Showing papers in "American Mineralogist in 2001"
TL;DR: In this paper, the authors measured high-resolution Fe K-edge XANES spectra of a series of crystalline Fe 2+ - and Fe 3+ -bearing model compounds in an effort to correlate characteristics of the pre-edge feature with oxidation state and local coordination environment of Fe atoms.
Abstract: High-resolution Fe K-edge XANES spectra of a series of crystalline Fe 2+ - and Fe 3+ -bearing model compounds were measured in an effort to correlate characteristics of the pre-edge feature with oxidation state and local coordination environment of Fe atoms. The model compounds comprise 30 natural minerals and synthetic compounds, with Fe coordination environments ranging from 4 to 12 O atoms for Fe 2+ , including 5-coordinated trigonal bipyramidal Fe 2+ , and from 4 to 6 O atoms for Fe 3+ . Most pre-edge spectra show two components (due to crystal-field splitting) that are located just above the Fermi level. The most useful characteristics of the Fe-K pre-edge for determining Fe oxidation state and coordination number are the position of its centroid and its integrated intensity. The separation between the average pre-edge centroid positions for Fe 2+ and Fe 3+ is 1.4 ± 0.1 eV. Thus, the position of the pre-edge feature can be used as a measure of the average Fe-redox state, with the average pre-edge position for mixed Fe 2+ -Fe 3+ compounds occurring between positions for Fe 2+ and Fe 3+ . The lowest pre-edge normalized heights and integrated intensities are observed for the most centrosymmetric sites of Fe, in agreement with previous studies (see Waychunas et al. 1983). Examination of the pre-edge features of mechanical mixtures of phases containing different proportions of Fe 2+ and Fe 3+ suggests that the pre-edge position and intensity for these mixtures can vary quite non-linearly with the average redox state of Fe. However, distinctly different trends of pre-edge position vs. pre-edge intensity can be observed, depending on the coordination environment of Fe 2+ and Fe 3+ , with an accuracy in redox determination of ±10 mol% provided that the site geometry for each redox state is known. These methods have been used to estimate the Fe 3+ /Fe 2+ ratio in 12 minerals (magnetite, vesuvianite, franklinite, rhodonite, etc.) containing variable/unknown amounts of Fe 2+ /Fe 3+ .
982 citations
TL;DR: In this article, the authors explore the implications of recent mineral physics measurements of diffusion coefficients and melting temperatures of lower mantle materials on the rheological and geothermal structure of Earth's lower mantle.
Abstract: We explore the implications of recent mineral physics measurements of diffusion coefficients and melting temperatures of lower mantle materials on the rheological and geothermal structure of Earth’s lower mantle. We show that MgSiO3 perovskite is significantly stronger than MgO periclase and therefore the rheology of the lower mantle depends strongly on the geometry of a weaker phase, periclase. We calculate viscosities of the lower mantle for two cases: (1) where periclase occurs as isolated grains and (2) where periclase occurs as continuous films, using mineral physics data and models of two-phase rheology. We find that the effective viscosity for the former is about ~10–1000 times higher than the latter. We therefore suggest that the rheology of the lower mantle is structure- and hence strain-dependent, leading to weakening at large strains due to the formation of continuous films of periclase. Overall depth variation of viscosity depends not only on the pressure dependence of creep but also on the geothermal gradient. Both MgSiO3 perovskite and periclase have relatively small activation energies ( E * = g R T m with g = 10–14, where R is the gas constant and T m is melting temperature), and therefore the depth variation of viscosity is rather small, even for a nearly adiabatic temperature gradient. However, the geothermal gradients consistent with the geodynamical inference of nearly depth-independent viscosity are sensitive to the pressure dependence of viscosity which is only poorly understood. A superadiabatic gradient of up to ~0.6 K/km is also consistent with mineral physics and geodynamical observations.
293 citations
TL;DR: In this paper, the authors used secondary ion mass spectrometry (SIMS) analyses to investigate the incorporation of Dy with no P added to the flux, the P to LREE excess, and the HREE to P excess in the doped zircon, and showed that Dy and P contributed to charge balance required by the excess Dy.
