Fetot Ratio Research Articles

Synthesis and Mössbauer characterization of Fe 1+x Cr 2-x O 4 (0 ≤ x ≤ 2/3) spinel single crystals

Periodico di Mineralogia (2011), 80, 1 (Special Issue), 39-55 - DOI: 10.2451/2011PM0004 Special Issue in memory of Sergio Lucchesi Synthesis and Mossbauer characterization of Fe 1+x Cr 2-x O 4 (0 ≤ x ≤ 2/3) spinel single crystals Marco Quintiliani 1 , Giovanni B. Andreozzi 1,* and Henrik Skogby 2 1 Dipartimento di Scienze della Terra, Sapienza Universita di Roma, Italy 2 Department of Mineralogy, Swedish Museum of Natural History, Stockholm, Sweden *Corresponding author: gianni.andreozzi@uniroma1.it Abstract Spinel single crystals belonging to the FeCr 2 O 4- Fe 3 O 4 join were synthesized by a flux growth method (Na 2 B 4 O 7 as flux, temperature range of 1200-900 °C and cooling rate of 4 °C/h). Crystals with compositions corresponding to Fe 1+x Cr 2-x O 4 (0 ≤ x ≤ 2/3) were obtained and successively investigated by a combined microchemical, diffractometric and spectroscopic approach. Fe-Cr-spinel single crystals produced are of different quality (in terms of size and shape). The flux content in the starting mixture showed to have the most relevant influence on the quality of synthetic products. Electron microprobe analysis evidenced a chemical heterogeneity for crystals of the same batch, with small crystals (down to 50 μm) being more homogeneous than the large ones. Mossbauer spectroscopy was used to determine the actual Fe 3+ /Fe tot ratios and highlighted the absence of magnetic phases. The combined chemical and spectroscopic approach allowed to exclude any deviation from stoichiometry due to oxidation. In contrast with most of the existing literature, Mossbauer spectroscopy indicated the Fe 2.5+ valence state to be present even for iron poor samples with composition close to the end member chromite. Key words : chromite-magnetite solid solution; synthesis, spinel single crystals; Mossbauer spectroscopy.

Mineralogical evidence for H2 degassing during serpentinization at 300 °C/300 bar

Hydrogen is produced in large amounts during hydrothermal alteration of peridotite in low-spreading-rate mid-ocean ridges. This production is directly linked to reducing conditions in hydrothermal fluids induced by the oxidation of Fe2+ in primary minerals (olivine and pyroxene) to Fe3+ in secondary minerals (magnetite and serpentine). A better knowledge of iron speciation in serpentine is therefore crucial to the quantification of hydrogen production during the serpentinization process. For the first time, we have determined the amount of ferric iron in altered peridotite as a function of alteration time. We investigated experimentally the alteration of powdered lherzolite in pure water at 300°C/300bar. For each experimental run (0, 7, 18, 34 and 70days), H2 degassing was measured using in-situ gas chromatography and the experimental products were analyzed using XRD, Raman and X-ray absorption spectroscopy at the iron K-edge. In parallel, aqueous solutions were analyzed by ICP-AES. Our results show quasi-complete serpentinization at 70days with replacement of primary olivine and pyroxene by secondary lizardite and magnetite. The Fe3+/Fetotal ratio is linearly dependent on the hydrogen production and ranges from 0 to 0.66 at the end of the experiment. Our results reveal strong variations in Fe3+ in serpentine for different alteration times, from 0 to 100% of ferric iron, including up to 12% of tetrahedral iron. Hydrogen was produced in three main stages: (1) a first stage during which the H2 production rate reaches a maximum at 18days and is controlled by the crystallization of magnetite, (2) an intermediate stage during which serpentine incorporates ferric iron and thus plays a major role (up to 50%) in the hydrogen formation, and (3) a final stage during which magnetite amount increases from ~2 to ~5% of the mineral assemblage. The last alteration stage is accompanied by a slight increase of the Fe3+/Fetotal ratio, while the rate of hydrogen production decreases at the end of the experiment. Consequently, variations of the ferric iron contents in natural oceanic peridotites may constitute a good indicator of the “hydrogen-potential” of various ultramafic hydrothermal fields.

