Showing papers in "American Mineralogist in 2008"
424 citations
TL;DR: In this paper, the authors performed a series of experiments that place better constraints on the range of oxygen fugacity imposed by this capsule material, on the Fe 3+ /Fe 2+ ratios in experimentally produced melts and minerals, and on the temperature reproducibility in Pt-graphite capsules.
Abstract: aB stra C t The Pt-graphite double-capsule technique is a very commonly used method in high-temperature, high-pressure experimental petrology, particularly for anhydrous experiments relevant to primitive basaltic magmas and mantle melting. We have performed a series of experiments that place better constraints on the range of oxygen fugacity imposed by this capsule material, on the Fe 3+ /Fe 2+ ratios in experimentally produced melts and minerals, and on the temperature reproducibility in Pt-graphite capsules. Oxygen fugacity in our piston-cylinder experiments using Pt-graphite capsules is CCO-0.7 (IW+1.5, QFM-2.2) at 1.5 GPa and 1360 °C. Comparison with other estimates and thermodynamic calculations indicate that a value of CCO-0.8 ± 0.3 can be used as a first approximation at least over the P-T range relevant for MORB and OIB magma generation (0.5–3.0 GPa, 1100–1500 °C). Under those conditions, the amount of Fe 3+ in silicate phases (pyroxenes, olivine, glass) and spinel is negligible (Fe 3+ / ΣFe < 0.05) and would not significantly affect thermodynamic properties. Significantly higher values of fO2 cannot be achieved using Pt-graphite or graphite only capsules, but fO2 can be tuned to lower values by using small pieces of PtFe alloys. The potential range of fO2 that can be reached in graphite or Pt-graphite capsules is CCO to CCO-4. Temperature reproducibility in piston-cylinder experiments has been examined and can be as low as ±10 °C. Finally, unless capsules are dried overnight at 400 °C before the experiment, small amounts of H 2O are always present in nominally dry experiments. These small amounts of H 2O should not, however, significantly change phase relations.
173 citations
TL;DR: In this article, the authors test the theory of light propagation in weakly absorbing anisotropic minerals using both polarized and unpolarized light and show that if the linear polarized maximum absorbance is smaller than 0.3, or if the ratio of maximum and minimum absorbances is close to unity, then the polarized maximum and the unpolarised absorbance are, to a good approximation, linearly proportional to thickness regardless of the direction of the incident light.
Abstract: The predictions of the theory of light propagation in weakly absorbing anisotropic minerals are tested against systematic measurements of the infrared absorbance spectra of calcite, olivine, and topaz oriented in both principal and random sections, using both polarized and unpolarized light. We show that if the linear polarized maximum absorbance is smaller than ~0.3, or if the ratio of maximum and minimum absorbance is close to unity, then (1) the polarized maximum and minimum absorbances as well as the unpolarized absorbance are, to a good approximation, linearly proportional to thickness, regardless of the direction of the incident light; (2) the angular variation of polarized light absorption is indistinguishable from the theoretical predictions within the uncertainty of the measurements; (3) for any section the unpolarized absorbance is the mean of the polarized maximum and minimum absorbance; and (4) the average unpolarized absorbance of randomly oriented grains is one third of the Total Absorbance (defined as the sum of the three principal absorbances). Therefore, calibrations relating Total Absorbance to absorber concentration in minerals that have been developed from measurements with polarized light parallel to the principal axes may be applied to measurements with unpolarized light on a population of randomly oriented sections. We show that 10 such measurements are sufficient to achieve a petrologically useful accuracy. The method enables water concentrations in nominally anhydrous minerals to be determined from samples where the preparation of oriented specimens is not feasible, such as high-pressure experimental runs and fine-grained mantle xenoliths. The method may also be used for obtaining quantitative measurements on low-symmetry minerals.
164 citations
TL;DR: In this paper, an Al/Si tetrahedral distribution in the glass network in different Qn species for silicon and essentially in Q4 and VAl for aluminum was proposed.
Abstract: The structure and properties of glasses and melts in the MgO-Al2O3-SiO2 (MAS) and CaO-MgO-Al2O3-SiO2 (CMAS) systems play an important role in Earth and material sciences. Aluminum has a crucial influence in these systems, and its environment is still questioned. In this paper, we present new results using Raman spectroscopy and 27Al nuclear magnetic resonance on MAS and CMAS glasses. We propose an Al/Si tetrahedral distribution in the glass network in different Qn species for silicon and essentially in Q4 and VAl for aluminum. For the CMAS glasses, an increase of VAl and VIAl is clearly visible as a function of the increase of Mg/Ca ratio in the (Ca,Mg)3Al2Si3O12 (garnet) and (Ca,Mg)AlSi2O8 (anorthite) glass compositions. In the MAS system, the proportion of VAl and VIAl increases with decreasing SiO2 and, similarly with calcium aluminosilicate glasses, the maximum of VAl is located in the center of the ternary system.
163 citations
TL;DR: In this article, an eclogite facies meta-plagiogranite from the Lanzo massif (western Alps, Italy) contains crystals of zircon intimately associated with allanite.
