Computer simulations of the interactions of the (0 1 2) and (0 0 1) surfaces of jarosite with Al, Cd, Cu and Zn

Jarosite is an important mineral on Earth, and possibly on Mars, where it controls the mobility of iron, sulfate and potentially toxic metals. Atomistic simulations have been used to study the incorporation of Al³⁺, and the M²⁺ impurities Cd, Cu and Zn, in the (0 1 2) and (0 0 1) surfaces of jarosite. The calculations show that the incorporation of Al on an Fe site is favorable on all surfaces in which terminal Fe ions are exposed, and especially on the (0 0 1) [Fe₃(OH)₃]⁶⁺ surface. Incorporation of Cd, Cu or Zn on a K site balanced by a K vacancy is predicted to stabilize the surfaces, but calculated endothermic solution energies and the high degree of distortion of the surfaces following incorporation suggest that these substitutions will be limited. The calculations also suggest that incorporation of Cd, Cu and Zn on an Fe site balanced by an OH vacancy, or by coupled substitution on both K and Fe sites, is unfavorable, although this might be compensated for by growth of a new layer of jarosite or goethite, as predicted for bulk jarosite. The results of the simulations show that surface structure will exert an influence on uptake of impurities in the order Cu > Cd > Zn, with the most favorable surfaces for incorporation being (0 1 2) [KFe(OH)₄]⁰ and (0 0 1) [Fe₃(OH)₃]⁶⁺.     

Holocene ochreous lacustrine sediments within the Famatina Belt,NW Argentina: A natural case for fossil damming of an acid drainage system

A 44 m-thick lacustrine succession of silty-clay banded ochres and subordinated sandstones, and conglomerates (known as the Corral Amarillo Formation) is superbly exposed within the Famatina Belt (Central Andes of Argentina) after deep entrenchment by the present-day Amarillo river due to strong recent uplifting and consequent relative drop in base level. The unusual ochreous-rich succession was produced by natural damming (3.48–3.54 ¹⁴C kyr BP) of an acid drainage system linked to the alteration cap of polymetallic deposits. Facies of silty-clay ochre (wet season) and banded ochre (dry season) from the paleolacustrine setting are composed of jarosite + goethite and goethite respectively. Geochemically, these layers record high concentrations of Fe₂O₃ (25–55 wt. %) and trace elements (Cu, Zn, Co, As, and Mo with mean concentrations of 2759; 2467; 109; 375 and 116 ppm, respectively). Their origin is inferred from a comparative analysis with the present-day Amarillo river, which has a pH of ∼3, (SO₄)²⁻ concentrations of ∼5000 mg/l, and jarosite as the dominant phase, in the upper catchments. Waters downstream have pH values of 3–4.5, (SO₄)²⁻ concentrations of ∼3000–480 mg/l, and schwertmannite as the dominant phase. Thus goethite in the paleolake facies is likely related to schwertmannite transformation by an aging process, whereas jarosite is probably transported from the river but could also be associated with post-depositional formation regulated by variations in grain size and the pore fluid chemistry. The Corral Amarillo Formation offers a Natural model, which may be employed to infer the effect on nature of acid drainage of mineralized areas.     

Catalytic ozonation of dimethyl phthalate by Ce-substituted goethite

Dimethyl phthalate (DMP) is ubiquitous in aquatic environments due to extensively used as plasticizer, which has received increasing attention in recent years. In this study, the catalytic ozonation of dimethyl phthalate was performed using Ce-substituted goethite as a novel catalyst, which was prepared by isomorphous substitution method. The specific surface area, pHpzc and surface hydroxyl density of the catalyst were determined. The catalyst was characterized using X-ray diffraction, scanning electron microscope and Fourier transform infrared spectroscopy. The removal efficiency of DMP was almost 100% after 30 min, and about 40% DMP was mineralized after 60 min, which was nearly four times higher than single ozonation. During catalytic ozonation process, anions (PO₄ ^3?, SO₄ ^2?, Cl^?) affected DMP degradation, indicating that surface hydroxyl groups on the surface of catalyst were main active sites. The electron transfer process by redox reaction between Ce³⁺/Ce⁴⁺, Fe²⁺/Fe³⁺ was proposed, and their interaction could also promote the formation of hydroxyl radicals. Ce-substituted goethite was an efficient catalyst for degradation of DMP by catalytic ozonation.     