Hydrogen content in clinopyroxene phenocrysts from Salina mafic lavas (Aeolian arc, Italy)

Clinopyroxene phenocrysts from the mafic calc-alkaline lavas of Salina (Aeolian arc, southern Tyrrhenian Sea, Italy) have been analysed to determine the hydrogen content and iron oxidation state of this early crystallized phase. The volcanic activity of Salina, starting at 168 ka and developed in several centres up to 24 ka, was dominated by calc-alkaline and high-K calc-alkaline basalts and andesites, with minor dacites and rhyolites. The presence of OH vibrational bands was detected in the IR spectra of clinopyroxenes phenocrysts from Corvo, Rivi-Capo (168–87 ka), Fossa delle Felci (108–59 ka) and Monte dei Porri (57 ka) eruptions. Corvo-Rivi-Capo basalts have clinopyroxenes with the lowest water contents 75–97 ppm H2O by weight, whereas an increase in the hydrogen contents of clinopyroxenes from Fossa delle Felci centre, with 171–286 ppm H2O by weight, and Monte dei Porri with 343–390 ppm H2O by weight, was observed. Mossbauer spectroscopy showed only a limited variation on the Fe3+/Fetot ratio of the studied samples, and a very similar atomic Fe3+ content (0.042–0.047 a.p.f.u.) suggesting that only minor variation on fO2 occurred during the crystallization of these clinopyroxenes. The water content of parental melts, calculated by applying an IVAl-dependent partition coefficient to the measured hydrogen contents of clinopyroxenes, is 0.4–0.8 wt% of water in melt for the Rivi-Capo-Corvo basalts, 0.5–3.7 wt% water in melt for Fossa delle Felci lavas and 1.6–2.6 wt% of water in melt for Monte dei Porri lavas. An increase in the maximum hydrogen contents of clinopyroxenes can be recognized during the evolution of the Salina volcano, with the highest hydrogen content measured in clinopyroxenes from Monte dei Porri where the eruptions were characterized by a high degree of explosivity, suggesting a key role of volatiles.

Electrical properties of Cr 2O 3–Fe 2O 3–P 2O 5 glasses. Part II

The electrical properties of xCr 2O 3–(40−x)Fe 2O 3–60P 2O 5, 0 ≤ x ≤ 10, (mol%) glasses have been investigated by impedance spectroscopy. In the glasses containing up to ~ 5 mol% Cr 2O 3, electrical conductivity increases due to the increase in the Fe 2+/Fe tot ratio. The conduction in these glasses is independent on Cr 2O 3 and it is controlled by the polaron hopping between iron ions. The electrical conductivity of the partially crystallized sample with 10 mol% Cr 2O 3 slightly increases although the Fe 2O 3 content and the fraction of Fe 2+ ions are smaller. This effect is related to the formation of highly disruptive regions around the β-CrPO 4 and Fe 3(P 2O 7) 2 crystallites in the glass matrix of this sample. These interconnected disruptive regions form the easy conductive pathways leading to a higher conductivity. Changes in the dielectric permittivity in chromium iron phosphate glasses are dependent on the Fe 2+/Fe tot ratio and, therefore, associated with the concentration of polarons related to Fe 2+ and Fe 3+ sites.

Detailed crystal chemistry and iron topochemistry of asbestos occurring in its natural setting: A first step to understanding its chemical reactivity

Five samples of tremolite asbestos from Italy and USA were fully characterized by ICP-MS, SEM, EMPA, FT-IR, MS, XRPD, to correlate crystal chemistry with chemical reactivity. Iron topochemistry was investigated in detail, due to the role of Fe in the aetiology of respiratory inflammatory diseases. The Italian tremolite samples have different Fe contents (San Mango > Ala di Stura > Rufeno > Castelluccio Superiore), and the USA sample from Maryland shows a value almost double that of the Italian samples. The bulk Fe 3+/Fe tot ratio was quantified by Mössbauer spectroscopy, and the values obtained range from 6% Fe tot (San Mango tremolite) to 24% Fe tot (Mt. Rufeno tremolite). A possible site distribution of Fe was derived from combining chemical, spectroscopic (Mössbauer) and structural (Rietveld refinement) data. For all samples Fe 2+ was disordered over M(1), M(2) and M(3) sites, whereas Fe 3+ was allocated to M(2). Production of HO° radical in the presence of hydrogen peroxide from the Italian samples is a measure of chemical reactivity, with the lowest value observed for the Castelluccio Superiore sample, and the highest value for the San Mango sample. Notably, HO° radical production is directly related to the Fe occupancy of the M(1) and M(2) octahedra, which are more exposed on the external surface of the fibers than M(3) octahedra and therefore have higher probability of being involved in surface reactions.

