The surface structure of Fe3O4(1 1 1) films as studied by CO adsorption

We have studied adsorption of CO on Fe₃O₄(1 1 1) films grown on a Pt(1 1 1) substrate by temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS) and high resolution electron energy loss spectroscopy (HREELS). Three adsorption states are observed, from which CO desorbs at ∼110, 180, and 230 K. CO adsorbed in these states exhibits stretching frequencies at ∼2115-2140, 2080 and 2207 cm⁻¹, respectively. The adsorption results are discussed in terms of different structural models previously reported. We suggest that the Fe₃O₄(1 1 1) surface is terminated by 1/2 ML of iron, with an outermost 1/4 ML consisting of octahedral Fe²⁺ cations situated above an 1/4 ML of tetrahedral Fe³⁺ ions, in agreement with previous theoretical calculations. The most strongly bound CO is assigned to adsorption to Fe³⁺ cations present on the step edges.     

Two earth years of Mössbauer studies of the surface of Mars with MIMOS II

The element iron plays a crucial role in the study of the evolution of matter from an interstellar cloud to the formation and evolution of the planets. In the Solar System iron is the most abundant metallic element. It occurs in at least three different oxidation states: Fe(0) (metallic iron), Fe(II) and Fe(III). Fe(IV) and Fe(VI) compounds are well known on Earth, and there is a possibility for their occurrence on Mars. In January 2004 the USA space agency NASA landed two rovers on the surface of Mars, both carrying the Mainz Mössbauer spectrometer MIMOS II. They performed for the first time in-situ measurements of the mineralogy of the Martian surface, at two different places on Mars, Meridiani Planum and Gusev crater, respectively, the landing sites of the Mars-Exploration-Rovers (MER) Opportunity and Spirit. After about two Earth years or one Martian year of operation the Mössbauer (MB) spectrometers on both rovers have acquired data from more than 150 targets (and more than thousand MB spectra) at each landing site. The scientific measurement objectives of the Mössbauer investigation are to obtain for rock, soil, and dust (1) the mineralogical identification of iron-bearing phases (e.g., oxides, silicates, sulfides, sulfates, and carbonates), (2) the quantitative measurement of the distribution of iron among these iron-bearing phases (e.g., the relative proportions of iron in olivine, pyroxenes, ilmenite and magnetite in a basalt), (3) the quantitative measurement of the distribution of iron among its oxidation states (e.g., Fe²⁺, Fe³⁺, and Fe⁶⁺), and (4) the characterization of the size distribution of magnetic particles. Special geologic targets of the Mössbauer investigation are dust collected by the Athena magnets and interior rock and soil surfaces exposed by the Athena Rock Abrasion Tool and by trenching with rover wheels. The Mössbauer spectrometer on Opportunity at Meridiani Planum, identified eight Fe-bearing phases: jarosite (K,Na,H3O)(Fe,Al)(OH)6(SO4)2, hematite, olivine, pyroxene, magnetite, nanophase ferric oxides (npOx), an unassigned ferric phase, and a metallic Fe–Ni alloy (kamacite) in a Fe–Ni-meteorite. Outcrop rocks consist of hematite-rich spherules dispersed throughout S-rich rock that has nearly constant proportions of Fe³⁺ from jarosite, hematite, and npOx (28%, 35%, and 19% of total Fe). Jarosite is mineralogical evidence for aqueous processes under acid–sulfate conditions because it has structural hydroxide and sulfate and it forms at low pH. Hematite-rich spherules, eroded from the outcrop, and their fragments are concentrated as hematite-rich soils (lag deposits) on ripple crests (up to 68% of total Fe from hematite). Olivine, pyroxene, and magnetite are primarily associated with basaltic soils and are present as thin and locally discontinuous cover over outcrop rocks, commonly forming aeolian bedforms. Basaltic soils are more reduced (Fe³⁺/Fe_total ～0.2?0.4), with the fine-grained and bright aeolian deposits being the most oxidized. Basaltic soil at Meridiani Planum and Gusev crater have similar Fe-mineralogical compositions. At Gusev crater, the Mössbauer spectrometer on the MER Spirit rover has identified 8 Fe-bearing phases. Two are Fe²⁺ silicates (olivine and pyroxene), one is a Fe²⁺ oxide (ilmenite), one is a mixed Fe²⁺ and Fe³⁺ oxide (magnetite), two are Fe³⁺ oxides (hematite and goethite), one is a Fe³⁺ sulfate (mineralogically not constrained), and one is a Fe³⁺ alteration product (npOx). The surface material in the plains have a olivine basaltic signature (Morris, et al., Science, 305: 833, 2004; Morris, et al., J. Geophys. Res., 111, 2006, Ming, et al., J. Geophys. Res., 111, 2006) suggesting physical rather than chemical weathering processes present in the plains. The Mössbauer signature for the Columbia Hills surface material is very different ranging from nearly unaltered material to highly altered material. Some of the rocks, in particular a rock named Clovis, contain a significant amount of the Fe oxyhydroxide goethite, α-FeOOH, which is mineralogical evidence for aqueous processes because it is formed only under aqueous conditions.     

