Occurrence of zinc in granitic biotites

37 granitic plutons in Great Britain and the United States were sampled and biotite separates prepared. These biotites were analysed for zinc and iron and a modal (point-count) analysis was conducted on the granitic samples. The results of these analyses were examined for differences that would characterise mineralised and non-mineralised granites, and for differences between biotites coexisting with muscovite and those coexisting with hornblende. The possibility of differentiating between mineralised and non-mineralised granites on the basis of the zinc content of biotites is rejected, but significant differences in composition are found between biotites from muscovite-biotite-granites and those from hornblende-biotite-granites. Muscovite-bearing granites have low biotite contents, these biotites having high iron and low zinc concentrations; hornblende-bearing granites have high biotite contents but these biotites, in contrast, have comparitively low iron and high zinc concentrations.

---

Zusammenfassung Biotitkonzentrate aus 37 granitischen Plutonen aus Großbritannien und den U.S.A. wurden auf ihre Zn- und Fe-Gehalte analysiert. Granite, mit denen Zn-Vererzungen zusammenhängen, können nach diesen Untersuchungen nicht aufgrund der Zn-Gehalte ihrer Biotite von Graniten ohne Zn-Vererzungen unterschieden werden. Die Biotite aus muskovit- und aus hornblendeführenden Graniten unterscheiden sich hingegen: Erstere haben hohe Fe- und niedrige Zn-Konzentrationen bei geringem Biotitanteil am Gestein; bei letzteren ist es umgekehrt.

     

鄂东南铜山口铜(钼)矿床黑云母矿物化学特征及其对岩石成因与成矿的指示

在鄂东南铜山口铜(钼)矿床存在着3种产状的黑云母,分别是赋存于花岗闪长斑岩矿物颗粒间的黑云母、钾长石斑晶内的黑云母以及蚀变带内的黑云母。本文运用电子探针(EMPA)和激光剥蚀-等离子体质谱(LA-ICP-MS)技术,对这3种不同产状的黑云母进行了原位成分分析。结果表明,3种产状的黑云母Ti的含量介于0.38～0.45之间,且Mg/(Mg+Fe)介于0.53～0.72之间,类似于岩浆成因黑云母的成分特征。3种产状的黑云母MgO和FeOT值差别较大,但Al2O3、TiO2、SiO2、Na2O和K2O值差别不明显。作为花岗闪长斑岩质熔体中Rb、Ba、Nb、Ta等不相容元素及Sc、V、Co、Ni、Cr等相容元素的主要载体,黑云母的U、Th、Pb、Sr、Zr、Hf、Y等元素含量显著低于主岩,而且高场强元素Nb、Ta受后期岩浆热液作用的影响较弱。黑云母并不是影响全岩稀土特征的主要矿物相。铜山口花岗闪长斑岩的形成与幔源岩浆作用关系密切,并可能与板块俯冲作用相关。黑云母含Cu量的高低并不是衡量侵入体是否成矿的有效指标,但体系中高的氧逸度(logfO2NiNiO+1),有利于斑岩铜矿的形成。与Cu成矿有关的黑云母具有高镁低铁的特征,与Sn成矿有关的黑云母则具有高铁低镁的特征。     

The degree of oxidation of iron in synthetic iron-magnesian biotites

Summary Chemical analytical and pyrolytical methods have been used to study the Fe⁺²/Fe⁺³ ratios and dehydroxylation reactions in synthetic biotites. It has been found for the biotites with Fe/(Fe + Mg) of 20 to 70 mole % that the oxidation degree decreases from 26 to 16% with increasing iron. Based on the measured amounts of water and hydrogen released during pyrolysis it is inferred that the deprotonization is a dominant reaction at low temperatures (T 600°C), accompanied by dehydration as the temperature increases. Depending on the composition, a complete dehydroxylation takes place at T 900 °C, and the measured amount of water corresponds to the iron oxidation degree in the starting samples. The results of this study have important implications with respect to determination of the formation conditions of biotite-bearing rocks, and also for improvement of the techniques for determination of different valence of iron and water.