Abstract: abst R a C t An eclogite facies meta-plagiogranite from the Lanzo massif (western Alps, Italy) contains crystals of zircon intimately associated with allanite. Zircon displays different microtextures ranging from pristine, euhedral, and magmatic to fractured, porous varieties with mosaic zoning, and pervasive recrystallization into euhedral microcrystals. Fractures and voids in the recrystallized zircon microcrystals are mainly filled by high-pressure Na-rich pyroxene. Electron backscattered diffraction analysis revealed a similar crystallographic orientation for primary magmatic zircon crystals and microcrystals, with less than 2° misorientation among neighboring microdomains. The textural change is coupled with chemical and isotopic modifications: recrystallized zircon domains contain significantly less Th and light- to mid-REE, but are richer in Sr than magmatic zircon crystals. Magmatic zircon preserves the protolith U-Pb age of 163.5 ± 1.7 Ma, whereas zircon microcrystals have a mean age of 55 ± 1 Ma. The coexisting allanite also contains inclusions of Na-rich pyroxene and has chemical features (elevated Sr and Ni contents and lack of Eu anomaly) indicating formation at high pressure. Despite being associated texturally with zircon, allanite yields a younger Th-Pb age of 46.5 ± 3.0 Ma, suggesting that the Lanzo unit remained at relatively high pressure conditions for ~8 m.y. Zircon recrystallization proceeded with volume reduction and loss of material to an alkaline metamorphic fluid that acted as the agent for a coupled dissolution-reprecipitation process. Recrystallization occurred with minimum transport, in a low-strain environment, and was not significantly enhanced by metamictization. The source of the fluid for zircon recrystallization is most probably related to prograde devolatilization reactions in the surrounding serpentinite.
152 citations
TL;DR: The Wills' Mineral Processing Technology as mentioned in this paper provides a fairly complete exposition of techniques and practices used in the ore processing industry to convert the raw ore feed to concentrate, with a new editor (T.J. Napier-Munn) and extended set of contributing authors.
Abstract: Wills’ Mineral Processing Technology compiles the major aspects of ore processing and segregation, from the influence of ore mineralogy to engineering and financial factors, as well as environmental concerns, into one source. Impressively, it largely succeeds in this aim, providing a fairly complete exposition of techniques and practices used in the ore processing industry to convert the raw ore feed to concentrate. It is unavoidably too brief in some aspects, but in general it provides a good introduction for students and others interested in the field of ore processing. Given that many of the techniques described are also in use in geology and related fields albeit on a laboratory scale, the book will appeal to a much wider audience.
The present book is the seventh edition of this work with a new editor (T.J. Napier-Munn) and extended set of contributing authors, with the aim of bringing the text up to date with current techniques and practices. Although largely successful in this aim, the editor decided to preserve as much as possible of the original, thereby restricting the scope of the revisions. This is unfortunate, as it has resulted in some lack of coherence in the book. Where several chapters are concise and well presented, others are wordy and contain minor typographic errors. Some chapters also dwell …
146 citations
TL;DR: The temperature dependence of the partitioning of Al2O3 between forsterite-rich olivine and Cr-rich spinels has been experimentally calibrated at 100 kPa between 1250 and 1450 °C under reducing conditions as discussed by the authors.
Abstract: The temperature dependence of the partitioning of Al2O3 between forsterite-rich olivine and Cr-rich spinels has been experimentally calibrated at 100 kPa between 1250 and 1450 °C under reducing conditions. For spinel with values of Y Cr between 0.07 and 0.69, and containing <0.1 Fe3+ atoms and <0.025 Ti atoms per 4 O atoms the experimental data can be fit to an equation:
\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[\mathit{T}({^\circ}C)\ =\ \frac{10\ 000}{0.512\ +\ 0.873\mathit{Y}\_{Cr}\ {-}\ 0.91ln(\mathit{K}\_{D})}\ {-}\ 273\] \end{document}
where Y Cr = Cr/(Cr + Al) in spinel in atomic proportions and K D = Al2O3ol/Al2O3sp in wt%. This equation reproduces the temperature of the experiment for the calibration data set within ±22 °C. Empirical tests suggest that this geothermometer has little pressure dependence within the range of pressures encountered in the crust and upper mantle.
142 citations
TL;DR: Coesite inclusions in garnet have been recognized in eclogitic rocks from western Tianshan, northwest China as discussed by the authors, showing the characteristic band at 522 cm -1 and subsidiary bands at 428, 326, 271, 178, 151, and 121 cm-1.
Abstract: Coesite inclusions in garnet have been recognized in eclogitic rocks from western Tianshan, northwest China. The coesite grains exhibit distinct radial cracks in host porphyroblastic garnet; some coesite relics are well preserved, whereas others are partially replaced by quartz. Coesite has been identified optically and then confirmed by in situ Raman spectroscopy, showing the characteristic band at 522 cm -1 and subsidiary bands at 428, 326, 271, 178, 151, and 121 cm -1 . The eclogitic rocks contain garnet, omphacite, and Na-Ca-amphibole, and they are rich in white mica (>30%) and graphite. Peak conditions of 570-630 °C and 2.7-3.3 GPa are constrained by garnet-clinopyroxene geothermometry and the occurrence of coesite. The presence of coesite and widespread quartz inclusions in garnet with radial cracks indicative of former coesite in these unique graphitic rocks confirms the previous suggestion of the UHP terrane for the western Tianshan, China.
127 citations
TL;DR: For example, in this paper, high-pressure phase equilibria with dynamic tectonic processes have illuminated the petrogenesis of the crust and provided new constraints on the constitution and evolution of the mantle.