Tourmaline in the Vetka porphyry copper-molybdenum deposit of the Chukchi Peninsula of Russia

Three generations of tourmaline have been identified in propylite in the Vetka porphyry copper-molybdenum deposit of the Chukchi Peninsula of Russia. Tourmaline-I is characterized by its Fetot/(Fetot + Mg) value, which ranges from 0.33 to 0.49. Tourmaline-II, which crystallizes at a lower temperature, overgrowing tourmaline-I or occurring as isolated crystals, is distinguished by a higher Fetot/(Fetot + Mg), which varies from 0.46 to 0.72. The Fetot/(Fetot + Mg) ratio in tourmaline-III, which overgrows tourmaline-II is lower (0.35–0.49), and is identical to that of the first tourmaline generation. This is probably caused by the beginning of sulfide deposition. Tourmalines in the deposit characterized by complex isomorphic substitutions can be attributed to the intermediate members of the dravite—“hydroxy-uvite”-“oxy-uvite” and schorl-“hydroxy-feruvite”-“oxy-feruvite” series. Tourmaline starts to crystallize at temperatures above 340°C. The fluid responsible for the tourmaline deposition was magmatic, with a significant admixture of meteoric water (δ18OH2O = −0.85 to −0.75‰). The high Fe3+/Fetot ratio (0.50) indicates high oxygen activity when the tourmaline precipitated. It has been established that the isomorphic substitution Fetot → Al is typomorphic of tourmalines from porphyry copper deposits worldwide.

Structural and spectroscopic characterization of a suite of fibrous amphiboles with high environmental and health relevance from Biancavilla (Sicily, Italy)

This study reports new spectroscopic and structural data of fibrous amphiboles from the volcanic area of Biancavilla (Sicily, Italy) that generated interest because of an anomalous increase of pleural mesothelioma of inhabitants. Each of the four samples is made of loose fibers, which show an edenite-winchite (fluorine) compositional trend, with significant tremolite component. Small amounts of iron (3.6–6.0 wt% FeO tot ) were identified in all samples, and the Fe 3+ /Fe tot ratios were evaluated by Mossbauer spectroscopy: two samples are characterized by Fe 3+ /Fe tot ratios between 50 and 70%, and the other two have Fe 3+ /Fe tot ratios higher than 90%. The OH-stretching region was investigated by FTIR, and no absorption bands were observed. Structural investigation was carried out by X-ray powder diffraction using the Rietveld method. Cell parameters, positional parameters for all the atoms, and site scattering for M1, M2, M3, M4, A, and A( m ) were refined. The most important differences with respect to prismatic fluoro-edenite are the decrease of β, a , and c with decreasing Ca content, A-site occupancy, and tetrahedral Al content, respectively. By combining chemical, spectroscopic, and structural data, possible site occupancies were obtained. In particular, it was found that Fe 2+ is distributed between M1 and M2 sites; moreover, for the two samples enriched in Fe 2+ , it is also present at M4. Fe 3+ is generally ordered at M2 site; however, for the two samples enriched in Fe 3+ , minor amounts are partially disordered between M1 and M3 sites. For the Biancavilla amphibole fibers, the large compositional variation observed in every sample makes the classification very difficult, so that the regulatory agencies would not classify as “asbestos” the whole mineral series, because of the large components of edenite and winchite in addition to tremolite. Many common features were found with respect to amphibole fibers from Libby, Montana, including Fe contents and oxidation state. Preliminary results of in vitro toxicological tests on Biancavilla fibers confirmed their high reactivity, and suggest that the samples with the highest Fe 2+ contents induce a rapid start to cell mortality.

From structure topology to chemical composition. VII. Titanium silicates: the crystal structure and crystal chemistry of jinshajiangite