Characterization of dimyristoylphosphatidylcholine liposome aggregates induced by dextran sulfate and La3+ by fluorescence spectroscopy

The addition of dextran sulfate (DS) to DMPC vesicles in the presence of di- and trivalent cations leads to a strong aggregation, resulting in a stack-like arrangement of the opposing membrane surfaces as shown by freeze-fracture electron microscopy. The strong aggregation is connected with a lipid mixing process, especially in the presence of La³⁺ (measured by the NBD/Rh assay). The extent of lipid mixing depends on the molecular weight of DS and size of the DMPC vesicles. Additionally, a decrease in the surface dielectric constant of DMPC vesicles [measured by the emission shift of the fluorescent probe, dansylphosphatidyl-ethanolamine (DPE)] was observed. A direct dependence on the molecular weight (MW) of DS exists: the higher their MW, the higher the blue emission shift of the DPE probe. The results are discussed in terms of the theory proposed by Ohki and Arnold, which connects the decrease of the surface dielectric constant with the interaction parameters of phospholipid membranes.     

Turn-off Fluorescence Chemosensor for Iron with Bis(2-aminoethyl)-2-(9-fluorenyl)malonamide Functionlized SBA-15

An bis(2-aminoethyl)-2-(9-fluorenyl)malonamide as fluorophore ligand was immobilized onto mesoporous silica type SBA-15 via post synthesis grafting. The obtained material was characterized by small and wide angle X-ray diffraction, N₂ adsorption–desorption, Fourier transform infrared spectroscopy, Raman spectroscopy and thermogravimetric analysis that indicate the successful immobilization of the ligand on the surface of mesoporous silica. The sensing ability of the obtained material was studied by addition of the cations Fe³⁺, Mg²⁺, Cr³⁺, Co²⁺, Ni²⁺, Cu²⁺, Hg²⁺ and Zn²⁺ to water suspensions of the assayed solid. Of all the cations tested addition of Fe³⁺ ion to a suspension of this material resulted in the largest decrease in the fluorescence intensity. Turn-off photoluminescence of this material was remarkably observed for iron ions in comparing of the other cations. A good linearity between the fluorescence intensity of this material and the concentration of Fe³⁺ ion is constructed, which enables it as a fluorescence chemosensor for detecting the Fe³⁺ ion with a suitable detection limit of 1.35?×?10^?5. It can be introduced as a novel fluorescent sensor in aqueous solution for a lot of practical applications in chemical, environmental and biological systems.     

The effect of [Fe<Superscript>3+</Superscript>]/[Fe<Superscript>2+</Superscript>] molar ratio and iron salts concentration on the properties of superparamagnetic iron oxide nanoparticles in the water/ethanol/toluene system

In this study, we developed a convenient one-pot method with sodium oleate as both the surfactant and precipitant to synthesize pure magnetite nanoparticles in the water/ethanol/toluene system. The initial molar ratio of [Fe³⁺]/[Fe²⁺] and the concentration of iron salts were changed in order to systematically investigate their influences on the chemical and physical properties of nanoparticles, such as the crystal structure, morphology, particle sizes, dispersion and magnetism. Samples were determined by XRD, XPS, FTIR, DLS, and VSM. The oleate coating steadily existed on the surface of the nanoparticles to profit them of excellent monodispersibility and stability in non-polar solvents with very narrow size distribution and extremely approximate mean diameters of ~7 nm. Particles consisted mainly of magnetite with a little or no maghemite phase with the molar ratio of [Fe³⁺]/[Fe²⁺] decreasing from 2:1 to 1:1, but they all exhibited superparamagnetism at room temperature. After the optimization, pure magnetite nanoparticles could be prepared with the saturation magnetization successfully increasing to 75 emu/g(Fe), when the molar ratio of [Fe³⁺]/[Fe²⁺] was 1.5:1 and the concentration of iron precursors was 95 mM.     