Abstract: Fifty years ago, geologic conditions attending the formation of blueschists, eclogites, and garnet lherzolites were not known. But, with the advent of high-pressure phase-synthesis equipment and precise calorimetry, minerals like jadeite, aragonite, pyrope, and the dense polymorphs of SiO 2 and C were shown to be stable at elevated pressures and relatively low temperatures. Metamorphic conditions required by P - T stabilities of these minerals reflect the operation of plate tectonics, lithospheric subduction, and inferred mantle convection. Integration of phase equilibria with dynamic tectonic processes has illuminated the petrogenesis of the crust. Combined with geochemical, geophysical, and isotopic data, high-pressure phase equilibria are also providing new constraints on the constitution and evolution of the mantle. Circumpacific blueschists and eclogites occur in penetratively sheared nappes that are overturned seaward, indicating 30–50 km descent of an oceanic plate during metamorphism before partial exhumation of mainly low-density crustal material. Neoblastic coesite and microdiamond inclusions in tough, rigid host minerals show that continental collision involves fragmentary recovery of subducted rocks from depths of 100–130 km, far deeper than traditionally thought. Even more surprising, garnet peridotites from the central Alps, western Norway, Bohemia, and China display intergrowths and exsolution lamellae reflecting the former existence of majoritic garnet, stishovite, and other phases requiring depths of origin >300 km. Exsolved nanominerals attest to the decompression of precursor phases that had formed at profound depths preceding mantle upwelling. Times of deep-seated storage and rates of exhumation remain as major problems. Fluid-rock and lithosphere-asthenosphere interactions have recycled volatiles to the deep Earth through subduction of both hydrous and nominally anhydrous minerals. Mantle petrochemistry and plume-plate dynamics control the evolving architecture of the Earth’s crust and the interdependent biosphere. Applications of advanced technologies to condensed materials are leading to a fuller understanding of the planetary interior in time and space.
127 citations
TL;DR: In this paper, the oxidation state of Fe in hydrous ultrapotassic (phono-tephritic) melts coexisting with mixed H 2 O-CO 2 fluids was studied experimentally at 1200 and 1250 °C and pressures from 50 to 500 MPa.
Abstract: The oxidation state of Fe in hydrous ultrapotassic (phono-tephritic) melts coexisting with mixed H 2 O-CO 2 fluids was studied experimentally at 1200 and 1250 °C and pressures from 50 to 500 MPa. The oxygen fugacity ( f O 2 ) varied from NNO −2.9 to NNO+2.6 in log f O 2 , relative to the Ni-NiO oxygen buffer (NNO), as imposed by external redox conditions in experimental vessels and internal variations in water activity from 0.05 to 1 inside the capsules. The Fe redox state of the quenched melts was determined by colorimetric wet-chemical analysis. This analytical method was optimized to measure the Fe 2+ /ΣFe ratio of milligram-sized samples within ±0.03 (2σ). The accuracy and precision was tested with international reference materials and with standards analyzed by other methods. The Fe 2+ /ΣFe ratio of the experimental glasses covered a range of 0.41 to 0.85. A small negative effect of dissolved water on Fe 2+ /ΣFe at given f O 2 was found, consistent with the thermodynamic model of Moretti (2005). No effect of pressure and temperature on the redox state of Fe was resolvable in the investigated P-T range. Compared to hydrous ferrobasaltic melts that were studied previously under similar conditions, systematically lower Fe 2+ /ΣFe ratios were found for the phono-tephritic melts, in particular at low oxygen fugacities. This effect is attributed to the much higher K 2 O contents of the phono-tephrite (7.5 compared to 0.3 wt%), but the difference in FeO T (7.8 wt% in the phono-tephrite and 12.9 wt% in the ferrobasalt) may have an influence as well. Comparison of the experimentally obtained relationship between log f O 2 and Fe 3+ /Fe 2+ for the studied hydrous ultrapotassic melts with commonly used empirical and thermodynamic models suggest that these models can be successfully applied to phono-tephritc melts, although such compositions were not implemented in the model calibrations. Furthermore, the new data can be used to improve the models with respect to the effects of compositional variables, such as H 2 O or K 2 O, on the redox state of Fe in silicate melts.
118 citations
TL;DR: In this paper, Thorite and uraninite inclusions are documented in metasomatized regions in three xenotime crystals and one monazite crystal collected from three granitic pegmatites hosted within an anorthosite intrusion located on the island of Hidra, southwestern Norway.
Abstract: Thorite and uraninite inclusions are documented in metasomatized regions in three xenotime crystals and one monazite crystal collected from three granitic pegmatites hosted within an anorthosite intrusion located on the island of Hidra, southwestern Norway. Formation of these inclusions is demonstrated to be the result of dissolution-reprecipitation processes that occurred in an effectively closed chemical system. The reaction was initiated by interaction between the early crystallizing orthophosphate minerals and the evolving pegmatite fluid. This fluid was dominated by H 2 O, but also contained F and minor amounts of Cl, and significant quantities of dissolved Na + and K + . This is an example of auto-metasomatism and highlights a natural case where Th-U-Si-enriched orthophosphate minerals have been partially altered such that the Th, U, and Si have been converted to secondary inclusions within the now Th-U-Si-depleted orthophosphate via reaction with fluids rich in alkali elements.
TL;DR: The origin of sulfite (S4+) species in silicate glasses was evaluated using XANES at the S K -edge as mentioned in this paper, and it was shown that S2− and S6+ are the only significant sulfite species occurring in silicates synthesized under geologically relevant conditions.
Abstract: The origin of sulfite (S4+) species in silicate glasses was evaluated using XANES at the S K -edge. Systematic investigations show that the presence of S4+ species in silicate glasses is an analytical artifact related to changes in the sulfur species caused by irradiation with an electron beam during EMPA or by irradiation with an intense focused X-ray beam during synchrotron analysis. The data shown here indicate that S2− and S6+ are the only significant sulfur species occurring in silicate glasses synthesized under geologically relevant conditions.
TL;DR: In this paper, the authors compare the sample form factor without adjustable parameters from powder X-ray diffraction data using a recently developed method with simulated (Debyesum method) theoretical sample form factors for the recently proposed structure.