The crystal structure of jinshajiangite, ideally BaNaTi 2 Fe 4 2+ (Si 2 O 7 ) 2 O 2 (OH) 2 F, a 10.6785(8), b 13.786(1), c 20.700(2) A, β 94.937(1)°, V 3035.93(6) A 3 , sp. gr. C 2/ m , Z = 8, D calc. 3.767 g/cm 3 , from Norra Karr, Tonkoping province, Sweden, has been refined to R 1 5.69 % on the basis of 3193 unique reflections (F O > 4σF). Electron microprobe analysis gave (wt%): SiO 2 27.56, Nb 2 O 5 0.12, TiO 2 18.36, ZrO 2 0.51, FeO 23.42, Fe 2 O 3 2.89 [the Fe 3+ /Fe tot ratio of 0.10(9) was determined by Mossbauer spectroscopy], MnO 5.13, MgO 0.44, CaO 2.52, BaO 10.24, K 2 O 1.95, Na 2 O 2.27, F 2.33, H 2 O 2.00 (calc. from structure refinement: OH + F = 3 apfu ), O = F − 0.98, total 98.76. The empirical formula is (Ba 0.58 K 0.36 ) ∑0.94 (Na 0.57 Ca 0.39 ) ∑0.96 (Fe 2.84 2+ Mn 0.63 Fe 0.32 3+ Mg 0.10 Zr 0.04 Na 0.07 ) ∑4.00 (Ti 2.00 Nb 0.01 ) ∑2.01 (Si 2 O 7 ) 2 O 2.12 (OH) 1.93 F 1.07 , calculated on the basis of 4 Si apfu . The crystal structure of jinshajiangite can be described as a combination of a TS block and an I block. The TS (titanium silicate) block consists of HOH sheets (H-heteropolyhedral, O-octahedral), and is a component of 27 Ti-disilicate minerals. In the O sheet, there are five [6]-coordinated M O sites occupied mainly by Fe 2+ and Mn 2+ , with minor Fe 3+ , Mg, Zr and Na with O –O> = 2.175 A. Five M O sites give ideally Fe 2+ 4 pfu . In the H sheet, there are three [6]-coordinated M H sites occupied solely by Ti (Ti = 2 apfu ), with H –O>. = 1.953 A, and four [4]-coordinated Si sites occupied solely by Si, with . = 1.619 A. The M H octahedra and (Si 2 O 7 ) groups constitute the H sheet. Linkage of H and O sheets via common vertices of M H octahedra and (Si 2 O 7 ) groups with M O (1–5) octahedra results in a TS block. The topology of the TS block is as in Group II of the Ti disilicates (Ti = 2 apfu ). There are six interstitial sites, three [9–10]-coordinated Ba-dominant A P sites with P –O> = 2.98 A and three [10]-coordinated Na-dominant B P sites with P –O> = 2.600 A. The total content of three A P sites sums to ~1 apfu = Ba 0.58 K 0.36 or ideally 1 Ba pfu . The total content of the three B P sites is Na 0.57 Ca 0.39 or ideally 1 Na pfu . Along c , the TS blocks link via common vertices of M H octahedra (as in astrophyllite-group minerals) and A P and B P sites which constitute the I block. Jinshajiangite is an Fe 2+ analogue of perraultite, ideally BaNaTi 2 Mn 4 2+ (Si 2 O 7 ) 2 O 2 (OH) 2 F, and its crystal structure is topologically identical to that of perraultite.

Druse clinopyroxene in D'Orbigny angritic meteorite studied by single‐crystal X‐ray diffraction, electron microprobe analysis, and Mössbauer spectroscopy

Abstract— The crystal structure of druse clinopyroxene from the D'Orbigny angrite, (Ca0.944 Fe2+0.042 Mg0.010Mn0.004) (Mg0.469Fe2+0.317Fe3+0.035Al0.125Cr0.010Ti0.044) (Si1.742Al0.258) O6, a = 9.7684(2), b = 8.9124(2), c = 5.2859(1) Å, β = 105.903(1)°, V = 442.58 Å3, space group C2/c, Z = 2, has been refined to an R1 index of 1.92% using single‐crystal X‐ray diffraction data. The unit formula, calculated from electron microprobe analysis, and the refined site scattering values were used to assign site populations. The distribution of Fe2+and Mg over the M1 and M2 sites suggests a closure temperature of 1000 °C. Mössbauer spectroscopy measurements were done at room temperature on a single crystal and a powdered sample. The spectra are adequately fit by a Voigt‐based quadrupole‐splitting distribution model having two generalized sites, one for Fe2+with two Gaussian components and one for Fe3+with one Gaussian component. The two ferrous components are assigned to Fe2+at the M1 site, and arise from two different next‐nearest‐neighbor configurations of Ca and Fe cations at the M2 site: (3Ca,0Fe) and (2Ca,1Fe). The Fe3+/Fetot ratio determined by Mössbauer spectroscopy is in agreement with that calculated from the electron microprobe analysis. The results are discussed in connection with the redox and thermal history of D'Orbigny.