---

Le degré d'oxidation du fer en biotites synthétiques contenants le fer et le magnésium

Résumé Des méthodes chimiques et pyrolitiques ont été utilisées pour l'étude des rapports Fe⁺²/Fe⁺³ et de la réaction de la déhydroxilation en biotites synthétiques. On a trouvé pour les biotites avec Fe/(Fe + Mg) de 20-70 mole % que le degré d'oxidation décroît à partir de 26 jusqu'à 16% pendent que le contenu du fer s'accroît. Sur la base de la quantité d'eau et hydrogène liberée pendant la pyrolyse, on infère que la déprotonisation est une réaction dominante à températures basses (T = 600°C), mais quand la température s'accroît, la déprotonisation est accopagnée de la déhydratation. Dépendant de la composition il y a une déhydroxilation complète à T = 900°C, et la quantité de l'eau mesurée correspond au degré d'oxidation du fer dans les specimens initials. Les résultats de cette étude infuencent la détermination des conditions de formation des roches contenantes biotite et aussi l'amélioration des techniques de la détermination du fer de valences différentes et de l'eau.

     

Biomineralization of lepidocrocite and goethite by nitrate-reducing Fe(II)-oxidizing bacteria: Effect of pH, bicarbonate, phosphate, and humic acids

Fe(III) solid phases are the products of Fe(II) oxidation by Fe(II)-oxidizing bacteria, but the Fe(III) phases reported to form within growth experiments are, at times, poorly crystalline and therefore difficult to identify, possibly due to the presence of ligands (e.g., phosphate, carbonate) that complex iron and disrupt iron (hydr)oxide precipitation. The scope of this study was to investigate the influences of geochemical solution conditions (pH, carbonate, phosphate, humic acids) on the Fe(II) oxidation rate and Fe(III) mineralogy. Fe(III) mineral characterization was performed using ⁵⁷Fe-Mössbauer spectroscopy and μ-X-ray diffraction after oxidation of dissolved Fe(II) within Mops-buffered cell suspensions of Acidovorax sp. BoFeN1, a nitrate-reducing, Fe(II)-oxidizing bacterium. Lepidocrocite (γ-FeOOH) (90%), which also forms after chemical oxidation of Fe(II) by dissolved O₂, and goethite (α-FeOOH) (10%) were produced at pH 7.0 in the absence of any strongly complexing ligands. Higher solution pH, increasing concentrations of carbonate species, and increasing concentrations of humic acids promoted goethite formation and caused little or no changes in Fe(II) oxidation rates. Phosphate species resulted in Fe(III) solids unidentifiable to our methods and significantly slowed Fe(II) oxidation rates. Our results suggest that Fe(III) mineralogy formed by bacterial Fe(II) oxidation is strongly influenced by solution chemistry, and the geochemical conditions studied here suggest lepidocrocite and goethite may coexist in aquatic environments where nitrate-reducing, Fe(II)-oxidizing bacteria are active.     

A reassignment of the optical absorption bands in biotites

The electronic absorption spectra of three biotites with largely differing Fe²⁺/Fe³⁺ ratios were studied before and after thermal dehydration and oxidation of divalent iron. Three absorption bands near 17,100, 20,500 and 24,100 cm^?1 and an absorption edge at slightly higher energies are assigned to trivalent iron present in clusters of strongly interacting ions. The presence of additional broad absorption bands due to intervalence transfer between Fe²⁺ and Fe³⁺ or Ti⁴⁺ in this region cannot be excluded for biotites with high Fe²⁺ concentrations. Three bands at lower energies show a satisfactory correlation with concentration of divalent iron and decrease in the same proportions with oxidation. We therefore assign them to split components of the spin-allowed ligand field transition of Fe²⁺ at the M ₁ and M ₂ sites. This contradicts the assignment of one of these bands to an intervalence charge transfer between Fe²⁺ and Fe³⁺ by previous authors. It is shown that there is no indisputable evidence against our assignment.     

