Formålet med denne Ph.d. afhandling er, at beskrive processer, der foregår hvor jernholdige mineraler er til stede. Arbejdet fokuseredes specielt på ferro-ferri hydroxyd-holdige mineraler, grøn rust, og deres mulige rolle i jerns kredsløb. Det første studium havde som formål at beskrive jernholdige mineraler i sprækker i granit ved Äspö, Sverige. Jernoxidernes struktur, kemiske sammensætning og stabile jern isotopforhold tillod, rekonstruktion af de palæo-redox- forhold i granittens sprækker. Der blev fundet 3 typer ferri oxider dannet i forskellige miljøer: 1) grovkornede hydrothermale hematitter, 2) meget fin kornede amorfe jern oxider, der var udfældet under boring af kernen, 3) krystalline jern oxider af intermediær størrelse, der blev dannet ved lave temperaturer (Når pH er neutral til let basisk vil oxidation af Fe(II)-holdige opløsninger udfælde grøn rust. I det andet studium benyttede jeg Røntgendiffraktion til, at identificere grøn rust i grundvandsområder med middel til relativt høj jern koncentration. Metoden jeg udviklede er rimelig enkel og bygger på at grøn rust stabiliseres, når det sidder på overflader af f.eks. glas eller glimmermineraler. Prøvetagningen skete i nærheden af Bornholms Lufthavn og i tunnelen til Äspö Hard Rock Laboratorium. Prøverne transporteredes under nitrogen til vores laboratorium hvor Røntgendiffraktion viste at prøvematerialet indeholdt GRCO3. Tilstedeværelsen af GR blev ligeledes bekræftet af et drastisk fald i toppenes intensitet efter et par dages kontakt med den almindelige atmosfære. Viden om den kemiske sammensætning af mineraler og deres krystalstruktur er essentiel, idet de bestemmer det faste stofs opførsel. En litteratur gennemgang i forbindelse med det tredje studium viste, at tidligere undersøgelser af sulfat-holdig GR benyttede strukturelle og sammensætningsmæssige parametre, der var fejlagtige eller mangelfulde. Jeg udførte grundige analyser af sulfat-holdige grøn rust med Røntgendiffraktion, Rietveld analyse, og Mössbauer spektroskopi. Sammenholdt med den kemiske sammensætning tillod det mig at revaluere strukturen og sammensætningen af stoffet. De nye resultater beviste at natrium er en essentiel strukturel komponent af sulfat-holdig grøn rust, GRNa,SO4. Den kemiske formel er NaFe(II)6Fe(III)3(SO4)2(OH)1812H2O, dens rum gruppe er P-3, og enhedscelle parametrene er a = 9.528(6) Å, c = 10.968(8) Å og Z = 1.Den sidste del af denne afhandling beskriver, hvordan GRNa,SO4 reagerer med andre redox-sensitive elementer. Selenium og neptunium er typiske henfaldsprodukter fra brugte radioaktive brændselsstænger. Disse og andre radioaktive elementer vil udgøre et alvorligt miljømæssigt problem, hvis der skulle opstå lækage fra fremtidige depoter til radioaktivt affald. For bedre at forstå de radioaktive elementers opførsel i naturen og deres vekselvirkning med depotets jernstrukturer undersøgte jeg, hvordan selenit (SeO32-) og neptunyl (NpO2+) reagerede med GRNa,SO4. Selenit reduceredes hurtigt af GR ved at reagere langs kanten af GR partiklerne. Her dannedes trigonalt elementært Se(0), mens den grønne rust omdannedes til goethit. Ved reaktion mellem neptunyl og GR foregik reduktion ligeledes omkring kanterne af den grønne rust. Neptunium reduceredes til tetravalent Np, mens GR omdannedes til goethit. Ved efterfølgende oxidation af reaktionsblandingen i luft forblev omkring 40% af neptunium i dets tetravalente form. Resultaterne indikerer, at Np(IV) enten er inkorporeret i goethitstrukturen eller eksisterer som en selvstændig oxid fase. Det skal dog noteres, at tilstedeværelsen af Np(IV) oxider er mindre sandsynligt, da denne fase let oxideres til pentavalent Np. I denne afhandling har jeg vist, at jernholdige mineraler kan bruges som indikatorer for redox-forhold, og at sulfat-holdige grøn rust indeholder monovalente kationer i mellemlaget, i dette tilfælde natrium. Grøn rust er således ikke udelukkende anion substituerende mineraler, men kan ligeledes potentielt udveksle kationer med omkringliggende væsker, som det ses for ler-mineraler. I grundvand transportmodeller er det nødvendigt at inkludere de parametre der beskriver grøn rust opførelse under grundvands forhold, da faserne gerne reagerer med en række giftige stoffer, hvorved de immobiliseres. Ud fra min forskning er det nu endnu tydeligere, at modeller der skal simulere eller forudsige grundvandstransport af giftige elementer, skal inkludere de termodynamiske og kinetiske parametre for grøn rust og dets reaktivitet med opløste stoffer. The purpose of this PhD thesis is to describe the processes that