Abstract: Recently, Michel et al. (2007a) have presented a structure for ferrihydrite that we show to be incorrect. We do this by comparing (1) the sample form factor without adjustable parameters from powder X-ray diffraction data using a recently developed method with (2) exactly simulated (Debyesum method) theoretical sample form factors for the recently proposed structure (including vacancy, particle size and shape, and positional disorder effects). Michel et al. (2007a) used pair distribution functions (PDFs) extracted from synchrotron diffraction data fitted with calculated PDFs using adjustable scale and peak shape parameters. The PDF method gives consistent short-range (coordination sphere) correlations but under-emphasizes intermediate-range correlations that represent more stringent constraints on the structure. Main characteristic diffraction peaks of six-line ferrihydrite (lines 2, 3, and 4) are not reproduced by the proposed structural model. We expect our method to offer rigorous tests of proposed structures of any nanocrystalline materials.
TL;DR: In this paper, the authors studied the properties of magnetosomes at several steps of maturity and found that the morphology of mature magnetosome is not exclusively determined by vectorial regulation at the organic boundaries or by molecular interaction with the magnetosOME membrane, but also by the rates of Fe uptake, which suggests that the expression of different faces is favored for different growth conditions.
Abstract: Magnetotactic bacteria are known to mediate the formation of intracellular magnetic nanoparticles in organelles called magnetosomes. These magnetite crystals are formed through a process called biologically controlled mineralization, in which the microorganisms exert a strict control over the formation and development of the mineral phase. By inducing magnetite nucleation and growth in resting, Fe-starved cells of Magnetospirillum gryphiswaldense, we have followed the dynamics of magnetosome development. By studying the properties of the crystals at several steps of maturity, we observed that freshly induced particles lacked a well-defined morphology. More surprisingly, although the mean particle size of mature magnetosomes is similar to that of magnetosomes formed by constantly growing and Fe-supplemented bacteria, we found that other physical properties such as crystal-size distribution, aspect ratio, and morphology significantly differ. Correlating these results with measurements of Fe uptake rates, we suggest that the expression of different faces is favored for different growth conditions. These results imply that the biological control over magnetite biomineralization by magnetotactic bacteria can be disturbed by environmental parameters. Specifically, the morphology of magnetite crystals is not exclusively determined by biological intervention through vectorial regulation at the organic boundaries or by molecular interaction with the magnetosome membrane, but also by the rates of Fe uptake. This insight may contribute to better define biomarkers and to an improved understanding of biomineralizing systems.
TL;DR: In this paper, visible, near-infrared, thermal, and Mossbauer spectroscopic data from the exposed, bright track soil at the “Paso Robles” site within Gusev crater, Mars, indicate the presence of Fe3+-sulfates and possibly Fe3-phosphates admixed with the host soil.
Abstract: Visible, near-infrared, thermal, and Mossbauer spectroscopic data from the exposed, bright track soil at the “Paso Robles” site within Gusev crater, Mars, indicate the presence of Fe3+-sulfates and possibly Fe3+-phosphates admixed with the host soil. When the spectroscopic analyses are combined with constraints imposed by chemical data, the determined dominant Fe3+-sulfate component is hydrous, and all of the spectroscopic methods suggest that it is probably ferricopiapite or some closely related, structurally similar species, possibly mixed with other Fe3+ sulfates such as butlerite or parabutlerite, or perhaps (para)coquimbite, fibroferrite, or metahohmanite. Such an assemblage is consistent with formation in a highly oxidized, relatively dehydrated environment with the bulk-sulfate assemblage having OH/(OH + 2SO4) of < ~0.4. Some Fe3+ is likely to be associated with phosphates in the soil in the form of ferristrunzite or strengite.
TL;DR: Apatite-hosting maskelynite and apatite within regions interstitial to cumulus olivine differ compositionally from that found within melt inclusions in the Chassigny martian meteorite as discussed by the authors.
Abstract: Apatite-hosting maskelynite and alkali maskelynite within regions interstitial to cumulus olivine differ compositionally from that found within melt inclusions in the Chassigny martian meteorite. Feldspar glass compositions within the interstitial regions evolve along a high-temperature crystallization path. Within the melt inclusions, feldspar glass shows evolution to low temperatures, extending into the subsolidus regime. Coupled with these differences in maskelynite compositions are differences in volatile abundance in apatite included within the maskelynite. Apatite found within the large olivine-hosted polyphase melt inclusions is uniformly fluorapatite, whereas that interstitial to cumulus olivine is chlor-fluorapatite.
We propose that the differences in maskelynite and apatite compositions within the melt inclusion and interstitial regions arose primarily from different crystallization conditions. Melt-inclusion maskelynite and apatite are consistent with nearly closed-system buildup of magmatic volatiles during crystallization within the melt inclusions, exsolution of a Cl-bearing fluid phase, and retention of this fluid phase within the melt inclusions into the hydrothermal regime. For the interstitial regions, however, the higher solidus temperatures of the interstitial melts, the early crystallization of feldspar with significant ternary component, and the Cl-rich nature of the apatite, all suggest open-system fluid migration through the cumulus pile, ingress of Cl-rich, H2O-poor brine from a hotter, less evolved portion of the magma plumbing system, and interaction of this Cl-rich fluid with melt prior to the crystallization of feldspar. Such processes of fluid migration through a cumulus pile have been suggested on Earth in layered intrusions, and apatite is the primary recorder of this process.
TL;DR: In this paper, the pressure volume equation of state (EoS) of single-crystal MgO has been studied in diamondanvil cells loaded with helium to 118 GPa and in a non-hydrostatic KCl pressure medium to 87 GPa using monochromatic synchrotron X-ray diffraction.
