Stable hydrogen isotopes in iron oxides—isotope effects associated with the dehydration of a natural goethite

The dehydration of a natural goethite to hematite is accompanied by a systematic hydrogen isotope fractionation. Closed system dehydration at, and below, 250°C results in a significantly greater degree of isotopic fractionation than does open system dehydration. This relationship is apparently reversed at 300°C. Both processes produce a progressive decrease in the $\text{D}\text{H}$ ratio of the mineral hydrogen with increasing degree of dehydration. At temperatures of 160°C to 250°C the closed system mineralvapor fractionation factor is independent of temperature, while above 250°C, it varies strongly with temperature. The mineral-vapor fractionation factor associated with open system dehydration appears to be independent of temperature over the interval 160°C to 300°C. The closed system $\text{D}\text{H}$ fractionation suggests that natural goethite undergoing dehydration in the presence of water can isotopically exchange with that water.CO₂ loss from goethite during dehydration is correlated with the loss of H₂O. The CO₃ is thought to be present in carbonates which exist as impurities in the goethite. Loss of both H₂O and CO₂ appears to be diffusion-controlled.     

The structure and transformation of the nanomineral schwertmannite: a synthetic analog representative of field samples

The phase transformation of schwertmannite, an iron oxyhydroxide sulfate nanomineral synthesized at room temperature and at 75 °C using H₂O₂ to drive the precipitation of schwertmannite from ferrous sulfate (Regenspurg et al. in Geochim Cosmochim Acta 68:1185–1197, 2004), was studied using high-resolution transmission electron microscopy. The results of this study suggest that schwertmannite synthesized using this method should not be described as a single phase with a repeating unit cell, but as a polyphasic nanomineral with crystalline areas spanning less than a few nanometers in diameter, within a characteristic ‘pin-cushion’-like amorphous matrix. The difference in synthesis temperature affected the density of the needles on the schwertmannite surface. The needles on the higher-temperature schwertmannite displayed a dendritic morphology, whereas the needles on the room-temperature schwertmannite were more closely packed. Visible lattice fringes in the schwertmannite samples are consistent with the powder X-ray diffraction (XRD) pattern taken on the bulk schwertmannite and also matched d-spacings for goethite, indicating a close structural relationship between schwertmannite and goethite. The incomplete transformation from schwertmannite to goethite over 24 h at 75 °C was tracked using XRD and TEM. TEM images suggest that the sample collected after 24 h consists of aggregates of goethite nanocrystals. Comparing the synthetic schwertmannite in this study to a study on schwertmannite produced at 85 °C, which used ferric sulfate, reveals that synthesis conditions can result in significant differences in needle crystal structure. The bulk powder XRD patterns for the schwertmannite produced using these two samples were indistinguishable from one another. Future studies using synthetic schwertmannite should account for these differences when determining schwertmannite’s structure, reactivity, and capacity to take up elements like arsenic. The schwertmannite synthesized by the Regenspurg et al. method produces a mineral that is consistent with the structure and morphology of natural schwertmannite observed in our previous study using XRD and TEM, making this an ideal synthetic method for laboratory-based mineralogical and geochemical studies that intend to be environmentally relevant.     

The nature of ordering and ordering defects in dolomite

The apparent lack of transformation-induced domains and stacking disorder defects in natural dolomites is considered in light of the different types of order in the dolomite structure. Experimentally produced twin domain boundaries and basal stacking defects are documented in a dolomite using high-resolution electron microscopy and electron diffraction techniques. Results reveal that upon cation ordering to form the dolomite structure, a twin domain is favored over an antiphase domain. The twin domain boundaries closely resemble antiphase boundaries (APB's) and are in contrast for superlattice reflections. However, background contrast within domains is shown to be different when imaged using certain fundamental reflections. The results allow speculation about the nature of ordering at low temperatures. Observations of twin domain boundaries in samples annealed at different temperatures allow estimation of the critical ordering temperature at 1,100°–1,150° C in stoichiometric dolomite.     