Abstract: Zircon crystals were grown from a Li-Mo flux (7.5 mol% Li2MoO4; 86.5 mol% MoO3) to which equal molar proportions ZrO2 and Li2SiO3 were added (3 mol% each). The crystals were initially grown undoped, but later Dy was added to the flux without any other charge-compensating species. With Dy and P added, in equal molar proportions, the zircon crystals incorporated 1.37 mol% (6.99 wt%) Dy and 1.36 mol% (1.33 wt%) P, a factor of 5.3 increase in Dy over the crystals grown without P intentionally added to the flux. The other P+REE-doped zircon crystals displayed an approximately 1000-fold increase in REE and P from La through Lu, a result of decreasing ionic radii. The incorporation of P5+ allowed, in a general sense, the xenotime-type coupled substitution of (REE3+, Y3+) + P5+ = Zr4+ + Si4+. From La to Nd, however, P exceeds REE, from Sm to Gd, the REE are generally equal to P, and from Tb to Lu the REE exceed P. The Y- and P-doped zircon behaved more like middle-REE (MREE)-doped zircon than heavy-REE (HREE)-doped zircon crystals in their ability to incorporate Y (and P) and to maintain charge neutrality. To investigate the incorporation of Dy with no P added to the flux, the P to LREE excess, and the HREE to P excess in the doped zircon, secondary ion mass spectrometry (SIMS) analyses were completed on selected crystals. In the Dy-doped crystals, the SIMS analyses revealed minor amounts of P, Li, and Mo in the crystals. These elements contributed to charge balance required by the excess Dy. In REE- and P-doped zircon, the SIMS analyses detected Li and Mo, and the Li and Mo may also provide charge balance for excess REE in the HREE+P-doped crystals.
230 citations
TL;DR: In this paper, the theoretical infrared spectrum of kaolinite [Al2Si2O5(OH)4, triclinic] was computed using ab initio quantum mechanical calculations.
Abstract: The theoretical infrared spectrum of kaolinite [Al2Si2O5(OH)4, triclinic] was computed using ab initio quantum mechanical calculations. Calculations were performed using the Density Functional Theory and the generalized gradient approximation. The low-frequency dielectric tensor of kaolinite was determined as a function of the light frequency using linear response theory. The IR spectrum was then calculated using a model that takes into account the shape and size of kaolinite particles. A remarkable agreement was obtained between theory and experiment, especially on the position of the stretching bands of OH groups. This agreement provides a firm basis for the interpretation of the IR spectrum of kaolinite in terms of structural parameters.
212 citations
TL;DR: In this article, the GASP geobarometer has been recalibrated using four recent garnet activity models, four analogous garnet-biotite temperature models, and two recent plagioclase activity models.
Abstract: The garnet-Al silicate-plagioclase (GASP) geobarometer has been recalibrated using four recent garnet activity models, four analogous garnet-biotite temperature models, and two recent plagioclase activity models. A typical sillimanite-bearing sample that formed at about 5.25 kbar, 575 °C shows a possible P range of ~0.7 kbar due to T error, ~1 kbar due to range of garnet activity model, ~0.9 kbar due to range of plagioclase activity model, and ~5.4 kbar due to range of experimental end-member reversals extended by one sigma.
Calibrations were further constrained with the kyanite-sillimanite (K-S) phase boundary such that the best fit of 76 pelitic schist samples from 11 localities provides an individual end-member calibration for each of the eight possible combinations of garnet and plagioclase activity models with the appropriate geothermometer. Samples with low grossular or anorthite component were rejected. The end-member calibrations are constrained to pass through the the best-determined portion of the GASP experimental reversals at 1230 °C, 26.6 kbar. These individual end-member calibrations provide self-consistent models that tend to compensate for error in the garnet and plagioclase activity expressions. The models were also tested on a set of 59 samples from the Alps.
The recommended calibration is the average garnet activity model and average garnet-biotite T model of Holdaway (2000), the Fuhrman and Lindsley (1988) plagioclase activity model, and H Grs = −6628521, S Grs = 258.76 to combine with the remaining phases in the Berman database to produce the optimum end-member GASP curve. These thermodynamic data are for the GASP geobarometer only. Error is about ±0.8 kbar absolute and about ±0.6 kbar relative. Geological error is the largest component of error in many of these samples. Care should be taken to be sure that analyzed plagioclase and biotite are near analyzed garnet, that the peak- T portions of garnet and plagioclase are selected, that the peak- T Al silicate is determined, and that the T calculated is the most accurate possible. These calibrations represent an improvement over previous published GASP calibrations. These eight models are available for distribution as three programs ( T , P , P - T intersection) for the DOS-based personal computer.
201 citations
TL;DR: X-ray absorption data were collected for a series of varnish and dendrite Mn oxide coatings on rock substrates containing a wide variety of mineralogies exposed to a variety of environments as discussed by the authors.