Iron Mössbauer redox and relation to technetium retention during vitrification

Technetium-99 is a long half-life radioactive fission product whose high volatility during waste vitrification (National Research Council, 2001; Migge, Mat Res Symp Proc 127:205–213, 1989) is a factor in the safe disposal of radioactive waste. Oxidation states of +7, +4 mostly, and some traces of metal have been observed in the glass (Migge, Mat Res Symp Proc 176:411–417, 1990; Darab and Smith, Chem Mater 8:1004–1021, 1996) with Tc7 + more volatile than Tc4 + , leading to a range of technetium retention in the glass varying from 0.8% to 92%. In the present experiments, a simulated high-sulfate Hanford tank waste was vitrified using a slurry fed joule heated furnace producing about 30 kg of glass per day. Various combinations of sucrose and urea were used as denitration and reducing agents in order to optimize sulfate and technetium retention. Since iron oxide is used as a glass additive, Fe-Mossbauer spectroscopy was a vital tool to monitor the melt redox through its Fe2 + /Fetot ratio. A Mossbauer glass redox scale was designed to relate the technetium speciation to its retention in the glass.

Sonication induced redox reactions of the Ojén (Andalucía, Spain) vermiculite

Sonication in a 1:1 mixture (volume ratio) of water and concentrated H 2O 2 (30%) is a soft method for particle size reduction of phyllosilicate minerals like vermiculites. Repeated sonication causes a particle size reduction to about 70 nm for the Santa Olalla and to 45 nm for the Ojén-vermiculite. In this context the question arises whether the strong oxidising effect of the hydrogen peroxide affects the oxidation state of the iron in the vermiculites. Therefore, the Fe 3+/Fe total ratio before and after sonication was determined by means of Mössbauer spectroscopy. Whereas this ratio was found to remain almost constant in the Santa Olalla vermiculite, it increased from 0.79 to 0.85 in case of the Ojén sample. In the latter case, the oxidation is accompanied by a decrease of the layer charge. Surprisingly, sonication in pure water leads to a decrease of the Fe 3+/Fe total ratio in the case of the Ojén-vermiculite, i.e., to an increase of the Fe 2+ fraction to roughly twice the value before sonication. Again the Fe 3+/Fe total ratio of the Santa Olalla vermiculite remains unchanged. The surface area S BET of the reduced Ojén-vermiculite amounts to 50 m 2/g, which is close to the value obtained in the presence of hydrogen peroxide. The results presented should be taken as a warning that particle size reduction by sonication may be accompanied by a change of the redox state and the layer charge of the material.

Spectroscopic studies of synthetic and natural ringwoodite, γ-(Mg, Fe)2SiO4

Synthetic ringwoodite γ-(Mg1−x Fe x )2SiO4 of 0.4 ≤ x ≤ 1.0 compositions and variously colored micro-grains of natural ringwoodite in shock metamorphism veins of thin sections of two S6-type chondrites were studied by means of microprobe analysis, TEM and optical absorption spectroscopy. Three synthetic samples were studied in addition with Mossbauer spectroscopy. The Mossbauer spectra consist of two doublets caused by VIFe2+ and VIFe3+, with IS and QS parameters close to those established elsewhere (e.g., O’Neill et al. in Am Mineral 78:456–460, 1993). The Fe3+/Fetotal ratio evaluated by curve resolution of the spectra, ranges from 0.04 to 0.1. Optical absorption spectra of all synthetic samples studied are qualitatively very similar as they are directly related to the iron content. They differ mostly in the intensity of the observed absorption features. The spectra consist of a very strong high-energy absorption edge and a series of absorption bands of different width and intensity. The three strongest and broadest absorptions of them are attributed to splitting of electronic spin-allowed 5 T 2g → 5 E g transitions of VIFe2+ and intervalence charge-transfer (IVCT) transition between ferrous and ferric ions in adjacent octahedral sites of the ringwoodite structure. The spin-allowed bands at ca. 8,000 and 11,500 cm−1 weakly depend on temperature, whilst the Fe2+/Fe3+ IVCT band at ~16,400 cm−1 displays very strong temperature dependence: i.e., with increasing temperature it decreases and practically disappears at about 497 K, a behavior typical for bands of this type. With increasing pressure the absorption edge shifts to lower energies while the spin-allowed bands shift to higher energy and strongly decreases in intensity. The IVCT band also strongly weakens and vanishes at about 9 GPa. We assigned this effect to pressure-induced reduction of Fe3+ in ringwoodite. By analogy with synthetic samples three broad bands in spectra of natural (meteoritic) blue ringwoodite are assigned to electronic spin-allowed transitions of VIFe2+ (the bands at ~8,600 and ~12,700 cm−1) and Fe2+/Fe3+ IVCT transition (~18,100 cm−1), respectively. Spectra of colorless ringwoodite of the same composition consist of a single broad band at ca. 12,000 cm−1. It is assumed that such ringwoodite grains are inverse (Fe, Mg)2SiO4-spinels and that the single band is caused by the split spin-allowed 5 E → 5 T 2 transition of IVFe2+. Ringwoodite of intermediate color variations between dark-blue and colorless are assumed to be partly inversed ringwoodite. No glassy material between the grain boundaries in the natural colored ringwoodite aggregates was found in our samples and disprove the cause of the coloration to be due to light scattering effect (Lingemann and Stoffler in Lunar Planet Sci 29(1308), 1998).