Surface properties of superparamagnetic iron oxide MR contrast agents: Ferumoxides,ferumoxtran, ferumoxsil

The surface properties of the active ingredients in AMI colloidal, superparamagnetic iron oxide magnetic resonance (MR) contrast agents are described. Scanning electron microscopy/energy dispersive X-ray elemental analyses and diffuse reflectance Fourier transform infrared spectroscopy (FTIR) spectra of ferumoxsil (AMI-121 drug substance) were consistent with the presence of a monolayer of H₂NCH₂CH₂NHCH₂CH₂CH₂Si(O⁻)₃ siloxane monomer or dimer. The X-ray photoelectron spectra (XPS) of ferumoxsil are also consistent with complete coverage of the iron oxide surface with a monolayer of siloxane. The static secondary ion mass spectra (SSIMS) of ferumoxsil showed that the siloxane film is covalently bonded (i.e., SiOFe bonds) to the iron oxide surface. The FTIR of ferumoxides (AMI-25) and Ferumoxtran (AMI-227) showed only adsorbed dextran. The XPS spectra of the dextrancoated colloids showed that Ferumoxtran has a thicker layer of dextran than ferumoxides iron oxide particles (∼5 and ∼3 nm, respectively). The SSIMS spectra of these dextran-coated colloids showed only low mass fragments due to the adsorbed dextran. The nature of the interactions of the dextran coating with the iron oxide surfaces of ferumoxides and Ferumoxtran is discussed.     

The interaction of oxygen molecules with iron films studied with MIES,UPS and XPS

X-ray Photoelectron Spectroscopy (XPS), Metastable Induced Electron Spectroscopy (MIES) and Ultraviolet Photoelectron Spectroscopy (UPS) were applied to study the interaction of oxygen molecules with iron films. Supplementarily, iron oxide was investigated for comparison.With XPS from the Fe 2p_3/2 range contributions of metallic Fe as well as Fe²⁺ and Fe³⁺ can be distinguished. During the interaction with oxygen an oxide film is formed on the iron surface. Nevertheless, XPS still shows metallic contributions even for a surface which is saturated with more than 10⁴ L. The oxide film hinders the dissociation of further impinging oxygen molecules.The interaction of He* atoms with iron oxide surfaces during MIES is dominated by Auger Neutralization. This surprising result follows from the high work function and the fact that intrinsic defects result in a Fermi level pinning to the conduction band.     

In situ preparation of high relaxivity iron oxide nanoparticles by coating with chitosan: A potential MRI contrast agent useful for cell tracking

Iron oxide nanocrystals are of considerable interest in nanoscience and nanotechnology because of their nanoscale dimensions, nontoxic nature, and superior magnetic properties. Colloidal solutions of magnetic nanoparticles (ferrofluids) with a high magnetite content are highly desirable for most molecular imaging applications. In this paper, we present a method for in situ coating of superparamagnetic iron oxide (SPIO) with chitosan in order to increase the content of magnetite. Iron chloride salts (Fe³⁺ and Fe²⁺) were directly coprecipitated inside a porous matrix of chitosan by Co-60 γ-ray irradiation in an aqueous solution of acetic acid. Following sonication, iron oxide nanoparticles were formed inside the chitosan matrix at a pH value of 9.5 and a temperature of 50 °C. The [Fe³⁺]:[Fe²⁺]:[NH₄OH] molar ratio was 1.6:1:15.8. The final ferrofluid was formed with a pH adjustment to approximately 2.0/3.0, alongside with the addition of mannitol and lactic acid. We subsequently characterized the particle size, the zeta potential, the iron concentration, the magnetic contrast, and the cellular uptake of our ferrofluid. Results showed a z-average diameter of 87.2 nm, a polydispersity index (PDI) of 0.251, a zeta potential of 47.9 mV, and an iron concentration of 10.4 mg Fe/mL. The MRI parameters included an R1 value of 22.0 mM⁻¹ s⁻¹, an R2 value of 202.6 mM⁻¹ s⁻¹, and a R2/R1 ratio of 9.2. An uptake of the ferrofluid by mouse macrophages was observed. Altogether, our data show that Co-60 γ-ray radiation on solid chitosan may improve chitosan coating of iron oxide nanoparticles and tackle its aqueous solubility at pH 7. Additionally, our methodology allowed to obtain a ferrofluid with a higher content of magnetite and a fairly unimodal distribution of monodisperse clusters. Finally, MRI and cell experiments demonstrated the potential usefulness of this product as a potential MRI contrast agent that might be used for cell tracking.     