Abstract: X-ray absorption data were collected for a series of varnish and dendrite Mn oxide coatings on rock substrates containing a wide variety of mineralogies exposed to a variety of environments. Near-edge spectra of the coatings indicate that the Mn-oxide phases present have Mn valences between 3+ and 4+, with average Mn valences for the varnishes closer to 4+ than those for the dendrites. Mn EXAFS data and analyses indicate that Mn-oxide structure types for the varnishes range, perhaps continuously, from large tunnel phases, similar to todorokite and romanechite, to layer phases, i.e., birnessite-family. Similar results were found for the dendrite samples, except that the variety of Mn-oxide phases is somewhat larger than those found for the varnishes. No correlations were found between Mn-oxide structure-type within these coatings and the corresponding substrate petrology.
195 citations
Journal Article•
TL;DR: Hill and Forti illustrate speleothems with some spectacular color photographs as discussed by the authors, showing how stalactites, stalagmites, flowstone, and a host of more intricate shapes formed from flowing, dripping, and seeping water and thus take on shapes dictated by the idiosyncrasies of the moving water.
Abstract: Think about all of those spectacular displays of mineral crystals that grace the world’s mineral museums. A great many of the large, faceted crystals grew in some sort of cavity over long periods of time. Such mineral crystals are discovered in vugs, pockets and cavities exposed in mines and quarries. The conditions under which they were found are very different from the conditions under which they grew. The fluids from which they grew are long gone, a challenge for mineralogists to deduce from the surviving clues of trace elements, isotopes, and fluid inclusions. Now, consider a cave. It also is a cavity, maintaining conditions of constant temperature, water vapor pressure, and carbon dioxide pressure for long periods of time. This is also an environment where minerals can grow. But in the case of many caves, human observers can watch the growth process in progress.
Some cave minerals appear as large faceted crystals. Some are nondescript crusts of little beauty but fascinating mineralogy. Most have formed from flowing, dripping, and seeping water and thus take on shapes dictated by the idiosyncrasies of the moving water. Such mineral deposits are known as speleothems: stalactites, stalagmites, flowstone, and a host of more intricate shapes. Hill and Forti illustrate speleothems with some spectacular color photographs. The authors have solicited contributions from cave photographers all over the world and without exception the …
190 citations
TL;DR: In this paper, focused ion beam (FIB) lift-out technique was used to extract electron transparent specimens with nanometer scale precision for transmission electron microscope (TEM) analysis.
Abstract: Argon ion milling is the conventional means by which mineral sections are thinned to electron transparency for transmission electron microscope (TEM) analysis, but this technique exhibits significant shortcomings. In particular, selective thinning and imaging of submicrometer inclusions during sample milling are highly problematic. We have achieved successful results using the focused ion beam (FIB) lift-out technique, which utilizes a 30 kV Ga + ion beam to extract electron transparent specimens with nanometer scale precision. Using this procedure, we have prepared a number of Earth materials representing a range of structures and compositions for TEM analysis. We believe that FIB milling will create major new opportunities in the field of Earth and planetary materials microanalysis, particularly with respect to ultraprecious mineral and rock samples.
189 citations
TL;DR: In this paper, the structural and compositional properties of pyroxenes were characterized by using Raman spectral features and peak positions of the pyroxene in the (Mg, Fe, Ca) quadrilateral with an accuracy of ± 0.1.
Abstract: This study reports the use of Raman spectral features to characterize the structural and compositional characteristics of different types of pyroxene from rocks as might be carried out using a portable field spectrometer or by planetary on-surface exploration. Samples studied include lunar rocks, martian meteorites, and terrestrial rocks. The major structural types of quadrilateral pyroxene can be identified using their Raman spectral pattern and peak positions. Values of Mg/(Mg + Fe + Ca) of pyroxene in the (Mg, Fe, Ca) quadrilateral can be determined within an accuracy of ± 0.1. The precision for Ca/(Mg + Fe + Ca) values derived from Raman data is about the same, except that corrections must be made for very low-Ca and very high-Ca samples. Pyroxenes from basalts can be distinguished from those in plutonic equivalents from the distribution of their Mg ′ [Mg/(Mg + Fe)] and Wo values, and this can be readily done using point-counting Raman measurements on unprepared rock samples. The correlation of Raman peak positions and spectral pattern provides criteria to distinguish pyroxenes with high proportions of non-quadrilateral components from (Mg, Fe, Ca) quadrilateral pyroxenes.