Mineralogical and chemical characteristics of some natural jarosites

This paper presents a detailed study of the mineralogical, microscopic, thermal, and spectral characteristics of jarosite and natrojarosite minerals. Systematic mineralogic and chemical examination of a suite of 32 natural stoichiometric jarosite and natrojarosite samples from diverse supergene and hydrothermal environments indicates that there is only limited solid solution between Na and K at low temperatures, which suggests the presence of a solvus in the jarosite-natrojarosite system at temperatures below about 140 °C. The samples examined in this study consist of either end members or coexisting end-member pairs of jarosite and natrojarosite. Quantitative electron-probe microanalysis data for several natural hydrothermal samples show only end-member compositions for individual grains or zones, and no detectable alkali-site deficiencies, which indicates that there is no hydronium substitution within the analytical uncertainty of the method. In addition, there is no evidence of Fe deficiencies in the natural hydrothermal samples. Hydronium-bearing jarosite was detected in only one relatively young supergene sample suggesting that terrestrial hydronium-bearing jarosites generally are unstable over geologic timescales.Unit-cell parameters of the 20 natural stoichiometric jarosites and 12 natural stoichiometric natrojarosites examined in this study have distinct and narrow ranges in the a- and c-cell dimensions. There is no overlap of these parameters at the 1σ level for the two end-member compositions. Several hydrothermal samples consist of fine-scale (2-10 μm) intimate intergrowths of jarosite and natrojarosite, which could have resulted from solid-state diffusion segregation or growth zoning due to variations in the Na/K activity ratio of hydrothermal solutions.     

Comparative planetary mineralogy: Implications of martian and terrestrial jarosite. A crystal chemical perspective

The importance of the discovery of jarosite at the Meridiani Planum region of Mars is discussed. Terrestrial studies demonstrate that jarosite requires a unique environment for its formation, crystallizing from highly acidic (pH < 4) S-rich brines under highly oxidizing conditions. A likely scenario for jarosite formation on Mars is that degassing of shallow magmas likely released SO₂ that reacted with aqueous solutions in shallow aquifers or on the martian surface. This interaction forms both H₂SO₄ and H₂S. A martian oxidant must be identified to both oxidize H₂S to produce the required acidity of the fluid, and to oxidize Fe²⁺ to Fe³⁺. We suggest that reactions involving both sulfur and the reduction of CO₂ to CO may provide part of the answer. The jarosite crystal structure is truly remarkable in terms of its tolerance for the substitution of a large number of different cations with different ionic radii and charges. The structure accommodates hydrogen, oxygen, and sulfur, the stable isotope systematics of which are strong recorders of low-temperature fluid-rock-atmosphere interactions. Jarosite has been proven to be a robust chronometer for Ar-Ar and K-Ar dating techniques, and there is every reason to believe that U-Pb, Rb-Sr, and Nd-Sm techniques for older jarosite from Mars will also be robust. Although the discovery of jarosite on Mars alone, with no other analytical measurements on the phase, has given us insights to martian surficial processes, the true power of jarosite can not be exploited until jarosite is sampled and returned from Mars. Mars sample return is a long way off but, until then, we should be vigilant about examining martian meteorites for alteration assemblages that contain jarosite. A suite of jarosite samples representing a significant time span on Mars may hold the key to reading the record of martian atmospheric evolution.     

The genesis of the west shropshire orefield: Evidence from fluid inclusions,sphalerite chemistry,and sulphur isotopic ratios