Mössbauer studies in the spinel system coxfe3−xo4

Mössbauer spectra have been obtained for the cobalt-iron spinels having the general formula CO_xFe_3?xO₄ for the whole range of composition, materials from x = 0 to x = 2.76 having been studied. The spectra fall into three groups: (A) Those for spinels for which 0 < χ < 1 which show two six-peak sets of lines corresponding to exchanging Fe²⁺/Fe³⁺ and to Fe³⁺ iron; (B) those for spinels containing Fe³⁺ iron only which show magnetic splitting 1 < χ < 2; (C) those for spinels containing Fe³⁺ iron only which show quadrupole splitting but no magnetic splitting (x = 2.3, 2.5 and 2.8). The significance of the various spectra in relation to the structure of these mixed oxides is discussed.     

Single-step preparation,characterization and photocatalytic mechanism of mesoporous Fe-doped sulfated titania

Mesoporous Fe-doped sulfated titania photocatalysts were prepared by one-step thermal hydrolysis of industrial titanyl sulfate and characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and N₂ adsorption–desorption techniques. The effects of the m(Fe)/m(TiO₂) on the structures of the titania photocatalysts were investigated. The photocatalytic activity of the mesoporous Fe-doped sulfated titania catalysts was evaluated using the photooxidation of methylene blue in aqueous solutions under UV light irradiation. The results indicated that Fe³⁺ substitutes Ti⁴⁺ in titania lattice, which induced the formation of oxygen vacancies. The oxygen vacancies are favorable to the dissociation adsorption H₂O and formation of surface hydroxyl group. Fe³⁺ captures the photoinduced electrons or holes that are conductive to the efficient separation of the photogenerated carriers, but too many doping Fe³⁺ will promote recombination of the photogenerated carrier. Sulfur species in the form of sulfate are incorporated into the network of TiOTi and coordinated to titania in bidentate model, resulting in the strong inductive effect, large specific surface area, and mesoporous structure. All these are beneficial to improve the photocatalytic activities of the mesoporous Fe-doped sulfated titania photocatalysts.     

Synthesis of iron oxide nanoparticles under oxidizing environment and their stabilization in aqueous and non-aqueous media

Synthesis of magnetite (Fe₃O₄) nanoparticles under oxidizing environment by precipitation from aqueous media is not straightforward because Fe²⁺ gets oxidized to Fe³⁺ and thus the ratio of Fe³⁺:Fe²⁺=2:1 is not maintained during the precipitation. A molar ratio of Fe³⁺:Fe²⁺ smaller than 2:1 has been used by many to compensate for the oxidation of Fe²⁺ during the preparation. In this work, we have prepared iron oxide nanoparticles in air environment by the precipitation technique using initial molar ratios Fe³⁺:Fe²⁺?2:1. The phases of the resulting powders have been determined by several techniques. It is found that the particles consist mainly of maghemite with little or no magnetite phase. The particles have been suspended in non-aqueous and aqueous media by coating the particles with a single layer and a bilayer of oleic acid, respectively. The particle sizes, morphology and the magnetic properties of the particles and the ferrofulids prepared from these particles are reported. The average particle sizes obtained from the TEM micrographs are 14, 10 and 9 nm for the water, kerosene and dodecane-based ferrofluids, respectively, indicating a better dispersion in the non-aqueous media. The specific saturation magnetization (σ_s) value of the oleic-acid-coated particles (∼53 emu/g) is found to be lower than that for the uncoated particles (∼63 emu/g). Magnetization σ_s of the dodecane-based ferrofluid is found to be 10.1 emu/g for a volume fraction of particles ?=0.019. Zero coercivity and zero remanance on the magnetization curves indicate that the particles are superparamagnetic (SPM) in nature.     