170 citations
TL;DR: In this paper, the dissolution behavior of two smectite minerals, hectorite (trioctahedral) and nontronite (dioctalite), was observed in situ, in acid solutions, using atomic force microscopy.
Abstract: The dissolution behavior of two smectite minerals, hectorite (trioctahedral) and nontronite (dioctahedral), was observed in situ, in acid solutions, using atomic force microscopy. As expected, the crystallites dissolved inward from the edges, and the basal surfaces appeared to be unreactive during the timescale of the experiments. The hectorite (010) faces appeared to dissolve about 6 × more slowly than the lath ends, usually broken edges. The edges visibly dissolved on all sides, and appeared to roughen somewhat. On the other hand, the (010), (110), and (11 ‐ 0) faces on nontronite crystals were exceptionally stable, so that any dissolution fronts originating at broken edges or defects would quickly become pinned along these faces, after which no more dissolution was observable. These observations can be explained by using periodic bond chain theory to predict the topology of the surface functional groups on the edge faces of these minerals. If a certain amount of predicted surface relaxation is allowed on the (110) and (11 ‐ 0) faces of nontronite, an important difference between the exceptionally stable faces and the others becomes apparent. That is, the oxygen sites connecting the octahedral and tetrahedral sheets are all fully bonded on the nontronite (010), (110), and (11 ‐ 0) edge faces, whereas all hectorite edge faces and nontronite broken edges would have coordinatively unsaturated connecting O atoms. This explanation for the differential reactivity of these crystal faces implies that the rate limiting step of the dissolution process is the breaking of bonds to connecting O atoms.
164 citations
TL;DR: In this paper, the authors show experimental evidence that the zoned Mg-Fe-Ca carbonates, magnetite, and Fe sulfides in Martian meteorite ALH84001 may have formed by simple, inorganic processes.
Abstract: We show experimental evidence that the zoned Mg-Fe-Ca carbonates, magnetite, and Fe sulfides in Martian meteorite ALH84001 may have formed by simple, inorganic processes. Chemically zoned carbonate globules and Fe sulfides were rapidly precipitated under low-temperature (150 °C), hydrothermal, and non-equilibrium conditions from multiple fluxes of Ca-Mg-Fe-CO2-S-H2O solutions that have different compositions. Chemically pure, single-domain, defect-free magnetite crystals were formed by subsequent decomposition of previously precipitated Fe-rich carbonates by brief heating to 470 °C. The sequence of hydrothermal precipitation of carbonates from flowing CO2-rich waters followed by a transient thermal event provides an inorganic explanation for the formation of the carbonate globules, magnetite, and Fe sulfides in ALH84001. In separate experiments, kinetically controlled 13C enrichment was observed in synthetic carbonates that is similar in magnitude to the 13C enrichment in ALH84001 carbonates.
TL;DR: In this article, phase transitions in calcite, a naturally occurring crystalline form of CaCO3, have been investigated by three different experimental techniques: (1) in-situ X-ray diffraction (XRD) using synchrotron radiation to 6 GPa and 1750 °C in a cubic anvil press; (2) Raman scattering to 10 GPa at room temperature using a diamond-anvil cell; and (3) postcompression XRD on samples retrieved after heat treatment at temperatures to 2000 °C and pressures to 9 GPa in
Abstract: Phase transitions in calcite, a naturally occurring crystalline form of CaCO3, have been investigated by three different experimental techniques: (1) in-situ X-ray diffraction (XRD) using synchrotron radiation to 6 GPa and 1750 °C in a cubic anvil press; (2) Raman scattering to 10 GPa at room temperature using a diamond-anvil cell; and (3) post-compression XRD on samples retrieved after heat treatment at temperatures to 2000 °C and pressures to 9 GPa in an octahedral anvil press. At room temperature, calcite I transformed into calcite II at 1.7 GPa and then to calcite III at ~2 GPa. Calcite III persisted to at least 10 GPa. Elevation of temperature at 3, 4, and 6 GPa caused a sequence of transitions: calcite III → aragonite → disordered calcite → liquid, and aragonite was retained upon rapid cooling of the liquid. The melting curve of disordered calcite increased with pressure following a relation: T m (°C) = 1338 + 82 P − 2.9 P 2 where P is in units of GPa.
TL;DR: In this paper, a sulfuric acid spring water with a pH of 3.37 to 2.89 was investigated to verify the formation processes of iron minerals and the effects of bacteria on their formation.