The Pb + Zn + Ba veins of West Shropshire, England, occupy fractures in Ordovician and Precambrian rocks of the Shelve Inlier. Precipitation of sphaleritic ores was succeeded by galena + baryte mineralization, with chalcopyrite also occurring late in the mineralizing episode. Three generations of sphalerite are recognized, and second being chemically zoned with distinct Fe + Cd-rich growth zones. Associated with these iron-rich bands are smaller (∼ 10 μm) zones rich in indium (>1.0 wt%) and copper, electron electron probe microanalyses suggesting coupled substitution of Cu⁺ + In⁺³ in the ZnS. Based on the sequence of growth zones and their chemistry a sphalerite stratigraphy can be recognized within the orefield. Fluid inclusions studies reveal the mineralizing fluids to be highly saline (18–30 wt% CaCl₂ equivalent) Na + Ca-richbrines, with mineralizing temperatures in the range 200–120°C. A trend from higher temperature-low salinity to low temperature-high salinity fluids with time is recognized. Sulphide sulphur isotopic ratios are consistent and suggest a ΔS³⁴S_H2S of the mineralizing fluid of 10%, while ³⁴S_baryte values are in the range + 14 to + 19%, indicating separate sulphate and sulphide sulphur sources. The mineralogical, fluid inclusion, and isotopic data suggest the saline fluids rose into an open plumbing system where mineral precipitation was mainly controlled by fluid cooling. The baryte however, formed due to mixing with oan overlying sulphate-bearing reservioir. Theree possible fluid sources are considered, namely: Lower Carboniferous seawater, basinal brines, and metamorphic fluids. However, the information available does not allow the source to be positively identified.     

The preparation and characterization of vanadium-substituted goethite: The importance of temperature

Despite a close geo-chemical association between vanadium (V) and iron (Fe) in natural environments, there is little research on the substitution of V in goethite. To assess the effect of temperature on V-substitution in goethite, a series of V-substituted goethite were prepared under varying synthesis temperatures, and analysed using wet chemical and multi-spectroscopic techniques. Vanadium substitution was inversely related to synthesis temperatures and was hindered by the oxidation of V³⁺ to V^4+/5+ as indicated by X-ray absorption near-edge spectroscopy. The presence of V (V⁵⁺ > V⁴⁺ > V³⁺) at high temperature hindered the nucleation of goethite and crystal growth along particular faces resulting in large-sized and twinned crystals as shown by transmission electron microscopy. The large-sized goethite crystals released more Fe (mmoles) per unit surface area during proton-promoted dissolution than the smaller-sized crystals, which could be due to distorted V^4+/5+ local coordination environments in the mineral structure. The dissolution studies showed a heterogeneous distribution of V and/or crystal defects in goethite crystals. The results show that low synthesis temperatures preserved the oxidation state of V³⁺, which has ionic radius and hydrolytic properties similar to Fe³⁺, and hence resulted in as much as 13.3 mol per cent substitution. The structural stability of the goethite decreased upon V-substitution in order; V³⁺ > V⁴⁺ > V⁵⁺. This research provides important information about the interaction between temperature, V incorporation, and crystal structure properties of goethite for V sequestration and other potentially toxic metal cations.     

Hydrothermal jarosite and hematite in a pyroxene-hosted melt inclusion in martian meteorite Miller Range (MIL) 03346: Implications for magmatic-hydrothermal fluids on Mars

Low-temperature aqueous processes have been implicated in the generation of jarosite and hematite on the martian surface, but little is known regarding the role that high-temperature magmatic fluids may have played in producing similar assemblages on Mars. We have identified jarosite and hematite in a clinopyroxene-hosted melt inclusion in martian meteorite MIL 03346 that shows evidence of having been hydrothermally precipitated. In addition to jarosite and hematite, the melt inclusion contains titanomagnetite, pyrrhotite, potassic-chlorohastingsite, an iron-rich silicate glass and possibly goethite. These phases were identified and characterized using scanning electron microscopy (SEM), con-focal Raman-spectroscopy and electron probe microanalysis (EPMA).Based on observed textural relationships and the compositions of the hosted phases, we report that the jarosite-bearing melt inclusion in MIL 03346 has recorded a fluid-rich history that began in the magmatic stage and continued to low-temperatures. This history begins at entrapment of a volatile-rich silicate melt that likely reached fluid-saturation after only minor crystallization within the melt inclusion. This fluid, rich in chlorine, reacted with surrounding silicate material to produce the potassic-chlorohastingsite. As cooling proceeded, the liquid phase eventually became more oxidized and reacted with the pyrrhotite. Sulfide oxidation resulted in SO₄²⁻ formation and concomitant acid production, setting the stage for jarosite formation once the fluid cooled beyond the upper thermal stability of jarosite (∼200 °C). As the fluid cooled below 200 °C, jarosite continued to precipitate with hematite and/or goethite until equilibrium was established or reactions became kinetically unfavorable.This work suggests an additional jarosite-hematite formation pathway on Mars; one that may be important wherever magmatic-hydrothermal fluids come into contact with primary sulfide grains at the martian surface or subsurface. Moreover, hydrothermal fluids rich in chlorine, sulfur, and iron are important for ore-forming processes on Earth, and their indirect identification on Mars may have important implications for ore-formation on Mars.     