Assessing the Redox Reactivity of Structural Iron in Smectites Using Nitroaromatic Compounds As Kinetic Probes

Structural Fe(II) in clay minerals is an important source of electron equivalents for the reductive transformation of contaminants in anoxic environments. We investigated which factors control the reactivity of Fe(II) in smectites including total Fe content Fe(II)/total Fe ratio, and excess negative charge localization using 10 nitroaromatic compounds (NACs) as reactive probe molecules. Based on evidence from this work and previous spectroscopic studies on Fe redox reactions in iron-rich smectites, we propose a kinetic model for quantifying the reactivity, abundance, and interconversion rates of two distinct Fe(II) sites in the minerals' octahedral sheet. Excellent agreement between observed biphasic NAC reduction kinetics and model fits points toward existence of two types of Fe(II) sites exhibiting reactivities that differ by 3 orders of magnitude in iron-rich ferruginous smectite (SWa-1) and Olberg montmorillonite. Low structural Fe content, as found in Wyoming montmorillonite (SWy-2), impedes the formation of highly reactive Fe sites and results in pseudo-first order kinetics of NAC reduction that originate from the presence of a single type of Fe(II) species of even lower reactivity. Similar correlations of one-electron reduction potentials of the NACs vs their second order reduction rate constants for all smectite suspensions suggest that contaminant-Fe(II) interactions were identical in all smectite minerals.

Weathering of Martian and Earth surface studied by Mössbauer spectroscopy

Martian regolith and Earth’s basaltoid samples have been investigated by means of Mossbauer spectroscopy. The identification of the same minerals: olivine, pyroxene, magnetite, hematite and confrontation of the Fe3 + /Fe2 + , Fe3 + /Fetot, Fe2 + /Fetot ratios are presented. Co-existence of olivine and hematite in Martian regolith, absent in presented by authors terrestrial samples has been tentatively explained.

Viscosity of andesite melts and its implication for magma mixing prior to Unzen 1991–1995 eruption

The viscosity of an iron-bearing melt with composition similar to Unzen andesite was determined experimentally in the high (10 9–10 10.5 Pa s) and low (5–1000 Pa s) viscosity range using a parallel plate viscometer and the falling sphere method, respectively. Falling sphere experiments were carried out in an internally heated argon pressure vessel and in a piston cylinder apparatus at 1323 to 1573 K and 200 to 2000 MPa. Creep experiments were performed in the temperature range of 747–845 K at 300 MPa. The water content of the melt varies from nominally dry to 6.2 wt.% H 2O. The Fe 2+/Fe tot ratio was determined for each sample in the quenched glass using a colorimetric method. Pressure has minor influence on the viscosity compared with the effect of temperature, water content (main compositional parameter controlling the viscosity) or with the Fe 2+/Fe tot ratio (especially important at low water content of the melt). Based on our new viscosity data and literature data with measured Fe 2+/Fe tot ratio we propose a new empirical equation to estimate the viscosity η (in Pa s) of andesitic melts as a function of temperature T (in K), water content w (in wt.%) and Fe 2+/Fe tot ratio. The derived relationship reproduces the experimental data (87 in total) in the viscosity range from 10 0.5 to 10 13 Pa s with a 1σ standard deviation of 0.17 log units. However, application of this calculation model is limited to Fe 2+/Fe tot Fe tot > 0.3 and to temperatures above T g. Moreover, in the high viscosity range the variation of viscosity with water content is constrained only by few experimental data and needs verification by additional measurements. The viscosity data are used to interpret mixing processes in the Unzen magma chamber prior to 1991–1995 eruption. We demonstrate that the viscosities of the rhyolite and andesite melts from the two end-member magmas are nearly identical prior and during mixing, enabling efficient magma mixing.