Abstract: Mineralogy and geochemistry of a sulfuric acid spring water with a pH of 3.37 to 2.89 were investigated to verify the formation processes of iron minerals and the effects of bacteria on their formation. To estimate the solubility of schwertmannite, experimental dissolution in 10.0 mM H2SO4 was conducted and this solubility data was used for geochemical modeling. Experimental incubation of the spring water containing bacteria was also performed and compared with a simulated abiotic system to evaluate the role of bacteria in the mineral formation. The spring water seeps through cracks of hydrothermally altered andesitic rocks containing pyrite, and precipitates schwertmannite and jarosite. Schwertmannite appears as a film-like thin layer floating on the water surface and composed of aggregates of spherical particles with diameters of 1 to 5 μ m. Jarosite is produced as a precipitate on submerged rock surfaces. The precipitate contains well crystallized jarosite spheres 5 to 10 μ m in diameter. Some ellipsoidal to rod shaped bacteria covered or decorated by poorly ordered iron minerals are also present in close association with the schwertmannite spheres. Results of the experimental incubation demonstrate that the oxidation rates of Fe2+ are 5.3 × 103 to 7.2 × 103 times greater than those of the simulated abiotic system, suggesting that the formation of the iron minerals is promoted by bacterial oxidation of Fe2+. The dissolution experiment indicates that the solubility product of the schwertmannite having an average chemical composition of Fe8O8(OH)5.9(SO4)1.05 is approximately log K s = 7.06 ± 0.09. Using this data, geochemical modeling reveals that the spring water is supersaturated with respect to schwertmannite and also goethite and jarosite, but undersaturated with respect to ferrihydrite. Additionally, it is confirmed that the bulk solution chemistry deviates slightly into the stability field of goethite rather than jarosite. This suggests that the aquatic environments in contact with the rock surfaces may be more acidic and/or enriched in SO2−4 relative to the bulk solution, which may eventually lead to the formation of jarosite instead of goethite.
TL;DR: In this article, the authors used electron microprobe and electron energy loss spectroscopy in the transmission electron microscope to estimate the oxygen fugacity of basaltic meteorites.
Abstract: The stoichiometry of titanomagnetite spinel in the martian basaltic meteorites is assessed using quantitative analysis of oxygen measured by electron microprobe and electron energy loss spectroscopy in the transmission electron microscope. The spinels are stoichiometric within the errors of the techniques, enabling the calculation of oxygen fugacity with confidence. The oxygen fugacity is calculated using the Ghiorso-Sack and Ca-QUIlF models, which also yield estimates of temperature. The oxygen fugacity of the martian basalts increases from 3 log units below the QFM buffer for QUE 94201 to QFM − 1.8 for EETA 79001 (both lithologies), to QFM − 1.0 for Shergotty, Zagami, and Los Angeles. Dar al Gani 476 spinels contain significant MgAl2O4 and FeCr2O4 components, complicating the use of Fe-Ti oxide models. The oxygen fugacity of Dar al Gani 476 is estimated to be 1.5 log units below QFM, on the basis of the Ghiorso-Sack model. The absolute error on the oxygen fugacity estimates is ± 0.5 log units; however, a consistent electron microprobe analytical routine was applied to all of the basalts, and the relative uncertainty is closer to 0.2 log units. Oxyexsolution has occurred in QUE 94201, but reconstruction of pre-exsolution titanomagnetite compositions permits the calculation of oxygen fugacity. Subsolidus reactions involving oxides and adjacent Fe-rich silicates are documented and the use of the Ca-QUIlF model for calculation of oxygen fugacity from these phases is discussed.
TL;DR: In this paper, a MORB-derived amphibolite has been used to determine fluid-absent solidus and supersolidus phase relationships, and the shape of the solidus was similar to previous estimations but the temperature interval between the high and low-pressure regions was strongly reduced.
Abstract: Experiments on a MORB (Mid Ocean Ridge Basalt)–derived amphibolite have been conducted in order to determine fluid–absent solidus and supersolidus phase relationships. The solidus has been determined in the range 4–14 kbar by using the incremental heating technique, tested against classical procedures. Peritectic clinopyroxene was produced below 10 kbar and epidote was the stable peritectic phase at pressures greater than 10 kbar for temperatures near the solidus. Garnet was the peritectic phase for temperatures above 800 °C and pressures of 10 to 14 kbar. The shape of the solidus was similar to previous estimations but the temperature interval between the high- and low-pressure regions was strongly reduced. With these new estimations, the possibilities for melting of the oceanic crust at P below 10 kbar are increased and, consequently, the number of tectonic scenarios needed for magma generation from amphibolite sources is enlarged. In the absence of free water, the solidus in the pressure region above 10 kbar is at higher T than previous estimations.