Heterogeneous distribution of phosphorus in olivine from otherwise well-equilibrated spinel peridotite xenoliths and its implications for the mantle geochemistry of lithium

The major- and trace-element abundances of the coexisting phases of four metasomatized spinel peridotite xenoliths from the Anakies locality (SE Australia) were determined by electron microprobe and laser-ablation ICP-MS. The compositions of all phases are remarkably homogeneous, with the exception of phosphorus (P), lithium (Li) and sodium (Na) in olivine. These three elements are enriched in large parts of most olivine crystals due to a second metasomatic episode. Apart from these elements, all phases are in mutual equilibrium with respect to both their major- and trace-element compositions. Li and Na show a strong correlation with P in olivine, although molar Li + Na are an order of magnitude less than molar P, indicating that the substitution mechanism of these elements is more complex than the simple charge-balanced coupled exchange ^IVSi⁴⁺ + ^VI(FeMg)²⁺ = ^IVP⁵⁺ + ^VI(LiNa)⁺. We suggest that Li and Na are decorating octahedral-site cation vacancies formed by the original incorporation of P. Elemental maps revealed that the P zoning patterns are concentric in a few large olivine porphyroblasts, but form irregular patches in most crystals. This distribution of P is proposed to be the result of a two-stage process, whereby the initial concentric zoning, caused by its exceptionally sluggish diffusion after metasomatic influx, is broken up by extensive sub-solidus deformation and recrystallization, attesting to large grain-scale strains even within the lithosphere. Such strains must be an efficient means of ensuring trace-element equilibrium during partial melting. The association of Li with P in olivine may help to explain the variability of Li abundances in mantle minerals and to interpret Li diffusion experiments and Li isotopic fractionation.     

K-H3OFe3(SO4)2(OH)6铁矾的XRD研究

根据X射线衍射(XRD)分析发现: A Fe₃(SO₄)₂(OH)₆(A=K⁺、H₃O⁺)系列铁钒的XRD数据十分相近,难以用XRD区别,需通过能谱(EDS)辅助分析,才能区分此类铁矾。另外,此类铁矾的003和107面网间距d随K⁺含量增大而增大,且呈一元三次方程的关系;而033和220面网间距d随K⁺含量增大而减小,呈一元二次方程的关系。对该现象从铁矾晶体结构方面进行解释:K⁺、H₃O⁺离子位于较大空隙中,且沿着Z轴方向排列,当K⁺、H₃O⁺离子之间相互替换时,会导致该铁矾晶体结构在Z轴方向有较明显的变化。     

Formation of tonalite in island arcs by seawater-induced anatexis of mafic rocks; evidence from the Lyngen Magmatic Complex, North Norwegian Caledonides

The Lyngen Magmatic Complex (LMC) of North Norway, consists of a western suite of layered gabbros of normal-mid oceanic ridge basalt (N-MORB) affinity and an eastern suite of layered gabbronorites, quartz-bearing gabbros and diorites/quartz-diorites of IAT (island-arc tholeiitte) to boninitic affinity. The boundary between the suites is defined by a large-scale ductile shear zone, the Rypdalen shear zone (RSZ). In this shear zone anatectic tonalites were generated by partial melting of the gabbro in the presence of an H₂O bearing fluid phase.Quartz from the tonalites contains early secondary and secondary liquid-dominated inclusions (88-99 wt.% H₂O), with an average salinity of 18 wt.% (calculated as NaCl_eq). Combined gas and ion chromatography shows that the major ions in the fluid are Cl⁻, Ca²⁺, Na⁺ with smaller amounts of K⁺, Mg²⁺, Sr²⁺, Br⁻ and NO₃⁻. The dominant non-H₂O volatile species is N₂ (0.5-10%), and small amounts of CO₂, CH₄ and other hydrocarbons are also present.The cation concentrations in the fluid are variable, due to element exchange during interaction of the fluids with the tonalites, amphibolites and metagabbros of the RSZ. The fluid contributed Na⁺ and K⁺ to the melt and gained Ca²⁺ in exchange, explaining the variable Na⁺/Ca²⁺ ratio of the fluid. The Br⁻ and Cl⁻ contents of the fluid inclusions plot on the same line as evaporating sea water, which strongly suggests a seawater origin for the fluid phase, and a seawater source fits well with other geochemical signatures and the tectonic setting of the LMC.It is suggested that seawater escaped from a subducting slab and was channelled along the Rypdalen shear zone. This caused anatexis of the gabbro, generating tonalitic melts at 0.5-0.9 GPa and 680-800°C.     

