ration of silica. 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