Lamellar exsolution systems in clinopyroxene

Exsolution systems in synthetic pyroxenes were studied by transmission electron microscopy. An iron free sample En₈₀Wo₂₀ was prepared by devitrifying glass at 1300°C. Samples with bulk composition En₅₀Fs₃₀Wo₂₀ and En₃₅Fs₃₈Wo₂₇ were given various but well-defined heat treatments. The exsolution systems observed cannot unambiguously be related to the heat treatment. Periodic lamellar exsolution was observed parallel to (001) and (100) with sharp satellite reflections in the diffraction diagram. In more complex exsolution systems coarse (100) lamellae were found together with fine lamellae parallel to (001) and (100). An unusual phenomenon occurs at a (100) twin boundary where both individuals display exsolution lamellae parallel to (001). Pigeonite lamellae in one twin meet augite lamellae of the other individual at the twin boundary and vice-versa. The precise matching is achieved by a change in width near the boundary. Smoothly curved phase boundaries are developed in the obtuse angle of crosshatched (100) and (001) pigeonite lamellae in augite, whereas the boundaries in the acute angle are straight with sharp edges. This is consistent with elastic energy constraints.     

Near-atomic images of interfaces between twin-related lamellae in a synthetic 6<Emphasis Type="Italic">H</Emphasis>-SiC sample

Transmission Electron Microscopy (TEM) and High Resolution TEM (HRTEM) investigations were used to study a complex micro-structure of twin-related lamellae of 15R SiC polytype, in syntactic coalescence with 6H-SiC. The analysis of the local stacking microstructure by means of HRTEM revealed that the perfect structure of 15R polytype, (23)₃ was locally interrupted by numerous adjoining stacking faults parallel to (0001) with stacking of the coupled (22) and (33) bilayers superimposed on the twin boundaries. HR images taken exactly across both the twin boundaries showed a zig-zag pattern (23) that switched to (32) by a twin coherent interfaces or, alternatively, a zig-zag pattern (32) which passed to (23) through an isolated (33), 6H like sequence. The selected area electron diffraction (SAED) patterns taken exactly above both the twin interfaces indicate classifying of the twin found in this study as a “Friedelian” reticular merohedric twinning. However, two indistinguishable twin operations matched the observed features: a reflection through rational plane (0001), and 180°-rotation around [0001]. Since individual Si and C atoms and even the SiC bilayer polarity could not be established from these HR images, the real twin law was deduced by taking into account that the coherent structural match at the interface could be guaranteed only by a 180°-rotation around [0001]. In order to explain the origin of the planar defects found in this sample, the growth mechanism and the influence of the low-energy stacking faults were considered.     

Structural changes of synthetic opal by heat treatment

The structural changes of synthetic opal by heat treatment up to 1,400 °C were investigated using scanning electron microscopy, X-ray diffraction, and Fourier transform infrared and Raman spectroscopies. The results indicate that the dehydration and condensation of silanol in opal are very important factors in the structural evolution of heat-treated synthetic opal. Synthetic opal releases water molecules and silanols by heat treatment up to 400 °C, where the dehydration of silanol may lead to the condensation of a new Si–O–Si network comprising a four-membered ring structure of SiO₄ tetrahedra, even at 400 °C. Above 600 °C, water molecules are lost and the opal surface and internal silanol molecules are completely dehydrated by heat effect, and the medium-temperature range structure of opal may begin to thermally reconstruct to six-membered rings of SiO₄ tetrahedra. Above 1,000 °C, the opal structure almost approaches that of silica glass with an average structure of six-membered rings. Above 1,200 °C, the opal changes to low-cristobalite; however, minor evidence of low-tridymite stacking was evident after heat treatment at 1,400 °C.     

Effect of the activation temperature over activated carbon production from castor cake and its evaluation as dye adsorbent

In this work, castor cake produced as a by-product in castor oil extraction was used for activated carbon production. Castor cake was chemically treated with a K₂CO₃ solution, and the effect of the pyrolysis temperature in the 500–900 °C range was studied. Materials were characterized by X-ray powder diffraction, thermogravimetric analysis, scanning electron microscopy and nitrogen adsorption–desorption at ?196 °C. Methylene blue adsorption was selected as a test probe to stress the removal capacity of the prepared materials. By the X-ray powder diffraction analysis, carbon obtaining in its graphite allotropic form together with other inorganic compounds was verified. Scanning electron microscopy images evidenced the generation of porosity in the thermally treated samples compared with the pristine compound. In addition, the specific surface area values augmented progressively with the thermal treatment increment achieving a value of 1015 m² g^?1 in the 900 °C calcined sample. Calcination at 800 °C and m/V = 0.003 ratio were the best parameter combination to achieve a 99.6% methylene blue uptake.     