Abstract: The pressure-volume equation of state (EoS) of single-crystal MgO has been studied in diamondanvil cells loaded with helium to 118 GPa and in a non-hydrostatic KCl pressure medium to 87 GPa using monochromatic synchrotron X-ray diffraction. A third-order Birch-Murnaghan fit to the nonhydrostatic P-V data (KCl medium) yields typical results for the initial volume, V0 = 74.698(7) A 3
TL;DR: In this paper, a theory of light propagation in anisotropic absorbing crystals is developed from Maxwell's equations to devise an expression for the transmittance of linearly polarized light traveling in an arbitrary direction in weakly absorbing media, and a procedure is proposed for estimating both total (A a + A b + A c) and principal absorbance from spectroscopic measurements of absorbance using unpolarized light on a set of randomly oriented crystals.
Abstract: A new approach to the use of spectroscopic absorbance measurements for anisotropic crystals allows results to be extracted using unpolarized light incident on random crystal orientations. The theory of light propagation in anisotropic absorbing crystals is developed from Maxwell’s equations to devise an expression for the transmittance of linearly polarized light traveling in an arbitrary direction in weakly absorbing media. This theory predicts the distribution of transmittance and absorbance as a function of direction and polarization angle of incident light. It is shown how a previously deduced empirical expression, commonly used in infrared spectroscopy, is a good approximation to the full theory under a wide range of conditions. The new theory shows that principal polarized absorbances correspond to the eigenvalues of an absorbance ellipsoid. An expression is derived for the unpolarized absorbance as a function of the angles describing incident light direction, A unpol(ϕ, ψ), and the principal polarized absorbances, A a, A b, A c in an anisotropic crystal
\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[\mathit{A}\_{unpol}({\phi},{\psi})\ =\ {\frac{1}{2}}[\mathit{A}\_{a}(cos^{2}{\phi}cos^{2}{\psi}\ +\ sin^{2}{\psi})\ +\ \mathit{A}\_{b}(cos^{2}{\phi}sin^{2}{\psi}\ +\ cos^{2}{\psi})\ +\ \mathit{A}\_{c}sin^{2}{\phi}].\] \end{document}
Integration of this expression over all incident angles leads to a simple relationship between total measured unpolarized absorbance and the three principal polarized absorbances. Using this theory, a procedure is proposed for estimating both total ( A a + A b + A c) and principal absorbances from spectroscopic measurements of absorbance using unpolarized light on a set of randomly oriented crystals.
TL;DR: In this article, the transformation of schwertmannite to goethite and/or hematite in high pH solutions was studied between 60 and 240 °C using synchrotron-based, in-situ energy-dispersive X-ray diffraction (EDXRD).
Abstract: The transformation of schwertmannite to goethite and/or hematite in high pH solutions was studied between 60 and 240 °C using synchrotron-based, in-situ energy-dispersive X-ray diffraction (EDXRD). Powder diffraction and electron microscopy indicate that the crystallization of hematite and goethite occurred via intermediate ferrihydrite. At temperatures ≤80 °C goethite was the only crystallization product, while at temperatures >80 °C goethite and hematite crystallized almost simultaneously. At temperatures ≥150 °C a secondary crystallization stage was observed in which goethite transformed to hematite. The activation energies of nucleation for goethite and hematite are 27 ± 1 and 25 ± 1 kJ/mol, respectively, while the activation energies of crystallization are 33 ± 1 and 28 ± 1 kJ/mol. Most of the sulfate was released from the schwertmannite during the early stages of crystallization with <5% of the sulfate remaining associated with the solid phase after crystallization was complete. Sulfate from the initial schwertmannite retarded the dissolution of ferrihydrite, which inhibited the nucleation and the early stages of goethite formation, but did not significantly affect the later stages of goethite crystallization. At high temperatures the presence of sulfate favored the crystallization of hematite over goethite. The activation energy of crystallization for the secondary transformation of goethite to hematite is 103 ± 3 kJ/mol.
TL;DR: In this paper, X-ray absorption spectroscopy (XAS) performed in situ at high temperature has been used to probe the local environment of low-Z elements (Al, Si, and Ca).
Abstract: Structural data on silicate, aluminate, and aluminosilicate melts are difficult to measure and understand at high temperature. X-ray absorption spectroscopy (XAS) performed in situ at high temperature has been used to probe the local environment of low-Z elements (Al, Si, and Ca). For fully tetrahedral network glasses, CaAl2Si2O8 (anorthite) and CaAl2O4, the modifications in the Al K-edge spectra with increasing temperature can be attributed to a structural rearrangement of the network or to an increase of fivefold-coordinated Al. For the Ca3Al2O6 composition, where Al is localized in a depolymerized tetrahedral site associated with non-bridging O atoms, XAS spectra at the Al K-edge are barely affected by temperature. Depending on the composition, Ca K-edge spectra investigated in these experiments allow us to follow changes in the distortion of the Ca sites in melts at high temperature. The structural modifications at both short and intermediate range upon melting are well shown by these XAS measurements.
TL;DR: In this article, the same authors compared natural and synthetic samples from mine waste deposits in Rio Tinto (Huelva, Spain), Ely Mine (Vermont), and a mineral collecting locality near Sharbot Lake (Ontario), as well as natural and synthesized samples documented in the literature.