Spurenelemente in Biotiten aus Graniten und Gneisen

91 biotites (53 from granites, 35 from highly metamorphic gneisses, 3 from redwitzites) were separated and analyzed for Fe, Mn, Zn, Cl, Sn, Ni, Co, Or, Cu, V, Mo, Pb. Biotites from gneisses contain much more Ni, Co, Cr, V but less Fe, Mn, Zn than those from granites. However, the distinction between biotites from gneisses and from granites on the basis of these elements is not certain. If a gneiss undergoes anatexis, the contents of Ni, Co, Cr, V, Zn and Sn of the preexistent biotite fractionate: Zn, Sn and Pe enter the anatectic melt readily while Ni, Co, Cr and V concentrate in the remaining matter (restite). Ni, Co, Cr and V are strongly positively correlated with one another but negatively with Fe and Zn, the latter being positively correlated with Pe. The chemical composition of biotites from granites depends not only on a potential degree of secondary decomposition into chlorite and muscovite but much more on the percentage of biotite in the rock: The more biotite, the higher the content of Ni, Co, Cr, V and the lower Fe, Zn and Sn in the biotite. Thus, it is possible to distinguish between normal and abnormal concentrations of an element in a biotite and in a rock. This might be useful in geochemical prospecting. Abnormal high concentrations of Sn and Zn were found in biotites from some granites which are connected with mineralizations of these elements. It is impossible hitherto to gain informations about the history and the parental material of a granitic magma from the minor elements in the rock or the biotite because their concentrations depend on how much biotite could be incorporated by the melt. The distribution coefficient of Cl between the lattice of 4 biotites and their fluid inclusions was determined to be 0,08.     

Ferromagnetic or antiferromagnetic Fe III spin configurations in sheet silicates

Mössbauer spectra of glauconite and nontronite recorded at temperatures down to 1.3K and in applied fields up to 4.5 T show that Fe III spin configurations are respectively ferromagnetic and antiferromagnetic. It is shown that in a particular material depending on the distribution and concentration of Fe III in the silicate sheet either mode might occur. A new model of competing nearest-neighbour (J ₁) and next-nearest-neighbour (J ₂) magnetic exchange interactions in the triangular lattice is introduced to account for the results. From available magnetic susceptibilities we estimate ∣J ₁∣～6∣J ₂∣. The results lead to the conclusion that the Fe III cations are highly ordered in glauconite and occupy cis sites so as to maximize their mutual separations.     

赤峰东南部建平群斜长角闪岩黑云母矿物化学特征及地质意义

利用电子探针、激光剥蚀-电感耦合等离子体质谱测试技术,对赤峰东南部建平群斜长角闪岩中黑云母的常量元素、微量(稀土)元素进行了测试分析。研究表明:赤峰东南部建平群斜长角闪岩中的黑云母主量元素以富Fe、Mg为特征,为高铁镁云母;黑云母稀土元素含量低,轻重稀土分馏较强,δEu、δCe均值为正,为选择Ce、Nd的配分型矿物;黑云母中Rb、Ba、Pb和Cs等大离子亲石元素富集,特别是Cs、Ba明显富集,而Sr略有亏损;高场强元素Zr、Hf、Sc等亏损,较富集的元素为U、Th、Nb、Ta元素;亏损的亲铁元素为Cr、Ni,而显著富集的元素为V、Ti;亲硫元素Cu亏损而Zn明显富集;分散元素Ga有明显的富集。     

Shock-induced mechanical deformations in biotites from crystalline rocks of the ries crater (Southern Germany)

Mechanical deformation features in shocked biotites from crystalline rocks of the Ries crater are: kink bands, planar elements, and plastic lattice deformations as determined by X-ray investigations.Kink bands can be observed in micas of various pressure histories (stages 0, I, II and less frequently stage III of shock metamorphism). Kink bands in shocked micas are less symmetrical than kinks of static origin. Asymmetry increases with increasing dynamic pressures. Moreover, kink band width is sensitive against changing peak pressures. Distribution of kinked and undistorted micas within a rock permits to fix the shock front direction. Shock-induced kinks in micas are produced by various gliding processes in the cleavage plane (001).Planar elements seldom occur in biotites of shock stages II and III and have never been described in endogenic rocks. Up to now orientations of planar elements parallel to (111), (1¯11), (112) and (11¯2) have been determined. Planar elements are interpreted as planes of plastic lattice gliding. {[110]} is supposed to be the main gliding direction. In the same pressure region other plastic lattice deformations have been determined. They are orientated parallel to (001), (100) and (¯132) or (201) which results from single crystal X-ray investigations and may represent planes of plastic lattice gliding. The dependency of formation of gliding planes and gliding directions on increasing dynamic pressures will be discussed.     