Ferrimagnetic minerals in red paleosols of Pleistocene Epoch, eastern China

The type,grain size and origin of ferimagnetic minerals separated from red paleosols of pleistocene Epoch(Q2)in eastern China ,were studied by using mineral magnetic measurement,X-ray powder diffraction and electron microscopy.Results showed that the iron oxider in red paleosols were composed of hematite(α-Fe2O3),maghemite(γ-Fe2O3) and goethite(α-FeOOH),Mineral magnetic parameters and X-ray diffraction patterns indicated that maghemite was the dominant remanence carrier in red paleosols,which is characterized by superparamagnetic(SP) and stable single domain(SSD) grains,The variations of magnetic susceptibility(χ） ,anhysteretic magnetic susceptibility(χRAM）and saturation isothermal remanent magnetization(SIRM) for red paleosols following heating to various temperatures showed two peak values at 700℃ and 900℃.The spherulitic magnetic particles measuring 250-1000μm in diameter in red paleosols were separated by the magnetic separation method,indicating that these magnetic particles were an assemblage of superparamagnetic and stable single domain ferrimagnetic grains,It is suggested that the ferrimagnetic minerals of red paleosols be a pedogenic ferrimagnetic component under high temperature and high humid conditions in the Pleistocene Fpoch(Q2).It is concluded that the magnetism characteristics of red paleosols can be used to evaluate the environmental changes of Quaternary in eastern China.     

An experimental study of the Fe oxidation states in garnet and clinopyroxene as a function of temperature in the system CaO–FeO–Fe2O3–MgO–Al2O3–SiO2: implications for garnet–clinopyroxene geothermometry

Samples with eclogitic composition in the system CaO–FeO–Fe₂O₃–MgO–Al₂O₃–SiO₂ were produced from various kinds of starting materials held in graphite-lined Pt capsules at a pressure of 2.5–3.0 GPa and temperatures of 800–1,300 °C using a piston-cylinder or Belt apparatus. Garnets and clinopyroxenes were characterized by analytical transmission electron microscopy and electron probe micro-analysis (EPMA). Fe³⁺/ΣFe ratios determined by electron energy-loss spectroscopy (EELS) decrease in clinopyroxene from 22.2 ± 3.4 % at 800 °C to 13.3 ± 5.4 % at 1,300 °C, while in garnet, they vary between 10.8 ± 1.5 and 15.4 ± 4.7 %, respectively. Temperature estimates according to Krogh (Contrib Mineral Petrol 99:44–48, 1988) reproduce the experimental temperature to ±60 °C without systematic deviations if total iron is used in the calculation. If only the Fe²⁺ content is used, which was obtained by combining EPMA and EELS results, the experimental temperature is underestimated by 33 °C on average at 800–1,200 °C and overestimated by 77 °C on average at 1,300 °C. These systematic deviations can be explained by the temperature-dependent ratio of Fe²⁺/ΣFe in garnet divided by that in clinopyroxene. Since the difference between the calculated and experimental temperature is relatively small, a Fe²⁺-based recalibration of the thermometer appears not to be necessary for the investigated system in the range of pressure, temperature and composition covered by the experiments of this study.     

Structure and chemistry of (111) twin boundaries in MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> spinel crystals from Mogok