Self-sealing isolation and immobilization: a geochemical approach to solve the environmental problem of waste acidic jarosite

Mimicking geochemical processes to solve environmental problems was implemented in dealing with waste acidic jarosite and alkaline coal fly ash. By placing these two chemically different materials adjacent to one another, a self-sealing layer was formed at the interface between both wastes, isolating and immobilizing chemical constituents in the process. A series of leaching experiments were performed on each material separately to study the release behavior of the principal constituents. Radiotracer experiments were conducted to explore diffusion and reaction of constituents such as Fe³⁺ in a combined jarosite/fly ash system. A model has been developed to simulate the coupled processes of diffusion and precipitation taking into account porosity change due to pore filling by precipitates. The formation of a self-sealing isolation layer in a hypothetical jarosite/fly ash disposal site was modelled. Leaching results indicate that the release of elements from jarosite is much larger than that from fly ash, and that the highly pH dependent release of Fe, Al, and Zn was controlled by the solubility of their hydroxides. Leaching results also suggest that precipitation reactions can be expected to occur at the interface between jarosite and alkaline coal fly ash where a large pH gradient exists. Radiotracer experiments showed that accumulation of constituents occurred at the interface. Modeled Fe³⁺ profiles in layered jarosite/fly ash were well validated by experiments. Modeling results also showed that with the accumulation of constituents at the interface, a new layer with low porosity was formed. Application of this model suggests that there is a potential use to form a self-sealing layer in jarosite/fly ash co-disposal sites.     

Trace and minor elements in sphalerite: A LA-ICPMS study

Sphalerite is an important host mineral for a wide range of minor and trace elements. We have used laser-ablation inductively coupled mass spectroscopy (LA-ICPMS) techniques to investigate the distribution of Ag, As, Bi, Cd, Co, Cu, Fe, Ga, Ge, In, Mn, Mo, Ni, Pb, Sb, Se, Sn and Tl in samples from 26 ore deposits, including specimens with wt.% levels of Mn, Cd, In, Sn and Hg. This technique provides accurate trace element data, confirming that Cd, Co, Ga, Ge, In, Mn, Sn, As and Tl are present in solid solution. The concentrations of most elements vary over several orders of magnitude between deposits and in some cases between single samples from a given deposit. Sphalerite is characterized by a specific range of Cd (typically 0.2-1.0 wt.%) in each deposit. Higher Cd concentrations are rare; spot analyses on samples from skarn at Baisoara (Romania) show up to 13.2 wt.% (Cd²⁺ ↔ Zn²⁺ substitution). The LA-ICPMS technique also allows for identification of other elements, notably Pb, Sb and Bi, mostly as micro-inclusions of minerals carrying those elements, and not as solid solution. Silver may occur both as solid solution and as micro-inclusions. Sphalerite can also incorporate minor amounts of As and Se, and possibly Au (e.g., Magura epithermal Au, Romania). Manganese enrichment (up to ∼4 wt.%) does not appear to enhance incorporation of other elements. Sphalerite from Toyoha (Japan) features superimposed zoning. Indium-sphalerite (up to 6.7 wt.% In) coexists with Sn-sphalerite (up to 2.3 wt.%). Indium concentration correlates with Cu, corroborating coupled (Cu⁺In³⁺) ↔ 2Zn²⁺ substitution. Tin, however, correlates with Ag, suggesting (2Ag⁺Sn⁴⁺) ↔ 3Zn²⁺ coupled substitution. Germanium-bearing sphalerite from Tres Marias (Mexico) contains several hundred ppm Ge, correlating with Fe. We see no evidence of coupled substitution for incorporation of Ge. Accordingly, we postulate that Ge may be present as Ge²⁺ rather than Ge⁴⁺. Trace element concentrations in different deposit types vary because fractionation of a given element into sphalerite is influenced by crystallization temperature, metal source and the amount of sphalerite in the ore. Epithermal and some skarn deposits have higher concentrations of most elements in solid solution. The presence of discrete minerals containing In, Ga, Ge, etc. also contribute to the observed variance in measured concentrations within sphalerite.     