Abstract: Members of the natrojarosite-hydronium jarosite [(Na,H 3 O)Fe 3 (SO 4 ) 2 (OH) 6 ] and jarosite-natrojarosite [(K,Na)Fe 3 (SO 4 ) 2 (OH) 6 ] solid-solution series were synthesized and investigated by Rietveld analysis of X-ray powder diffraction data. The synthesized samples have full Fe occupancy, where in many previous studies there were significant vacancies in the B site. Well-defined trends can be seen in the unit-cell parameters across the solid-solution series in the synthetic samples. The majority of the samples in this study were directly synthesized under hydrothermal conditions at 140 °C. End-member natrojarosite was synthesized using a two-step method, where the initial sample was heated in a 1.0 m H 2 SO 4 −0.5Na 2 SO 4 solution at 200 °C for 3 days, yielding a sample with 100% Na occupancy. Many of the samples were initially zoned and required grinding and re-heating in the reactant solution for homogenization. Substitution of H 3 O and K into natrojarosite changes unit-cell parameters in a linear fashion. The unit-cell parameters presented here are significantly different than the majority of previous studies on synthetic samples, as samples in the current study have full Fe occupancy and the Na-K jarosite series has no H 3 O substitution in the A site. Substitution in the A site mainly affects unit-cell parameter c with little change in a . As Na occupancy increases there is a decrease in A-O2 and A-O3 distances and a consequent increase in Fe-O2 and Fe-O3 distance leading to an overall decrease in unit-cell parameter c in both the Na-H 3 O and Na-K jarosite series. The synthetic samples are compared to natural samples from mine waste deposits in Rio Tinto (Huelva, Spain), Ely Mine (Vermont), and a mineral collecting locality near Sharbot Lake (Ontario), as well as natural and synthetic samples documented in the literature. Based on unit-cell parameters many of the natural samples appear to have full Fe occupancy and correlate well with the synthetic samples from this study. The infrared spectra of the samples were analyzed, and there is a gradual change in the spectral features across the solid-solution series between end-members. The results from this study will aid in the interpretation of the possible chemical compositions of natural jarosite group minerals in mine waste and on Mars.
TL;DR: In this article, different techniques have been used to characterize the physical and chemical structure of the red coral calcitic skeleton, showing that each layer is made of fibers, organized or not in fan-shaped structures; and submicrometer (apparently mono-) crystalline units.
Abstract: Different techniques have been used to characterize the physical and chemical structure of the red coral calcitic skeleton. A section normal to the axis of the skeleton shows a medullar zone surrounded by a circular domain composed of concentric rings. Growth rings are revealed by the cyclic variation of organic matter (OM) and Mg/Ca ratio. These growth rings are annual; thus, both OM and Mg/Ca ratio can be used to date red coral colonies. Growth rings display wavelets. The internal structure of each wavelet results from the stacking of layers with tortuous interfaces. Tortuosity is due to the presence of microprotuberances. Interfaces between layers may display sharp discontinuities indicative of interruption of the mineralizing process. SEM and TEM studies show that each layer is made of (1) fibers, organized or not in fan-shaped structures; and (2) submicrometer (apparently mono-) crystalline units. Fibers are superstructures made of submicrometer units possibly assembled by an oriented aggregation mechanism. HRTEM studies show that in spite of displaying single-crystal scattering behavior, the submicrometer crystalline units are made of 2–5 nm nanograins again possibly aggregated by a mechanism of oriented attachment. Thus, submicrometer crystalline units and polycrystalline fibers can be both defined as mesocrystals. The red coral skeleton is a hierarchically organized organic-inorganic composite that exhibits porosity and structural and compositional order on length scales from the nanoscale to the macroscale.
TL;DR: In this article, the authors measured water concentration profiles around spherulites in obsidian by synchrotron Fourier transform infrared spectroscopy and determined the kinetics of spherule growth in obsidians from Krafla volcano, Iceland.
Abstract: We determined the kinetics of spherulite growth in obsidians from Krafla volcano, Iceland. We measured water concentration profiles around spherulites in obsidian by synchrotron Fourier transform infrared spectroscopy. The distribution of OH? groups surrounding spherulites decreases exponentially away from the spherulite-glass border, reflecting expulsion of water during crystallization of an anhydrous paragenesis (plagioclase + SiO2 + clinopyroxene + magnetite). This pattern is controlled by a balance between the growth rate of the spherulites and the diffusivity of hydrous solute in the rhyolitic melt. We modeled advective and diffusive transport of the water away from the growing spherulites by numerically solving the diffusion equation with a moving boundary. Numerical models fit the natural data best when a small amount of post-growth diffusion is incorporated in the model. Comparisons between models and data constrain the average spherulite growth rates for different temperatures and highlight size-dependent growth among a small population of spherulites.
TL;DR: In this article, high-pressure X-ray diffraction was carried out on a single crystal of gypsum compressed in a diamond anvil cell, and axial compressibility values, fitting data collected up to 3.94 GPa, were β0aEoS = 6.1(1) and β0cEoS= 5.6(1)-10−3 GPa−1.