Magnetic properties of sheet silicates; 2:1 layer minerals

Susceptibility, magnetisation and Mössbauer measurements are reported for a representative selection of 2:1 layer phyllosilicates. Eight samples from the mica, vermiculite and smectite groups include examples diluted in iron which are paramagnetic at all temperatures, as well as iron-rich silicates which order magnetically below 10 K. Anisotropic susceptibility of crystals of muscovite, biotite and vermiculite is quantitatively explained with a model where the Fe²⁺ ions lie in sites of effective trigonal symmetry, the trigonal axis lying normal to the sheets. The ferrous ground state is an orbital singlet. Ferric iron gives an isotropic contribution to the susceptibility. Fe²⁺-Fe²⁺ exchange interactions are ferromagnetic with y ～ 2 K, whereas Fe³⁺-Fe³⁺ coupling is antiferromagnetic in the purely ferric minerals. A positive paramagnetic Curie temperature for glauconite may be attributable to Fe²⁺ → Fe³⁺ charge transfer. Magnetic order was found to set in inhomogeneously for glauconite at 1–7 K. One biotite sample showed an antiferromagnetic transition at T _N =7 K marked by a well-defined susceptibility maximum. Its magnetic structure, consisting of ferromagnetic sheets with moments in their planes coupled antiferromagnetically by other, weak interactions, resembles that found earlier for the 1:1 mineral greenalite.     

Mössbauer and X-ray studies on the oxidation of annite and ferriannite

Various indirect approaches have been used to estimate the amount of the oxy end member in biotites. Since this could be used to indicate the formation temperatures and the oxygen fugacities of biotite bearing rocks, its determination requires careful experimental work. The oxidation of the iron micas annite and ferriannite has been studied by Mössbauer effect (ME) and X-ray diffraction (XRD). A model that relates ME parameters to reaction mechanism in biotites has been proposed according to which oxidation at M2 is relatively easier than oxidation at M1, a result which agrees well with the site assignment adopted. Heating synthetic ferriannite at 300° C produced an oxidation ratio that corresponds to the oxy-ferriannite end member where the ME spectra show that the two ferrous peaks are well resolved. Finally, ME and XRD results show that the c-axis decreases by 0.001 Å for every percent oxybiotite added.     

Activity-composition relationship in Tschermak's substituted Fe biotites at 700°C, 2 kbar

The reaction-displacement technique was applied to the end-member reaction annite = sanidine + magnetite + H₂ in order to determine the activity of the annite component (a _Ann) in iron biotites with variable degrees of the Tschermak's substitution (^[6]Fe + ^[4]Si = ^[6]Al + ^[4]Al). Based on the simplified relation a _Ann = f _{H 2}/foH₂ (foH₂ = hydrogen fugacity of the end-member reaction at P, T), two types of experiments were performed at 700°C / 2 kbar: Type I used Fe-Al biotites of known starting composition together with sanidine + magnetite + H₂O. This assemblage was exposed to various f _{H 2} conditions (f _{H 2} < foH₂) produced in the pressure vessel either by using different ratios of water/oil as pressure medium (f _{H 2} in this case was measured by the hydrogen sensor technique), or by the Ni′NiO buffer. The composition of the Fe-Al biotites changed through incorporation or release of the annite component in response to the externally imposed f _{H 2}. By using opposite biotite starting compositions, the equilibrium composition as a function of f _H2 was bracketed. For type II, f _{H 2} in equilibrium with a specific combination of fine-grained Fe-Al biotite (+ sanidine + magnetite + H₂O) was measured internally by application of the hydrogen sensor technique. Both type I and type II experiments yield consistent results demonstrating that a fine-grained assemblage of Fe-Al biotite (+ sanidine + magnetite + H₂O) is able to act as a sliding-scale buffer. The final chemical composition of the Fe-Al biotite after the experiments was determined by electron microprobe and Mössbauer spectroscopy. The ^[4]Al and ^[6]Al in the biotites are coupled according to the Tschermak's substitution. In the tetrahedral sheet 0.1 Al-atoms per formula unit are present in excess to the amount required to balance ^[6]Al, and all Fe-Al biotites contain 8–10% Fe³⁺. Therefore, they are not members of the pure annite - siderophyllite join, but have an almost constant amount (15 Mol%) of two additional Fe³⁺-bearing components (ferri-siderophyllite and a vacancy end-member). The volume - composition relationship obtained does not indicate excess molar volumes of mixing for the annite (Ann) - siderophyllite (Sid) binary. The data are consistent with a molar volume of annite of 15.46 ± 0.02 Jbar^–1 and of 15.06 ± 0.02 Jbar^–1 for siderophyllite. The experimentally determined activity - composition relation shows that biotites on the join annite - siderophyllite deviate negatively from ideality. A symmetric interaction parameter W_AnnSid is sufficient to represent the data within error. This was constrained as: W _AnnSid = –29 ± 4 kJmol^–1. This is in contradiction to empirical interaction parameters derived from natural assemblages for this binary that predict positive deviation from ideality. Reasons for this discrepancy are discussed.     