take place where iron-containing minerals are present. The work was especially focused on the ferro-ferric hydroxide minerals, the green rust series, and their possible role in the geochemical iron-cycle. The goal of the first study was to describe the iron-containing minerals in the granitic fractures at Äspö, Sweden. The structure, chemical composition and stable iron isotope ratio of the ferric oxides allowed reconstruction of the paleo-redox conditions in the granitic fractures. I found 3 types of formation environments: 1) course-grained hydrothermal hematites, 2) very fine-grained amorphous iron-oxides that had precipitated during drilling and 3) crystalline iron-oxides of intermediate size which we interpreted to form at low temperatures (< 10° C) at a depth of less than ~100 m below surface. When pH is neutral to slightly basic Fe(II)-containing solutions oxidise, green rust precipitates. In the second study, X-ray diffraction allowed identification of green rust in groundwater from areas with medium to relatively high iron concentration. The method I developed is relatively simple and is based on the observation that green rust is stabilised, when it attaches to substrates such as glass and mica. The sampling was done from the site close to the airport of Bornholm and in the Äspö hard rock laboratory tunnel. The samples were transported to the laboratory under nitrogen where the X-ray diffraction showed that the material contained GRCO3. The presence of green rust was verified by a drastic decrease in the intensity of the GR peaks when the samples had been exposed to air for a couple of days.Knowledge about the chemical composition of minerals and their crystal structures is essential for use in databases and, because composition determines the behaviour of a solid. A literature review showed that previous reports of sulphate-containing green rust have presented structural and compositional parameters that have been erroneous or lacking. The third study was a detailed examination of sulphate-containing green rust by X-ray diffraction, Rietveld analysis and Mössbauer spectroscopy. Using newly determined data for chemical composition from pure samples, we were able to determine details in the structure and composition of the solid. The new results proved that sodium is essential for sulphate-containing green rust, GRNa,SO4. The chemical composition is NaFe(II)6Fe(III)3(SO4)2(OH)1812H2O, its space group is P-3, and its cell parameters are: a = 9.528(6) Å, c = 10.968(8) Å and Z = 1. The last chapter describes how GRNa,SO4 reacts with other redox-sensitive elements. Selenium and neptunium are typical daughter products of spent radioactive fuel rods. These and other radioactive elements pose a serious environmental risk for long term storage, if leaks develop in the waste repository. To better understand the behaviour of these radioactive elements in the natural environment and their reactivity with the repository iron structures, I examined how selenite (SeO32-) and neptunyl (NpO2+) react with GRNa,SO4. Selenite was reduced readily by reaction at the edge of the GR particles. It converted to trigonal elemental Se(0), while the GR transformed to goethite. When neptunyl reacted with GR, it was reduced to tetravalent Np, and the GR also oxidised to goethite. Following complete oxidation of the green rust suspension slurry, approximately 40% of the Np remained in the tetravalent redox state. The results indicate that Np(IV) is either incorporated in the goethite structure, or it exists as discrete Np(IV)-oxides. The presence of Np(IV) oxides is less likely, because neptunium dioxide are readily oxidise to pentavalent Np.In this thesis, I have shown that Fe-containing minerals can be used as indicators of redox conditions and that green rust sulphate contains a monovalent cation in the interlayer, namely sodium. Thus, green rust is not solely an anionic exchanger, but can also exchange cations with the surrounding solutions, similar to what is observed for clays. In groundwater transport modelling, parameters describing the behaviour of green rust under aquifer conditions must be included because the compounds react readily with a number of toxic components. In some cases, they are immobilised whereas in others it is possible that they are mobilised by colloidal transport. From my research, it is now even clearer that models intended to simulate or predict groundwater transport of toxic elements should include thermodynamic and kinetic parameters for green rust and its reactivity with dissolved compounds.