Different isotope and chemical patterns of pyrite oxidation related to lag and exponential growth phases of Acidithiobacillus ferrooxidans reveal a microbial growth strategy

The solution chemistry during the initial (slow increase of dissolved iron and sulfate) and main stage (rapid increase of dissolved iron and sulfate) of pyrite leaching by Acidithiobacillus ferrooxidans (Af) at a starting pH of 2.05 shows significant differences. During the initial stage, ferrous iron (Fe2+) is the dominant iron species in solution and the molar ratio of produced sulfate (SO42−) and total iron (Fetot) is 1.1, thus does not reflect the stoichiometry of pyrite (FeS2). During the main stage, ferric iron (Fe3+) is the dominant iron species in solution and the SO42−:Fetot ratio is with 1.9, close to the stoichiometry of FeS2. Another difference between initial and main stage is an initial trend to slightly higher pH values followed by a drop during the main stage to pH 1.84. These observations raise the question if there are different modes of bioleaching of pyrite, and if there are, what those modes imply in terms of leaching mechanisms.Different oxygen and sulfur isotope trends of sulfate during the initial and main stages of pyrite oxidation confirm that there are two pyrite bioleaching modes. The biochemical reactions during initial stage are best explained by the net reaction FeS2+3O2⇒Fe2++SO42−+SO2(g). The degassing of sulfur dioxide (SO2) acts as sink for sulfur depleted in 34S compared to pyrite, and is the cause of the SO42−:Fetot ratio of 1.1 and the near constant pH. During the exponential phase, pyrite sulfur is almost quantitatively converted to sulfate, according to the net reaction FeS2+15/4O2+1/2H2O⇒Fe3++2SO42−+H+. We hypothesize that the transition between the modes of bioleaching of pyrite is due to the impact of the accumulation of ferrous iron, which induces changes in the metabolic activity of Af and may act as an inhibitor for the oxidation of sulfur species. This transition defines a fundamental change in the growth strategy of Af. A mode, where bacteria gain energy by oxidation of elemental sulfur to sulfite but show little growth is switched into a mode, where bacteria gain a smaller amount of energy by the oxidation of ferrous iron, but induce much faster pyrite leaching rates due to the production of ferric iron.

Red-staining of the wall rock and its influence on the reducing capacity around water conducting fractures

Red-staining and alteration of wall rock is common around water conducting fractures in the Laxemar–Simpevarp area (SE Sweden), which is currently being investigated by the Swedish Nuclear Fuel and Waste Management Co. (SKB) in common with many other places. Red-staining is often interpreted as a clear sign of oxidation but relevant analyses are seldom performed. The area is dominated by Palaeoproterozoic crystalline rocks ranging in composition from quartz monzodiorite to granite. In this study wall rock samples have been compared with reference samples from within 0.1 to 1 m of the red-stained rock, in order to describe mineralogical and geochemical changes but also changes in redox conditions. A methodology for tracing changes in mineralogy, mineral and whole rock chemistry and Fe3+/Fetot ratio in silicates and oxides in the red-stained wall rock and the reference rock is reported. The results show that the red-stained rock adjacent to the fractures displays major changes in mineralogy; biotite, plagioclase and magnetite have been altered and chlorite, K-feldspar, albite, sericite, prehnite, epidote and hematite have been formed. The changes in chemistry are however moderate; K-enrichment, Ca-depletion and constant Fetot are documented. The Fe3+/Fetot ratio in the oxide phase is higher in the red-stained samples whereas the Fe3+/Fetot ratio in the silicate phase is largely similar in the wall rock and the reference samples. Because most of the Fe is hosted in the silicate phase the decrease in reducing capacity (Fe2+), if any, in the red-stained wall rock is very small and not as high as macroscopic observations might suggest. Instead, the mineralogical changes in combination with the modest oxidation and formation of minute hematite grains in porous secondary minerals in pseudomorphs after plagioclase have produced the red-staining. Increased porosity is also characteristic for the red-stained rock. Moderate alteration in the macroscopically fresh reference rock shows that the hydrothermal alteration reaches further from the fracture than the red-staining. The extent of the red-staining can therefore not be used in the same way as the extent of the alteration adjacent to a fracture. The increase in porosity in the red-stained rock may result in enhanced retention of radio-nuclides due to an increased sorptivity and diffusion close to the fracture. The hydrothermal alteration causing the red-staining is thought to have occurred at temperatures of about 250–400 °C, based on the secondary mineralogy. The major part of this alteration in the area is assumed to be related to fluid circulation associated with the intrusion of the Mesoproterozoic Götemar and Uthammar granites nearby.