Geochemistry of biotites from granitic rocks,Northern Portugal

The biotites from a series of rocks ranging in composition from tonalite to granite have been analysed for both major and trace elements.The relations between chemical composition and paragenesis of the biotites are studied. Most biotites co-exist with potassium feldspar and ilmenite. Variations in composition can be correlated with the occurrence of amphibole, primary muscovite and aluminosilicates in the rocks.Variation diagrams of the trace element contents and element ratios of biotite are compared to those of the host rocks. Fractionation of elements can be defined more accurately as the influence of other mineral phases is eliminated.Variations in the proportions of the octahedrally co-ordinated Al, Ti and Fe³⁺ are correlated with the conditions of crystallization and comparisons made with biotites from other suites of calc-alkali rocks.In the light of the experimental data available, the petrographic observations and the chemical data it is apparent that biotites crystallized from systems in which fO₂ was buffered, its values remaining close to that of the buffer FMQ. From the same data, a temperature of 800°C for fO₂ = 10^?14to 10^?15 bars is deduced as prevalent during the crystallization of the tonalites while for the granites, at a temperature of crystallization of 680°C, fO₂ = 10^?16to 10^?18 bars.A calc-alkali trend of fractionation is therefore apparent with decreasing fO₂ while fH₂O₂ remains relatively high.     

Dependence of microbial magnetite formation on humic substance and ferrihydrite concentrations

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

新疆玉海-三岔口铜矿黑云母矿物化学特征及成岩成矿意义

黑云母的化学成分对于揭示结晶条件、岩石成因、成矿演化以及含矿性评价等具有重要的指示意义.利用电子探针（EPMA）对新疆玉海和三岔口铜矿中的黑云母进行成分分析,结果表明与成矿相关岩体的黑云母具有富镁贫铁的特征,而无矿化岩体中的黑云母则呈现富铁贫镁的特征.分析结果显示矿化岩体中黑云母为再平衡镁质黑云母,而无矿化岩体中黑云母为原生铁质黑云母;它们的寄主岩石均属于I型花岗岩,形成于与俯冲相关的构造背景,但是与矿化相关岩体的岩浆源区为壳幔混合来源,而无矿岩体的源区为地壳来源,形成过程中有新生地壳组分的混染;两类黑云母的结晶温度为529~677 ℃,寄主岩体的固结压力为1.1~2.8 kbar,均形成于高氧逸度条件;此外,黑云母的Mg/Fe和Fe2+/（Fe2++Mg2+）还可以区分斑岩型铜矿的含矿性.      

Iron(III) accumulations in inland saline waterways,Hunter Valley,Australia: Mineralogy,micromorphology and pore-water geochemistry