Surface complexation model for strontium sorption to amorphous silica and goethite

Strontium sorption to amorphous silica and goethite was measured as a function of pH and dissolved strontium and carbonate concentrations at 25°C. Strontium sorption gradually increases from 0 to 100% from pH 6 to 10 for both phases and requires multiple outer-sphere surface complexes to fit the data. All data are modeled using the triple layer model and the site-occupancy standard state; unless stated otherwise all strontium complexes are mononuclear. Strontium sorption to amorphous silica in the presence and absence of dissolved carbonate can be fit with tetradentate Sr²⁺ and SrOH⁺ complexes on the β-plane and a monodentate Sr²⁺complex on the diffuse plane to account for strontium sorption at low ionic strength. Strontium sorption to goethite in the absence of dissolved carbonate can be fit with monodentate and tetradentate SrOH⁺ complexes and a tetradentate binuclear Sr²⁺ species on the β-plane. The binuclear complex is needed to account for enhanced sorption at hgh strontium surface loadings. In the presence of dissolved carbonate additional monodentate Sr²⁺ and SrOH⁺ carbonate surface complexes on the β-plane are needed to fit strontium sorption to goethite. Modeling strontium sorption as outer-sphere complexes is consistent with quantitative analysis of extended X-ray absorption fine structure (EXAFS) on selected sorption samples that show a single first shell of oxygen atoms around strontium indicating hydrated surface complexes at the amorphous silica and goethite surfaces.     

Stable isotope evidences of jarosite–barite mineralization in the Rangan rhyolitic dome NE Isfahan,Iran

The Rangan area is part of Cenozoic magmatic belt of central Iran. Eocene volcanic flows and pyroclastic rocks are intruded by a Neogene rhyolitic dome along the major Qom–Zefreh fault. The dome is pervasively hydrothermally altered. The main mineral assemblage is jarosite+barite+pyrite+quartz+sericite. This assemblage indicates acid sulphate or advanced argillic alteration. Sulfur and oxygen isotope data (δ³⁴S & δ¹⁸O (SO₄)) obtained from jarosite and barite indicate a mixing episode during the evolution of hydrothermal system and reflect the overlapping of two distinct sources of acid sulphate alteration in Rangan, i.e., a magmatic–hydrothermal fluid reacting with steam-heated meteoric water. Considering the position of brittle–ductile transition and major fault movements, jarosite and barite seemingly precipitated from rapid injection of magmatic–hydrothermal fluids into the upper portions of a steam-heated environment.     

Distinctive crystal chemistry and site configuration of the clinopyroxene from alkali basaltic rocks

A crystal chemical investigation of clinopyroxenes from a suite of nepheline-bearing lavas located in the Nyambeni Range of Kenya has delineated the polyhedral site configurations and related intracrystalline relationships. These are distinct from those determined for the clinopyroxene in an analogous suite of leucite-bearing lavas from the Sabatini volcanoes in the Roman Region of Italy (Dal Negro et al. 1985).The Nyambeni clinopyroxene, varying from salite to hedenbergite, preferentially accepts Na in the M2 site to balance increasing Fe²⁺ and Si, respectively, whereas the Sabatini clinopyroxene is confined within the salite field and preferentially accepts Al^iv to balance the effect of increasing (Fe³⁺+Ti⁴⁺+Al^vi+Cr³⁺)_M1.The Fe²⁺/Fe³⁺ and K/Na ratios of the host rocks emerge as significant factors in determining the different polyhedral configurations and evolutions of the clinopyroxene from the two lava suites, respectively. The resulting Mg-Fe²⁺ order-disorder relationships in M1–M2 are also distinct in the two clinopyroxenes. A high degree of MgFe²⁺ order in M1–M2 corresponds to the largest configurational, hence energetic, difference between M1 and M2 in the Nyambeni clinopyroxene, whereas the converse applies to the Sabatini clinopyroxene.In view of the significant crystal chemical differences and distinct evolution trends, it is proposed that salites from alkali volcanic rocks may be referred to as Nyambeni-type or Sabatini-type, respectively.     