The atomic scale structure and chemistry of (111) twins in MgAl₂O₄ spinel crystals from the Pinpyit locality near Mogok (Myanmar, formerly Burma) were analysed using complementary methods of transmission electron microscopy (TEM). To obtain a three-dimensional information on the atomic structure, the twin boundaries were investigated in crystallographic projections and Using conventional electron diffraction and high-resolution TEM (HRTEM) analysis we have shown that (111) twins in spinel can be crystallographically described by 180° rotation of the oxygen sublattice normal to the twin composition plane. This operation generates a local hcp stacking in otherwise ccp lattice and maintains a regular sequence of kagome and mixed layers. In addition to rotation, no other translations are present in (111) twins in these spinel crystals. Chemical analysis of the twin boundary was performed by energy-dispersive X-ray spectroscopy (EDS) using a variable beam diameter (VBD) technique, which is perfectly suited for analysing chemical composition of twin boundaries on a sub-nm scale. The VBD/EDS measurements indicated that (111) twin boundary in spinel is Mg-deficient. Quantitative analyses of HRTEM (phase contrast) and HAADF-STEM (Z-contrast) images of (111) twin boundary have confirmed that Mg²⁺ ions are replaced with Be²⁺ ions in boundary tetrahedral sites. The Be-rich twin boundary structure is closely related to BeAl₂O₄ (chrysoberyl) and BeMg₃Al₈O₁₆ (taaffeite) group of intermediate polysomatic minerals. Based on these results, we conclude that the formation of (111) twins in spinel is a preparatory stage of polytype/polysome formation (taaffeite) and is a result of thermodynamically favourable formation of hcp stacking due to Be incorporation on the {111} planes of the spinel structure in the nucleation stage of crystal growth. The twin structure grows as long as the surrounding geochemical conditions allow its formation. The incorporation of Be induces a 2D-anisotropy and exaggerated growth of the crystal along the (111) twin boundary.     

The effect of deformation on the TitaniQ geothermobarometer: an experimental study

We performed high strain (up to 47 %) axial compression experiments on natural quartz single crystals with added rutile powder (TiO₂) and ~0.2 wt% H₂O to investigate the effects of deformation on the titanium-in-quartz (TitaniQ) geothermobarometer. One of the objectives was to study the relationships between different deformation mechanisms and incorporation of Ti into recrystallized quartz grains. Experiments were performed in a Griggs-type solid-medium deformation apparatus at confining pressures of 1.0–1.5 GPa and temperatures of 800–1,000 °C, at constant strain rates of 1 × 10^?6 or 1 × 10^?7 s^?1. Mobility of Ti in the fluid phase and saturation of rutile at grain boundaries during the deformation experiments are indicated by precipitation of secondary rutile in cracks and along the grain boundaries of newly recrystallized quartz grains. Microstructural analysis by light and scanning electron microscopy (the latter including electron backscatter diffraction mapping of grain misorientations) shows that the strongly deformed quartz single crystals contain a wide variety of deformation microstructures and shows evidence for subgrain rotation (SGR) and grain boundary migration recrystallization (GBMR). In addition, substantial grain growth occurred in annealing experiments after deformation. The GBMR and grain growth are evidence of moving grain boundaries, a microstructure favored by high temperatures. Electron microprobe analysis shows no significant increase in Ti content in recrystallized quartz grains formed by SGR or by GBMR, nor in grains grown by annealing. This result indicates that neither SGR nor moving grain boundaries during GBMR and grain growth are adequate processes to facilitate re-equilibration of the Ti content in experimentally deformed quartz crystals at the investigated conditions. More generally, our results suggest that exchange of Ti in quartz at low H₂O contents (which may be realistic for natural deformation conditions) is still not fully understood. Thus, the application of the TitaniQ geothermobarometer to deformed metamorphic rocks at low fluid contents may not be as straightforward as previously thought and requires further research.     

Meteoric incursion and oxygen fronts in the Dalradian metamorphic belt, southwest Scotland: a new hypothesis for regional gold mobility

Post-metamorphic quartz veins which occur over hundreds of square kilometres in the biotite zone of the Dalradian metamorphic belt consist of three principal types: anhedral quartz with pyrite, anhedral quartz with hematite, and prismatic quartz with hematite or rutile. The oxide minerals in anhedral veins have formed by oxidation of pre-existing sulphides, and gold was mobilized during this oxidation. Anhedral quartz veins formed from an aqueous fluid with up to 5 wt% dissolved salts and 16 wt% CO₂ at about 300 °C. Texturally later prismatic quartz crystals formed from a compositionally similar fluid which was undergoing phase separation at the H₂O-CO₂ solvus at 160–200 °C and 500 to 1200 bars fluid pressure. Oxygen isotope ratios for quartz from the veins range from 12.0 to 15.3‰, with hematite-bearing veins generally isotopically heavier than pyrite-bearing veins. Calculated fluid oxygen isotope ratios range from + 8‰ for pyrite-bearing veins to -2‰ for late prismatic crystals. The mineralizing fluid contained a substantial component of meteoric water whose isotopic and chemical composition evolved with progressive water-rock interaction. Evolution of meteoric fluid composition involved migration of oxidation and oxygen isotope fronts in the down-flow direction as head-driven water passed through structurally controlled fractures in the schist pile. A gold solubility trough occurs for the observed fluid in the oxidation frontal zone. Gold remobilization and reprecipitation occurred progressively as the oxidation front migrated through the schist pile.     