Discharge of Fe(II)-rich groundwaters into surface-waters results in the accumulation of Fe(III)-minerals in salinized sand-bed waterways of the Hunter Valley, Australia. The objective of this study was to characterise the mineralogy, micromorphology and pore-water geochemistry of these Fe(III) accumulations. Pore-waters had a circumneutral pH (6.2–7.2), were sub-oxic to oxic (Eh 59–453 mV), and had dissolved Fe(II) concentrations up to 81.6 mg L⁻¹. X-ray diffraction (XRD) on natural and acid-ammonium-oxalate (AAO) extracted samples indicated a dominance of 2-line ferrihydrite in most samples, with lesser amounts of goethite, lepidocrocite, quartz, and alumino-silicate clays. The majority of Fe in the samples was bound in the AAO extractable fraction (Fe^Ox) relative to the Na-dithionite extractable fraction (Fe^Di), with generally high Fe^Ox:Fe^Di ratios (0.52–0.92). The presence of nano-crystalline 2-line ferrihydrite (Fe₅HO₃·4H₂O) with lesser amounts of goethite (α-FeOOH) was confirmed by scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDX), and transmission electron microscopy (TEM) coupled with selected area electron diffraction (SAED). In addition, it was found that lepidocrocite (γ-FeOOH), which occurred as nanoparticles as little as ∼5 lattice spacings thick perpendicular to the (0 2 0) lattice plane, was also present in the studied Fe(III) deposits. Overall, the results highlight the complex variability in the crystallinity and particle-size of Fe(III)-minerals which form via oxidation of Fe(II)-rich groundwaters in sand-bed streams. This variability may be attributed to: (1) divergent precipitation conditions influencing the Fe(II) oxidation rate and the associated supply and hydrolysis of the Fe(III) ion, (2) the effect of interfering compounds, and (3) the influence of bacteria, especially Leptothrix ochracea.     

Discrimination between primary magmatic biotites, reequilibrated biotites and neoformed biotites

The ternary diagram TiO₂–FeO*–MgO (FeO* = FeO + MnO) is proposed as a quantitative objective tool for distinguishing between primary magmatic biotites and those that are more or less reequilibrated, or possibly neoformed, by or within a hydrothermal fluid. The limit of the domains of the primary magmatic biotites, the reequilibrated biotites and the neoformed biotites were determined on the basis of optical, paragenetic and chemical criteria. To cite this article: H. Nachit et al., C. R. Geoscience 337 (2005).     

Relative contributions of abiotic and biological factors in Fe(II) oxidation in mine drainage

The oxidation of Fe(II) is apparently the rate-limiting step in passive treatment of coal mine drainage. Little work has been done to determine the kinetics of oxidation in such field systems, and no models of passive treatment systems explicitly consider iron oxidation kinetics. A Stella II^™ model using Fe(II)_init concentration, pH, temperature, Thiobacillus ferrooxidans and O₂ concentration, flow rate, and pond volume is used to predict Fe(II) oxidation rates and concentrations in seventeen ponds under a wide range of conditions (pH 2.8 to 6.8 with Fe(II) concentrations of less than 240 mg L⁻¹) from 6 passive treatment facilities. The oxidation rate is modeled based on the combination of published abiotic and biological laboratory rate laws. Although many other variables have been observed to influence Fe(II) oxidation rates, the 7 variables above allow field systems to be modeled reasonably accurately for conditions in this study.Measured T. ferrooxidans concentrations were approximately 10⁷ times lower than concentrations required in the model to accurately predict field Fe(II) concentrations. This result suggests that either 1) the most probable number enumeration method underestimated the bacterial concentrations, or 2) the biological rate law employed underestimated the influence of bacteria, or both. Due to this discrepancy, bacterial concentrations used in the model for pH values of less than 5 are treated as fit parameters rather than empirically measured values.Predicted Fe(II) concentrations in ponds agree well with measured Fe(II) concentrations, and predicted oxidation rates also agree well with field-measured rates. From pH 2.8 to approximately pH 5, Fe(II) oxidation rates are negatively correlated with pH and catalyzed by T. ferrooxidans. From pH 5 to 6.4, Fe(II) oxidation appears to be primarily abiotic and is positively correlated with pH. Above pH 6.4, oxidation appears to be independent of pH. Above pH 5, treatment efficiency is affected most by changing design parameters in the following order: pH>temperature≈influent Fe(II)>pond volume≈O₂. Little to no increase in Fe(II) oxidation rate occurs due to pH increases above pH 6.4. Failure to consider Fe(II) oxidation rates in treatment system design may result in insufficient Fe removal.     