Abstract: High-pressure X-ray diffraction was carried out on a single crystal of gypsum compressed in a diamond anvil cell. The sample maintained its crystal structure up to 4.0 ± 0.1 GPa. The fit of pressure dependence of the unit-cell volume to the third-order Birch-Murnaghan equation yielded K T0 = 44(3) GPa and (∂ K T/∂ P ) = 3.3(3), where K T0 and (∂ K T/∂ P ) are the isothermal bulk modulus and its pressure derivative in ambient conditions. The axial compressibility values, fitting data collected up to 3.94 GPa, were β0aEoS = 6.1(1) and β0cEoS = 5.6(1) 10−3 GPa−1. The value of β0b EoS was 6.2(8) 10−3 GPa−1 fitting the data collected up to 2 GPa, due to non-linearity above this pressure; axial compressibility of gypsum is almost isotropic (β0a: β0b: β0c = 1:1:0.9). This behavior is partly unexpected for a layered mineral based on alternate layers of Ca- and S-polyhedral chains separated by interlayers occupied by water molecules. Above 4.0 GPa the compression curve of gypsum shows a discontinuity with a 2.5% contraction in volume. Structural refinements indicate that SO4 volume and average S-O bond distances remain almost unchanged from room pressure to 3.9 GPa [range 1.637(4)–1.66(9) A3; 1.4733–1.48 A]. The SO4 tetrahedron undergoes distortion: the smaller distance decreases from 1.4731(9) to 1.45(2) A and the larger increases from 1.4735(9) to 1.51(2) A. In contrast, the calcium polyhedra show expected high-pressure behavior, becoming more regular and decreasing in volume from 25.84(8) A3 at ambient P to 24.7(1) A3 at 3.9 GPa. The largest variations were observed in the interlayer region where the water molecules are located. Along the b axis, the two structural layers have very different compressibilities: the polyhedral layer is almost incompressible in the pressure range studied, whereas water layer compressibility is 9.7(3) 10−3 GPa−1, about twice that of the other two lattice parameters. At ambient conditions, water molecules form weak hydrogen bonds with the O atoms of Ca and S polyhedra. With increasing pressure, the weakest hydrogen bond becomes the strongest one: from 0.001 to 4 GPa, the distance changes from 2.806(1) to 2.73(2) A for OW-H1···O2, and from 2.883(2) to 2.69(3) A for OW-H2···O2. Structure refinements show that water remains in the structure when P increases. The observed distortion of sulfate tetrahedra explains the splitting of the ν1 sulfate stretching mode, and the various measured compressibilities of the two hydrogen bonds and the coalescence of the Raman stretching mode observed at pressures over 5 GPa.
TL;DR: In this article, three post-spinel structures of CaMn2O4, CaFe2O 4 and CaTi 2O4 have been proposed as post-spinel phases.
Abstract: Three structures of CaMn2O4, CaFe2O4, and CaTi2O4 have been proposed as post-spinel phases. Because these structures are very similar, several ambiguities and inconsistencies appear in high-pressure studies, leading to many problems that are yet to be solved. Systematic powder diffraction studies related to these three phases were conducted under high pressure using synchrotron radiation. All three samples have further high-pressure polymorphs. CaMn2O4 transforms to the CaTi2O4-type structure at about 30 GPa. The MnO6 octahedron in the lower-pressure structure is distorted by the Jahn-Teller effect. A new phase was observed at pressures above 50 GPa during compression of CaFe2O4. Rietveld profile fitting analysis of diffraction data at 63.3 GPa demonstrated that the high-pressure structure, with space group Pnam , is produced via a martensitic transformation by displacing atoms in every third layer perpendicular to the c axis. CaTi2O4 also has a new high-pressure polymorph above 39 GPa with space group Bbmm . The most probable post-spinel candidate in the mantle is the CaTi2O4-type structure. The CaMn2O4-type structure is only formed at high pressure from spinel phases with atoms susceptible to Jahn-Teller distortion.
TL;DR: In this article, the authors synthesize hematite spherules whose mineralogic, chemical, and crystallographic properties are strikingly similar to those for the hematites in lag deposits on the surface of Mars.
Abstract: We synthesized hematite spherules whose mineralogic, chemical, and crystallographic properties are strikingly similar to those for the hematite-rich spherules in lag deposits on the surface and embedded in outcrops at Meridiani Planum, Mars. The spherules were synthesized in the laboratory along with hydronium jarosite and minor hydronium alunite from Fe-Al-Mg-S-Cl acid-sulfate solutions under hydrothermal conditions. The reaction sequence was (1) precipitation of hydronium jarosite; (2) jarosite dissolution and precipitation of hematite spherules; and (3) precipitation of hydronium alunite upon depletion of hydronium jarosite. The spherules exhibit a radial growth texture with the crystallographic c axis aligned along the radial direction, so that thermal emission spectra have no hematite emissivity minimum at ~390 cm−1. Our experiments show that hydrothermal, acid-sulfate solutions are a pathway for formation of jarosite and the hematite spherules at Meridiani Planum, Mars.
TL;DR: The atomic arrangements of two natural samples of whitlockite and two synthetic samples of merrillite have been determined in space group R3c by X-ray diffraction methods; the high quality structure refinements yielded R < 0.019 as mentioned in this paper.
Abstract: The atomic arrangements of two natural samples of whitlockite, a synthetic whitlockite specimen, a synthetic whitlockite specimen heated at 500 °C, and a synthetic merrillite specimen (formed through dehydrogenation of synthetic whitlockite by heating at 1050 °C for 24 h) have been determined in space group R3c by X-ray diffraction methods; the high-quality structure refinements yielded R < 0.019. Whitlockite, ideally Ca 18 Mg 2 (PO 4 ) 12 [PO 3 (OH)] 2 and merrillite, ideally Ca 18 Na 2 Mg 2 (PO 4 ) 14 , are similar phases that differ by the lack of hydrogen and the concomitant addition of charge-balancing sodium (or calcium) in merrillite. The atomic arrangements of whitlockite and merrillite contain a structural unit consisting of a [(Mg,Fe)(PO 4 ) 6 ] 16 2 complex anion that forms a "bracelet-and-pinwheel" arrangement. The central octahedral cation and the six coordinating phosphate tetrahedra form a pinwheel, and in whitlockite and merrillite the pinwheels are not polymerized; the structural units are linked by interstitial complexes. In unsubstituted merrillite (assuming no Na or REE substituents for Ca), the interstitial complex has a formula of [Ca 19 (PO 4 ) 2 ] 32+ , and in whitlockite, the terrestrial phase in which hydrogen is accommodated, the interstitial unit has the formula [Ca 18 (PO 3 [OH]) 2 ] 32+ , yielding the charge-balancing relationship [H (whit) ↔ Ca 0.5(merr) ] 2 . Whitlockite and merrillite are perhaps the only phases that form a solid solution with terrestrial and extra-terrestrial end-members that differ by structural adjustments that result from the accommodation of hydrogen in the terrestrial phase. The results of the study also suggest that in terrestrial samples of whitlockite, a merrillite component of the solid solution is common, but that extraterrestrial samples of merrillite are devoid of any whitlockite component.