In situ high-temperature X-ray diffraction and spectroscopic study of fibroferrite,FeOH(SO<Subscript>4</Subscript>)·5H<Subscript>2</Subscript>O

The thermal dehydration process of fibroferrite, FeOH(SO₄)·5H₂O, a secondary iron-bearing hydrous sulfate, was investigated by in situ high-temperature synchrotron X-ray powder diffraction (HT-XRPD), in situ high-temperature Fourier transform infrared spectroscopy (HT-FTIR) and thermal analysis (TGA-DTA) combined with evolved gas mass spectrometry. The data analysis allowed the determination of the stability fields and the reaction paths for this mineral as well as characterization of its high-temperature products. Five main endothermic peaks are observed in the DTA curve collected from room T up to 800 °C. Mass spectrometry of gases evolved during thermogravimetric analysis confirms that the first four mass loss steps are due to water emission, while the fifth is due to a dehydroxylation process; the final step is due to the decomposition of the remaining sulfate ion. The temperature behavior of the different phases occurring during the heating process was analyzed, and the induced structural changes are discussed. In particular, the crystal structure of a new phase, FeOH(SO₄)·4H₂O, appearing at about 80 °C due to release of one interstitial H₂O molecule, was solved by ab initio real-space and reciprocal-space methods. This study contributes to further understanding of the dehydration mechanism and thermal stability of secondary sulfate minerals.     

Goethite Morphology and Composition in Banded Iron Formation, Orissa, India

Iron ore deposits are generally described in terms of size, grade and chemical composition rather than the mineralogical and microstructural characteristic of different ore types. It is essential, however, to know the morphology, microstructure and chemical composition of individual minerals for optimum mineral processing. Goethite is reported to occur as a ubiquitous phase in many iron ore types and is particularly abundant in the Precambrian banded iron ore formation of north Orissa, India. Goethite from the Bonai–Keonjhar Belt in Orissa has been examined in terms of its morphology and microstructure in relation to chemical composition. Electron microscopy indicated several goethite morphotypes including botryoidal, nodular, spheroidal, platy, stalactitic and flaky. These different morphotypes display intergranular, intragranular, wedge, reniform, comb, prismatic, cavity-line and bead microstructures. In situ analysis using electron probe microanalyzer indicated a wide compositional variation among the different morphotypes and microstructures. Goethite replacing hematite is generally devoid of deleterious elements while re-precipitated goethite generally contains adsorbed alumina, silica and/or phosphorus. Nodular goethite commonly has a high phosphorus level while botryoidal, spheroidal and platy goethite often contains increased combined alumina and silica. Goethite having a reniform, wedge, intergranular or intragranular microstructure is highly water bearing and cryptocrystalline in nature. During dehydration, bead, comb, cavity-lined or prismatic goethite develop, which are more crystalline and which have a higher iron concentration. Goethite with a wedge, prismatic or bead-type microstructure has a higher adsorption of silica (2–4%), while goethite having an intergranular, bead or prismatic microstructure invariably contains appreciable phosphorus, generally at levels deleterious to processing.     

Fluids in the peridotite–water system up to 6 GPa and 800°C: new experimental constrains on dehydration reactions