Analysis of XPS spectra of Fe and Fe ions in oxide materials

Samples of the iron oxides Fe_0.94O, Fe₃O₄, Fe₂O₃, and Fe₂SiO₄ were prepared by high temperature equilibration in controlled gas atmospheres. The samples were fractured in vacuum and high resolution XPS spectra of the fractured surfaces were measured. The peak positions and peak shape parameters of Fe 3p for Fe²⁺ and Fe³⁺ were derived from the Fe 3p XPS spectra of the standard samples of 2FeO·SiO₂ and Fe₂O₃, respectively. Using these parameters, the Fe 3p peaks of Fe₃O₄ and Fe_1−yO are analysed. The results indicate that high resolution XPS techniques can be used to determine the Fe²⁺/Fe³⁺ ratios in metal oxides. The technique has the potential for application to other transition metal oxide systems.     

Superparamagnetic silica nanoparticles with immobilized metal affinity ligands for protein adsorption

Superparamagnetic silica-coated magnetite (Fe₃O₄) nanoparticles with immobilized metal affinity ligands were prepared for protein adsorption. First, magnetite nanoparticles were synthesized by co-precipitating Fe²⁺ and Fe³⁺ in an ammonia solution. Then silica was coated on the Fe₃O₄ nanoparticles using a sol–gel method to obtain magnetic silica nanoparticles. The condensation product of 3-Glycidoxypropyltrimethoxysilane (GLYMO) and iminodiacetic acid (IDA) was immobilized on them and after charged with Cu²⁺, the magnetic silica nanoparticles with immobilized Cu²⁺ were applied for the adsorption of bovine serum albumin (BSA). Scanning electron micrograph showed that the magnetic silica nanoparticles with an average size of 190 nm were well dispersed without aggregation. X-ray diffraction showed the spinel structure for the magnetite particles coated with silica. Magnetic measurement revealed the magnetic silica nanoparticles were superparamagnetic and the saturation magnetization was about 15.0 emu/g. Protein adsorption results showed that the nanoparticles had high adsorption capacity for BSA (73 mg/g) and low nonspecific adsorption. The regeneration of these nanoparticles was also studied.     

Electron Paramagnetic Resonance and Mössbauer Studies of Fe3+ Ions in the Fe2O3 [dbnd] MoO3 System

The electron paramagnetic resonance (EPR) and Mössbauer spectra of ironmolybdenum mixed oxides system have been investigated. Both EPR and Mössbauer measurements revealed the formation of ferric molybdate, Fe₂ (MoO₄)₃ and the existence of two different Fe³⁺ species. The EPR spectra of Fe₂ (MoO₄)₃, show two resonance lines of Fe³⁺ ions indicating that a strong exchange interaction is dominates in pure iron molybdate. Mösbauer measurements revealed that the isomer shift (I. S) increases with increasing the iron content. A maximum of quadrupole splitting (Q. S) is observed at the stoichiometric concentration corresponding to the formation of Fe₂ (MoO₄)₃. The effect of a catalytaic dehydration process on the redox behaviour of Fe³⁺ located in iron molybdate frame work is investigated. Results indicate that the catalytaic dehydration of 2-proppanol over this catalyst reduces Fe³⁺ ions to Fe²⁺ leading to the formation of the inactive phase FeMoO₄. the essential role of oxygen, in the alcohol gas feed, in regeneration the activity of the catalysts was demonstrated.     

Variety of available coordination sites for extra-framework iron in LTA and MFI zeolites

Variations in coordination states of extra-framework iron are studied in low iron content ferrisilicates (Si/⁵⁷Fe ≈ 200) during various in situ treatments. In Fe-LTA complete Fe³⁺ ? Fe^2?+? reversibility is observed. In Fe-MFI extra-framework iron can be stabilized in Fe^2?+? state in spite of ambient oxidizing conditions (N₂O, 620 K). Further, in Fe-MFI simultaneous stabilization of Fe^2?+? and Fe^3?+? may take place providing centres for redox catalytic processes.     

Mössbauer effect in inorganic glasses

⁵⁷Fe- and¹¹⁹Sn-Mössbauer effects become a very effective tool for the local structural study of inorganic glasses. Fe³⁺ and Sn⁴⁺ occupy network former (NWF) sites in several oxide glasses, while they are present at interstitial sites as network modifier (NWM) in phosphate, germanate and sulfate glasses. Characteristic-ray or thermal neutron irradiation effect indicates the structural role of Mössbauer ions; an increase in the 4s electron density or a reduction of Fe³⁺ to Fe²⁺ is observed due to the charge transfer from oxygen to Fe³⁺ when iron occupies NWF sites. By contrast, oxidation of Fe²⁺ to Fe³⁺ takes place when iron occupies NWM sites. Debye temperatures ( _D) obtained from low-temperature Mössbauer measurements are higher than 280 K when Fe³⁺ and Sn⁴⁺ are covalently bonded to oxygen at NWF sites, while the _D is lower than 270 K when these ions are ionically bonded to oxygen or halogen at NWM sites. A linear relationship between the glass transition temperatureT _g and the quadrupole splitting () of Fe³⁺, named T_g- rule, also indicates the structural role of Fe³⁺, i.e.,T _g versus plot yields a large slope of 680°C/mm s^–1 when Fe³⁺ occupies NWF sites, whereas it is only 35°C/mm s^–1 when the iron is present at NWM sites.     