Preparation of Fe(II)-montmorillonite by reduction of Fe(III)-montmorillonite with ascorbic acid

A new and convenient method is described here to prepare Fe(II)-montmorillonite (mont) using Fe(III)-mont as the starting material. Fe(III)-mont was treated with ascorbic acid (vitamin C) solution at 70 °C for ~ 12 h in Ar atmosphere. All of the adsorbed iron(III) was reduced to form Fe(II)-mont. The presence of any associated Fe-oxide phase in the starting material, Fe(III)-mont, was rather advantageous to reach stoichiometric Fe(II)-mont. The Fe 2+/Fe total ratio of the iron extracted from freshly prepared Fe(II)-mont was found to be close to unity. The total amount of iron extracted in 1 M NH 4Cl was equivalent to the cation exchange capacity of the parent montmorillonite (Kunipia F montmorillonite) in terms of Fe(II), provided there was sufficient iron(III) in the starting material. The basal spacing at a relative humidity (RH) of 40% was 14.7 Å, typical of montmorillonite with divalent interlayer cations. FTIR spectra showed no significant changes in the basic clay mineral structure. The structure was further proved by Mössbauer spectra at room temperature (RT). Thus Fe(II)-mont preparation as described here was found to be highly convenient for routine preparation in the ambient atmospheric condition, also for large quantities. The feasibility of solid-state preparation of Fe(II)-mont was also demonstrated. The stability of Fe(II)-mont was proved in dispersion in water under partially deoxygenated condition. There was about 30% and 70% oxidation of interlayer Fe(II) ions in about 47 days at RT and 65 °C. Thus, the interlayer Fe(II) ions were gradually oxidized (i.e., not rapidly, as anticipated). For long-time preservation of the Fe(II)-mont sample, it is essential to keep deoxygenated, low RH (< 40%) conditions and without exposing to elevated temperatures.

Devitrification and High Temperature Properties of Mineral Wool

Mineral wool products can be used for thermal and acoustic insulation as well as for fire protection. The high temperature properties and the crystallization behaviour (devitrification) of the amorphous fibres during heating have been examined. Commercial stone wool and commercial hybrid wool (stone wool produced by a glass wool process) have been compared, as well as specially produced stone wool fibres. The fibres differed in chemical compositions and degree of oxidation given by Fe3+/Fetotal ratios. The materials were studied by thermal stability tests, X-ray diffraction, Mössbauer spectroscopy, secondary neutral mass spectroscopy, differential scanning calorimetry and thermal gravimetric analysis. When stone wool fibres were heated at 800 °C in air, oxidation of Fe2+ to Fe3+ occurred simultaneously with migration of divalent cations (especially Mg2+) to the surface. Decreasing Fe3+/Fetotal ratios resulted in increasing migration and improved thermal stability. The cations formed a surface layer mainly consisting of MgO. When heated to above 800 °C, bulk crystallization of the fibres took place with diopside and nepheline as the main crystalline phases. Commercial stone wool and the specially made fibres were considerably more temperature stable than the commercial hybrid wool. Commercial hybrid wool has a high Fe3+/Fetotal ratio of 65% resulting in less migration of cations during heat treatment.

Electronic conductivity in zinc iron phosphate glasses

The electrical properties of (40−x)ZnO–xFe2O3–60P2O5 (x=10, 20, 30mol%) glasses were measured by impedance spectroscopy in the frequency from 0.01Hz to 4MHz and the temperature range from 303 to 473K. It was shown that the dc conductivity strongly depends on the Fe2O3 content and Fe(II)/Fetot ratio. The increase in dc conductivity for these glasses is attributed to the increase in Fe2O3 content from 10 to 30mol%. With increasing Fe(II) ion content from 6% to 17% the dc conductivity increases. This indicated that the conductivity arises mainly from polaron hopping between Fe(II) and Fe(III) ions suggesting an electron conduction in these glasses. By applying scaling on conductivity data measured at different temperatures, single master curve was obtained for each glass. On the other hand, deviation from the master curve at high frequencies was observed for glasses with different compositions. This deviation originates from a various mobility of charge carriers in different glass structures. Raman spectra showed the change of structure, from metaphosphate to pyrophosphate, with increasing Fe2O3 content from 10 to 30mol%.