The phase assemblages and compositions in a K-free lherzolite + H₂O system were determined between 4 and 6 GPa and 700–800°C, and the dehydration reactions occurring at subarc depth in subduction zones were constrained. Experiments were performed on a rocking multi-anvil apparatus using a diamond-trap setting. The composition of the fluid phase was measured using the recently developed cryogenic LA–ICP–MS technique. Results show that, at 4 GPa, the aqueous fluid coexisting with residual lherzolite (~85 wt% H₂O) doubles its solute load when chlorite transforms to the 10-Å phase between 700 and 750°C. The 10-Å phase breaks down at 4 and 5 GPa between 750 and 800°C and at 6 GPa between 700 and 750°C, leaving a dry lherzolite coexisting with a fluid phase containing 58–67 wt% H₂O, again doubling the total dissolved solute load. The fluid fraction in the system increases from 0.2 when a hydrous mineral is present to 0.4 when coexisting with a dry lherzolite. Our data do not reveal the presence of a hydrous peridotite solidus below 800°C. The directly measured fluid compositions demonstrate a fundamental change in the (MgO + FeO) to SiO₂ mass ratio of fluid solutes occurring at a depth of ca. 120–150 km (in the temperature window of 700–800°C), from (MgO–FeO)-dominated at 4 GPa [with (MgO + FeO)/SiO₂ ratio of 1.41–1.56] to SiO₂-dominated at 5–6 GPa (ratios of 0.61–0.82). The mobility of Al₂O₃ increases by more than one order of magnitude across this P–T interval and demonstrates that Al₂O₃ is compatible in an aqueous fluid coexisting with the anhydrous ol-opx-cpx ± grt assemblage. This shift in the fluid composition correlates with changes in the phase assemblage of the residual silicates. The hitherto unknown fundamental change in (MgO + FeO)/SiO₂ ratio and prominent increase in Al₂O₃ of the aqueous fluid with progressive subduction will likely inspire novel concepts on mantle wedge metasomatism by slab fluids.     

Experimental and petrological constraints on local-scale interaction of biotite-amphibole gneiss with H2O-CO2-（K, Na）Cl fluids at middle-crustal conditions： Example from the Limpopo Complex, South Africa

Reaction textures and fluid inclusions in the～2.0 Ga pyroxene-bearing dehydration zones within the Sand River biotite-hornblende orthogneisses（Central Zone of the Limpopo Complex） suggest that the formation of these zones is a result of close interplay between dehydration process along ductile shear zones triggered by H₂O-CO₂-salt fluids at 750—800℃and 5.5—6.2 kbar.partial melting,and later exsolution of residual brine and H₂O-CO₂ fluids during melt crystallization at 650—700℃.These processes caused local variations of water and alkali activity in the fluids,resulting in various mineral assemblages within the dehydration zone.The petrological observations are substantiated by experiments on the interaction of the Sand River gneiss with the H₂O-CO-2-（K,Na）Cl fluids at 750 and 800℃and 5.5 kbar.It follows that the interaction of biotite-amphibole gneiss with H₂O-CO₂-（K.Na）Cl fluids is accompanied by partial melting at 750—800℃.Orthopyroxene-bearing assemblages are characteristic for temperature 800℃and are stable in equilibrium with fluids with low salt concentrations,while salt-rich fluids produce clinopyroxene-bearing assemblages.These observations arc in good agreement with the petrological data on the dehydration zones within the Sand River orthoeneisses.     

The thermal stability of sideronatrite and its decomposition products in the system Na2O–Fe2O3–SO2–H2O

The thermal stability of sideronatrite, ideally Na₂Fe³⁺(SO₄)₂(OH)·3(H₂O), and its decomposition products were investigated by combining thermogravimetric and differential thermal analysis, in situ high-temperature X-ray powder diffraction (HT-XRPD) and Fourier transform infrared spectroscopy (HT-FTIR). The data show that for increasing temperature there are four main dehydration/transformation steps in sideronatrite: (a) between 30 and 40 °C sideronatrite transforms into metasideronatrite after the loss of two water molecules; both XRD and FTIR suggest that this transformation occurs via minor adjustments in the building block. (b) between 120 and 300 °C metasideronatrite transforms into metasideronatrite II, a still poorly characterized phase with possible orthorhombic symmetry, consequently to the loss of an additional water molecule; X-ray diffraction data suggest that metasideronatrite disappears from the assemblage above 175 °C. (c) between 315 and 415 °C metasideronatrite II transforms into the anhydrous Na₃Fe(SO₄)₃ compound. This step occurs via the loss of hydroxyl groups that involves the breakdown of the [Fe³⁺(SO₄)₂(OH)] _∞ ^2? chains and the formation of an intermediate transient amorphous phase precursor of Na₃Fe(SO₄)₃. (d) for T > 500 °C, the Na₃Fe(SO₄)₃ compound is replaced by the Na-sulfate thenardite, Na₂SO₄, plus Fe-oxides, according to the Na₃Fe³⁺(SO₄)₃ → 3/2 Na₂(SO₄) + 1/2 Fe₂O₃ + SO_x reaction products. The Na–Fe sulfate disappears around 540 °C. For higher temperatures, the Na-sulfates decomposes and only hematite survives in the final product. The understanding of the thermal behavior of minerals such as sideronatrite and related sulfates is important both from an environmental point of view, due to the presence of these phases in evaporitic deposits, soils and sediments including extraterrestrial occurrences, and from the technological point of view, due to the use of these materials in many industrial applications.     