Iron isotope fractionation during microbially stimulated Fe(II) oxidation and Fe(III) precipitation

Interpretation of the origins of iron-bearing minerals preserved in modern and ancient rocks based on measured iron isotope ratios depends on our ability to distinguish between biological and non-biological iron isotope fractionation processes. In this study, we compared ⁵⁶Fe/⁵⁴Fe ratios of coexisting aqueous iron (Fe(II)_aq, Fe(III)_aq) and iron oxyhydroxide precipitates (Fe(III)_ppt) resulting from the oxidation of ferrous iron under experimental conditions at low pH (<3). Experiments were carried out using both pure cultures of Acidothiobacillus ferrooxidans and sterile controls to assess possible biological overprinting of non-biological fractionation, and both SO₄²⁻ and Cl⁻ salts as Fe(II) sources to determine possible ionic/speciation effects that may be associated with oxidation/precipitation reactions. In addition, a series of ferric iron precipitation experiments were performed at pH ranging from 1.9 to 3.5 to determine if different precipitation rates cause differences in the isotopic composition of the iron oxyhydroxides. During microbially stimulated Fe(II) oxidation in both the sulfate and chloride systems, ⁵⁶Fe/⁵⁴Fe ratios of residual Fe(II)_aq sampled in a time series evolved along an apparent Rayleigh trend characterized by a fractionation factor α_{Fe(III)aq-Fe(II)aq} ∼ 1.0022. This fractionation factor was significantly less than that measured in our sterile control experiments (∼1.0034) and that predicted for isotopic equilibrium between Fe(II)_aq and Fe(III)_aq (∼1.0029), and thus might be interpreted to reflect a biological isotope effect. However, in our biological experiments the measured difference in ⁵⁶Fe/⁵⁴Fe ratios between Fe(III)_aq, isolated as a solid by the addition of NaOH to the final solution at each time point under N₂-atmosphere, and Fe(II)_aq was in most cases and on average close to 2.9‰ (α_{Fe(III)aq-Fe(II)aq} ∼ 1.0029), consistent with isotopic equilibrium between Fe(II)_aq and Fe(III)_aq. The ferric iron precipitation experiments revealed that ⁵⁶Fe/⁵⁴Fe ratios of Fe(III)_aq were generally equal to or greater than those of Fe(III)_ppt, and isotopic fractionation between these phases decreased with increasing precipitation rate and decreasing grain size. Considered together, the data confirm that the iron isotope variations observed in our microbial experiments are primarily controlled by non-biological equilibrium and kinetic factors, a result that aids our ability to interpret present-day iron cycling processes but further complicates our ability to use iron isotopes alone to identify biological processing in the rock record.     

Rb-Sr and K-Ar systems of biotite in surface environments regulated by weathering processes with implications for isotopic dating and hydrological cycles of Sr isotopes

Biotite is widely used for Rb-Sr and K-Ar isotopic dating and influences Sr isotope geochemistry of hydrological regimes. The isotopic system of biotite behaves diversely in response to surface weathering; i.e. the complete preservation of original Rb-Sr and K-Ar isotopic ages or dramatic reduction. In this study, we have explored the relation between the behavior of isotopic systems and complex weathering processes of biotites in the weathering profiles distributed on the Mesozoic granitoids in South Korea. In the lower parts of the profiles, biotite in the early stages of weathering was transformed into either oxidized biotite or hydrobiotite, with a mass release of ⁸⁷Sr and ⁴⁰Ar forced by the rapid oxidation of ferrous iron. During the transformation to oxidized biotite, ⁸⁷Sr and ⁴⁰Ar were preferentially released relative to Rb and K, respectively, via solid-state diffusion through the biotite lattice, resulting in a drastic reduction of original isotopic age. The reduction of Rb-Sr age was greater than that of K-Ar age because K was preferentially released over Rb whereas ⁸⁷Sr and ⁴⁰Ar were released proportionally to each other. However, during the transformation of biotite to hydrobiotite (i.e., to regularly interstratified biotite-vermiculite), ⁸⁷Sr, Rb, ⁴⁰Ar, and K were completely retained in the alternating biotite interlayer, and thus the original isotopic age can be preserved. In the upper parts of the profiles, where iron oxidation was almost completed, ⁸⁷Sr, Rb, ⁴⁰Ar, and K were gradually and proportionally released, with no further significant change in isotopic age during the gradual transformation of the early-formed oxidized biotite into hydrobiotite and vermiculite or during their final decomposition to kaolinite. The ratios and amounts of isotopes released from weathered biotites are dependent upon the degree of iron oxidation and the pathways of mineralogical transformation. Regional and local variations in isotopic systems affected by particular weathering processes should be considered when dating biotite or biotite-bearing rocks in weathering environments, modeling the transfer of Sr isotopes to hydrologic regimes, and tracking the provenance of sediments.