Oscillatory zoning in garnet from the Willsboro Wollastonite Skarn, Adirondack Mts, New York: a record of shallow hydrothermal processes preserved in a granulite facies terrane

Oscillatory zoning in low δ¹⁸O skarn garnet from the Willsboro wollastonite deposit, NE Adirondack Mts, NY, USA, preserves a record of the temporal evolution of mixing hydrothermal fluids from different sources. Garnet with oscillatory zoning are large (1–3 cm diameter) euhedral crystals that grew in formerly fluid filled cavities. They contain millimetre‐scale oscillatory zoning of varying grossular–andradite composition (X_Adr = 0.13–0.36). The δ¹⁸O values of the garnet zones vary from 0.80 to 6.26‰ VSMOW and correlate with X_Adr. The shape, pattern and number of garnet zones varies from crystal to crystal, as does the magnitude of the correlated chemistry changes, suggesting fluid system variability, temporal and/or spatial, over the time of garnet growth. The zones of correlated Fe content and δ¹⁸O indicate that a high Fe³⁺/Al, high δ¹⁸O fluid mixed with a lower Fe³⁺/Al and δ¹⁸O fluid. The high δ¹⁸O, Fe enriched fluids were likely magmatic fluids expelled from crystallizing anorthosite. The low δ¹⁸O fluids were meteoric in origin. These are the first skarn garnet with oscillatory zoning reported from granulite facies rocks. Geochronologic, stable isotope, petrologic and field evidence indicates that the Adirondacks are a polymetamorphic terrane, where localized contact metamorphism around shallowly intruded anorthosite was followed by a regional granulite facies overprint. The growth of these garnet in equilibrium with meteoric and magmatic fluids indicates an origin in the shallow contact aureole of the anorthosite prior to regional metamorphism. The zoning was preserved due to the slow diffusion of oxygen and cations in the large garnet and protection from deformation and recrystallization in zones of low strain in thick, rigid, garnetite layers. The garnet provide new information about the hydrothermal system adjacent to the shallowly intruded massif anorthosite that predates regional metamorphism in this geologically complex, polymetamorphic terrane.     

Oxidative Ce3+ sequestration by fungal manganese oxides with an associated Mn(II) oxidase activity

Sequestration of Ce³⁺ by biogenic manganese oxides (BMOs) formed by a Mn(II)-oxidizing fungus, Acremonium strictum strain KR21-2, was examined at pH 6.0. In anaerobic Ce³⁺ solution, newly formed BMOs exhibited stoichiometric Ce³⁺ oxidation, where the molar ratio of Ce³⁺ sequestered (Ce_seq) relative to Mn²⁺ released (Mn_rel) was maintained at approximately two throughout the reaction. A similar Ce³⁺ sequestration trend was observed in anaerobic treatment of BMOs in which the associated Mn(II) oxidase was completely inactivated by heating at 85 °C for 1 h or by adding 50 mM NaN₃. Aerobic Ce³⁺ treatment of newly formed BMO (enzymatically active) resulted in excessive Ce³⁺ sequestration over Mn²⁺ release, yielding Ce_seq/Mn_rel > 200, whereas heated or poisoned BMOs released a significant amount of Mn²⁺ with lower Ce³⁺ sequestration efficiency. Consequently, self-regeneration by the Mn(II) oxidase in newly formed BMO effectively suppressed Mn²⁺ release and enhanced oxidative Ce³⁺ sequestration under aerobic conditions. Repeated treatments of heated or poisoned BMOs under aerobic conditions confirmed that oxidative Ce³⁺ sequestration continued even after most Mn oxide was released from the solid phase, indicating auto-catalytic Ce³⁺ oxidation at the solid phase produced through primary Ce³⁺ oxidation by BMO. From X-ray diffraction analysis, the resultant solid phases formed through Ce³⁺ oxidation by BMO under both aerobic and anaerobic conditions consisted of cerianite with crystal sizes of 5.00–7.23 Å. Such nano-sized CeO₂ (CeO_2,BMO) showed faster auto-catalytic Ce³⁺ oxidation than that on well-crystalized cerianite under aerobic conditions, where the normalized pseudo-first order rate constants for auto-catalytic Ce³⁺ oxidation on CeO_2,BMO was two orders of magnitude higher. Consequently, we concluded that Ce³⁺ contact with BMOs sequesters Ce³⁺ through two oxidation paths: primary Ce³⁺ oxidation by BMOs produces nano-sized crystalline cerianite, and subsequent auto-catalytic Ce³⁺ oxidation efficiently occurs using dissolved oxygen as the oxidizing agent. Pretreatment of newly formed BMOs with La³⁺ solution resulted in decreased rate constants for primary Ce³⁺ oxidation by BMO due to site blocking by La³⁺ sorption. The results presented herein increase our understanding of the role of BMO in oxidative Ce³⁺ sequestration process(es) through enzymatic and abiotic paths in natural environments and provide supporting evidence for the potential application of BMOs towards the recovery of Ce³⁺ from contaminated waters.     