Atomic structures and energies of grain boundaries in Mg2SiO4 forsterite from atomistic modeling

Grain boundaries influence many physical and chemical properties of crystalline materials. Here, we perform molecular dynamics simulations to study the structure of a series of [100] symmetric tilt grain boundaries in Mg₂SiO₄ forsterite. The present results show that grain boundary energies depend significantly on misorientation angle. For small misorientation angles (up to 22°), grain boundary structures consist of an array of partial edge dislocations with Burgers vector $\frac{1}{2}[001]$ associated with stacking faults and their energies can be readily fit with a model which adds the Peach-Koehler equation to the Read-Shockley dislocation model for grain boundaries. The core radius of partial dislocations and the spacing between the partials derived from grain boundary energies show that the transition from low- to high-angle grain boundaries occurs for a misorientation angle between 22° and 32°. For high misorientation angles (32.1° and 60.8°), the cores of dislocations overlap and form repeated structural units. Finally, we use a low energy atomic configuration obtained by molecular dynamics for the misorientation of 12.18° as input to simulate a high-resolution transmission electron microscopy (HRTEM) image. The simulated image is in good agreement with an observed HRTEM image, which indicates the power of the present approach to predict realistic atomic structures of grain boundaries in complex silicates.     

Genesis of superimposed hypogene and supergene Fe orebodies in BIF at the Madoonga deposit, Yilgarn Craton, Western Australia

The Madoonga iron ore body hosted by banded iron formation (BIF) in the Weld Range greenstone belt of Western Australia is a blend of four genetically and compositionally distinct types of high-grade (>55 wt% Fe) iron ore that includes: (1) hypogene magnetite–talc veins, (2) hypogene specular hematite–quartz veins, (3) supergene goethite–hematite, and (4) supergene-modified, goethite–hematite-rich detrital ores. The spatial coincidence of these different ore types is a major factor controlling the overall size of the Madoonga ore body, but results in a compositionally heterogeneous ore deposit. Hypogene magnetite–talc veins that are up to 3 m thick and 50 m long formed within mylonite and shear zones located along the limbs of isoclinal, recumbent F₁ folds. Relative to least-altered BIF, the magnetite–talc veins are enriched in Fe₂O_3(total), P₂O₅, MgO, Sc, Ga, Al₂O₃, Cl, and Zr; and depleted in SiO₂ and MnO₂. Mafic igneous countryrocks located within 10 m of the northern contact of the mineralised BIF display the replacement of primary igneous amphibole and plagioclase, and metamorphic chlorite by hypogene ferroan chlorite, talc, and magnetite. Later-forming, hypogene specular hematite–quartz veins and their associated alteration halos partly replace magnetite–talc veins in BIF and formed during, to shortly after, the F₂-folding and tilting of the Weld Range tectono-stratigraphy. Supergene goethite–hematite ore zones that are up to 150 m wide, 400 m long, and extend to depths of 300 m replace least-altered BIF and existing hypogene alteration zones. The supergene ore zones formed as a result of the circulation of surface oxidised fluids through late NNW- to NNE-trending, subvertical brittle faults. Flat-lying, supergene goethite–hematite-altered, detrital sediments are concentrated in a paleo-topographic depression along the southern side of the main ENE-trending ridge at Madoonga. Iron ore deposits of the Weld Range greenstone belt record remarkably similar deformation histories, overprinting hypogene alteration events, and high-grade Fe ore types to other Fe ore deposits in the wider Yilgarn Craton (e.g. Koolyanobbing and Windarling deposits) despite these Fe camps being presently located more than 400 km apart and in different tectono-stratigraphic domains. Rather than the existence of a synchronous, Yilgarn-wide, Fe mineralisation event affecting BIF throughout the Yilgarn, it is more likely that these geographically isolated Fe ore districts experienced similar tectonic histories, whereby hypogene fluids were sourced from commonly available fluid reservoirs (e.g. metamorphic, magmatic, or both) and channelled along evolving structures during progressive deformation, resulting in several generations of Fe ore.