TL;DR: In this paper, the Mie-Gruneisen formalism is used to fit a Birch-Murnaghan equation of state to high-temperature (T), high-pressure (P) X-ray diffraction unit-cell volume (V) measurements on synthetic goethite (alpha-FeOOH) to combined conditions of T = 23-250o C and P = 0-29.4 GPa.
Abstract: The Mie-Gruneisen formalism is used to fit a Birch-Murnaghan equation of state to high-temperature (T), high-pressure (P) X-ray diffraction unit-cell volume (V) measurements on synthetic goethite (alpha-FeOOH) to combined conditions of T = 23-250o C and P = 0-29.4 GPa. We find the zero-pressure thermal expansion coefficient of goethite to be alpha0 = 2.3 (+-0.6) x 10-5 K-1 over this temperature range. Our data yield zero-pressure compressional parameters: V0 = 138.75 (+- 0.02) Angstrom3, bulk modulus K0 = 140.3 (+- 3.7) GPa, pressure derivative K0' = 4.6 (+- 0.4), Gruneisen parameter gamma0 = 0.91 (+- 0.07), and Debye temperature Theta0 = 740 (+- 5) K. We identify decomposition conditions for 2alpha-FeOOH --> alpha-Fe2O3 + H2O at 1 - 8 GPa and 100-400oC, and the polymorphic transition from alpha-FeOOH (Pbnm) to epsilon-FeOOH (P21mn). The non-quenchable, high-pressure epsilon-FeOOH phase P-V data are fitted to a second-order (Birch) equation of state yielding, K0 = 158 (+- 5) GPa and V0 = 66.3 (+- 0.5) Angstrom3.
TL;DR: In this paper, the Fe3+reducing bacteria Geobacter sulfurreducens and Shewanella oneidensis were used to synthesize spinel ferrite nanoparticles of the general chemical formula M x Fe3-x O4, where M is either Co, Ni, Mn, Zn or a combination of Mn and Zn.
Abstract: Ferrimagnetic nanoparticles have many uses in industry including in magnetic recording media and transformers, however these particles are often expensive to synthesize. In this study, the Fe3+-reducing bacteria Geobacter sulfurreducens and Shewanella oneidensis were used to synthesize spinel ferrite nanoparticles of the general chemical formula M x Fe3– x O4, where M is either Co, Ni, Mn, Zn, or a combination of Mn and Zn. This was done at ambient temperatures through the dissimilatory reduction of Fe3+-oxyhydroxides containing the appropriate substitutional cations. A combination of L -edge and K -edge X-ray absorption spectroscopy (XAS) and L -edge X-ray magnetic circular dichroism (XMCD) was used to determine the site occupancies, valence, and local structure of the Fe and substitutional cations within the spinels. The Ni and Co ferrites produced using each bacterium were very similar and therefore this study concludes that, despite the difference in reduction mechanism of the bacteria used, the end-product is remarkably unaltered. Nickel ferrites contained only Ni2+, with at least 80% in Oh coordination. Cobalt ferrites contained only Co2+ but with a significant proportion (up to 45%) in Td coordination, showing a slight preference for Td sites. The Mn-ferrites contained Mn2+ only on the Oh sites but a mixture of Mn2+ and Mn3+ on Td sites when the amount of Mn exceeded 3% (compared to the amount of Fe) or some Zn was also present. This study successfully produced a range of nanoparticulate ferrites that could be produced industrially using relatively environmentally benign methodologies.
TL;DR: A suite of pure magnesian wadsleyite (β-Mg 2 SiO 4 ) samples containing 0.005, 0.38, 1.18 and 1.66 wt% H 2 O was studied by single-crystal X-ray diffraction to determine the effects of hydration on cation ordering and crystal symmetry as discussed by the authors.
Abstract: A suite of pure magnesian wadsleyite (β-Mg 2 SiO 4 ) samples containing 0.005, 0.38, 1.18, and 1.66 wt% H 2 O was studied by single-crystal X-ray diffraction to determine the effects of hydration on cation ordering and crystal symmetry. Separate compressibility experiments were carried out to 9.6 GPa to determine the effects of hydration on isothermal equations of state. Crystal-structure refinements at ambient conditions show cation vacancies order onto the M3 site. The most hydrous sample (1.6 wt% H 2 O) displayed monoclinic symmetry with β = 90.090(7)°, whereas the samples with lower content were statistically orthorhombic. The density of wadsleyite decreases with increasing water content at STP according to the empirical relation, ρ = 3.470(2) – 0.046(2) C H 2 O g/cm 3 (with C H 2 O in wt% H 2 O). Bulk moduli and pressure derivatives of wadsleyite are K T0 = 173(5) GPa, K 0 ′ = 4.1(15) for 0.005 wt% H 2 O; K T0 = 161(4) GPa, K 0 ′ = 5.4(11) for 0.38 wt% H 2 O; K T0 = 158(4) GPa, K 0 ′ = 4.2(9) for 1.18 wt% H 2 O; and K T0 = 154(4) GPa, K 0 ′ = 4.9(11) for 1.66 wt% H 2 O. Variation of the bulk modulus of wadsleyite with water content is non-linear, which may be attributable to softening of the structure by ordering of vacancies onto two non-equivalent M3 sites (M3a and M3b) and an accompanying dilution of orthorhombic symmetry.