Multiproxy analysis of a new terrestrial and a marine Cretaceous-Paleogene (K-Pg) boundary site from New Zealand

An integrated study of palynology, Mössbauer spectroscopy, mineralogy and osmium isotopes has led to the detection of the first K-Pg boundary clay layer in a Southern Hemisphere terrestrial setting. The K-Pg boundary layer was independently identified at centimetre resolution by all the above mentioned methods at the marine K-Pg boundary site of mid-Waipara and the terrestrial site of Compressor Creek (Greymouth coal field), New Zealand. Mössbauer spectroscopy shows an anomaly of Fe-containing particles in both K-Pg boundary sections: jarosite at mid-Waipara and goethite at Compressor Creek. This anomaly coincides with a turnover in vegetation indicated by an interval dominated by fern spores and extinction of key pollen species in both sections. In addition to the terrestrial floristic changes, the mid-Waipara section reveals a turnover in the dinoflagellate assemblages and the appearance of global earliest Danian index species. Geochemical data reveal relatively small iridium enrichments in the boundary layers of 321 pg/g at mid-Waipara and 176 pg/g at Compressor Creek. Unradiogenic ¹⁸⁷Os/¹⁸⁸Os values of the boundary clay reveal the presence of a significant extraterrestrial component. We interpret the accumulation of Fe nano-phases at the boundary as originating from both the impactor and the crystalline basement target rock. The goethite and jarosite are interpreted as secondary phases formed by weathering and diagenesis. The primary phases were probably controlled by the initial composition of the vapor plume and condensation kinetics rather than condensation thermodynamics. This investigation indicates that identification of Fe in nano-phases by Mössbauer spectroscopy is an accurate and cost-effective method for identifying impact event horizons and it efficiently complements widely used biostratigraphic and geochemical methods.     

A comparison of metal attenuation in mine residue and overburden material from an abandoned copper mine

The metal attenuation capacities of secondary acid mine water precipitates is dependent upon such factors as pH, ionic strength, the presence of competing ions, and tailings mineralogy. At the abandoned Spenceville Cu mine in Nevada County, California, approximately 6800 m³ of jarosite overburden and 28,000 m³ of hematite residue are potential sources of heavy metals loading to infiltrating surface waters. A column study was performed to assess the ability of the overburden and the residue to attenuate heavy metals from acidic mine drainage. The study information was needed as part of a remedial design for the abandoned mine, and was designed to simulate a worst-case scenario to examine the plausibility of backfilling a large open pit with the waste materials. Ten pore volumes of acidic mine drainage were allowed to pass through the materials, and the column effluents were analyzed for dissolved Fe, Al, Ca, Mg, Na, K, Mn, Cu, Zn, Pb and Ni using ICP-AES. The oxidation-reduction potential (Eh) was measured with a combination PtAg/AgCl electrode and also calculated from Fe(II) and Fe(III) measurements using the Nernst equation. Ion activities in solution and saturation index (SI) values for various solid phases were calculated using the geochemical speciation model MINTEQA2, and mineralogical compositions of fine (< 2 mm) and coarse ( > 2 mm) fractions were determined by XRD. Geochemical modeling of the column effluent compositions indicate that goethite, jarosite, jurbanite and gypsum are potential solid phases that may control metal solubilities in the column effluents. Excellent agreement was observed between the measured Eh values and those calculated from the activity ratio of Fe²⁺(aq) to Fe³⁺(aq). The large attenuation capacities for Cu and Zn exhibited by the jarosite overburden also suggest that solid solution substitution plays a large role in controlling metal concentrations in the pore waters. Relatively little metal attenuation, however, was provided by the hematite residue.