ODMR and EPR investigations of Fe centers in KTaO3

The analysis of EPR spectra obtained from iron doped KTaO₃ crystals in the as-grown state revealed three dominant iron centers: Fe³⁺-O_I, axial Fe-centers with spinS = 3/2 and rhombic Fe³⁺. By comparison with data from literature possible assignments for the center withS = 3/2 are discussed. For the rhombic species the temperature dependence of the main parameters of the Spin- Hamiltonian was measured. The result makes it most plausible that only one rhombic iron center exists in KTaO₃, in contrast with literature. The understanding of the EPR spectra allows us to assign transitions, observed at very low magnetic fields by optically detected magnetic resonance (ODMR), to this rhombic Fe center. On this basis, the magnetic circular dichroism (MCD) of this defect could be identified using the method of tagged-MCD. This spectrum is compared to the tagged-MCD of Fe³⁺-O₁ and of axial Fe⁴⁺ centers, which may be generated metastably by optical charge transfer. Considerably different structures in the MCD spectra of both Fe³⁺ centers indicate different local surroundings and electronic states.Dedicated to O. F. Schirmer on the occasion of his 60th birthday     

Mössbauer investigation of temperature transformations in bacterial ferrihydrite

Ferrihydrite nanoparticles formed as a result of the microorganism activity have been studied using Mössbauer spectroscopy, X-ray powder diffraction analysis, and X-ray fluorescence analysis. Three positions of trivalent iron with nonoverlapping ranges of quadrupole splittings have been revealed in bacterial ferrihydrite: QS{Fe³⁺(1)} = 0.49–0.83 mm/s, QS{Fe³⁺(2)} = 0.84–1.10 mm/s, and QS{Fe³⁺(3)} = 1.25–1.73 mm/s. It has been experimentally demonstrated that the Fe³⁺(3) positions are the centers of nucleation of the hematite phase in the course of heat treatment.     

Optical absorption of MgO: Fe

The optical absorption spectra of as-grown, oxidized and reduced MgO: Fe crystals at low temperatures are reported. It is found that reduction at 1500°C converts more than 99% of the iron to Fe²⁺ revealing optical spectra due to Fe²⁺ in both cubic and non-cubic sites. Oxidation, however, converts only 60–70% of the iron to Fe³⁺ and no clear Fe³⁺ crystal-field spectrum is observed. A cubic Fe²⁺ band at 4.9 eV exhibits a progression of 13 vibronic sidebands typical of a Jahn-Teller distortion.     

A Raman spectroscopic study of copiapites Fe2+Fe3+(SO4)6(OH)2 · 20H2O: environmental implications

Raman spectroscopy has been used to study selected mineral samples of the copiapite group. Copiapite (Fe²⁺Fe³⁺(SO₄)₆(OH)₂ · 20H₂O) is a secondary mineral formed through the oxidation of pyrite. Minerals of the copiapite group have the general formula AFe₄(SO₄)₆(OH)₂ · 20H₂O, where A has a + 2 charge and can be either magnesium, iron, copper, calcium and/or zinc. The formula can also be B_2/3Fe₄(SO₄)₆(OH)₂ · 20H₂O, where B has a + 3 charge and may be either aluminium or iron. For each mineral, two Raman bands are observed at around 992 and 1029 cm⁻¹, assigned to the (SO₄)²⁻ν₁ symmetric stretching mode. The observation of two bands provides evidence for the existence of two non‐equivalent sulfate anions in the mineral structure. Three Raman bands at 1112, 1142 and 1161 cm⁻¹ are observed in the Raman spectrum of copiapites, indicating a reduction of symmetry of the sulfate anion in the copiapite structure. This reduction in symmetry is supported by multiple bands in the ν₂ and ν₄(SO₄)²⁻ spectral regions. Copyright © 2010 John Wiley & Sons, Ltd.