TL;DR: In this paper, a single-crystal X-ray diffraction (XRD) was performed on Zircon crystals synthesized in a Li-Mo oxide melt and doped with trivalent lanthanides and Y (REE), both with and without P, were examined.
Abstract: Zircon crystals synthesized in a Li-Mo oxide melt and doped with trivalent lanthanides and Y (REE), both with and without P, were examined by single-crystal X-ray diffraction (XRD). REE are incorporated into the Zr site in the zircon structure, and some Zr appears to be displaced to the Si site. Crystals doped with middle REE (MREE, Sm to Dy) and Y, plus P follow the xenotime substitution (REE 3+ + P 5+ = Zr 4+ + Si 4+ ) rather closely, whereas crystals doped with heavy REE (HREE, Er to Lu) deviate from the xenotime substitution, having REE:P atomic ratios significantly greater than one. Xenotime substitution requires that P 5+ replace Si 4+ , but this substitution becomes limited by strain at the Si site in HREE-doped crystals. As Si sites become saturated with P 5+ , additional charge balance in synthetic zircon crystals may be provided by Mo 6+ and Li + from the flux entering interstitial sites, accounting for an additional 0.3 to 0.6 at% HREE beyond that balanced by P 5+ ions. Heavy REE are more compatible in the zircon structure than are LREE and MREE, and HREE substitution is ultimately limited by the inability of the zircon structure to further accommodate charge-compensating elements. Thus the limit on REE concentrations in zircon is not a simple function of REE 3+ ionic radii but depends in a complex way on structural strain at Zr and Si sites, which act together to limit REE and P incorporation. The mechanisms that limit the coupled xenotime substitution change from LREE to HREE. This change means that REE fractionation in zircon may vary according to the availability of charge-compensating elements. REE partition coefficients between zircon and melt must also depend in part on the availability of charge-compensating elements and their compatibility in the zircon structure.
TL;DR: In this paper, the effect of metamictization on the weathering of zircon in detrital continental sediments and tropical soils of the Amazon basin, Brazil was investigated.
Abstract: We have investigated the effect that metamictization has on the weathering of zircon in detrital continental sediments and tropical soils of the Amazon basin, Brazil. The degree of radiation damage in the near-surface region of the zircon grains was determined by Raman microprobe. In each of the four series investigated (i.e., sediment, podzol, topsoil, and subsoil horizons of lateritic soil), the degree of radiation damage ranges from less than 1014 to ~3.5 × 1015 α−decay/mg. The maximum degree of radiation damage coincides with the first percolation threshold of the metamictization process at ~3.5 × 1015 α-decay/mg. Below this threshold, amorphous volumes in the structure of damaged zircon are not connected to each other. The ranges of U, Th, and Pb contents (in ppm) measured by proton induced X-ray emission (PIXE) microanalysis are 100 < U < 7000, 100 < Th < 18000, and 100 < Pb < 1300. Chemical ages, assessed from U, Th, and total-Pb, range between 0.15 Ga and 2.8 Ga. This range is roughly consistent with the ages reported for the Precambrian shields of the Amazon basin (0.45–3.5 Ga). Corresponding radiation doses range between <2 × 1015 and 3 × 1016 α-decay/mg. Comparison of calculated doses with the degree of structural damage indicates that most of the zircon grains have experienced significant annealing. However, the degree of annealing differs from one grain to another. Thus, the acute maximum limit observed for the degree of radiation damage of the whole zircon series is better explained by low-temperature alteration or weathering processes than by thermal resetting. Following this interpretation, our results provide evidence for a dramatic decrease in the chemical durability of zircon in natural weathering environments when the radiation dose exceeds 3.5 × 1015 α-decay/mg. Below the first percolation threshold, the zircon population survives the soil formation intact, but more damaged zircons are dissolved during weathering/ alteration processes.
TL;DR: In this article, chemical compositions of reintegrated perthitic, mesoperthitic and antiperthitic feldspars were calculated from the modal proportions and the chemical analyses of host and lamellar domains formed through exsolution.
Abstract: Ultrahigh metamorphic temperatures attained in the mid- to lower-crust have been assessed by examining the mineral chemistry of ternary feldspars with relatively coarse exsolution lamellae from the Archean Napier Complex, East Antarctica. Chemical compositions of re-integrated perthitic, mesoperthitic and antiperthitic feldspars are calculated from the modal proportions and the chemical analyses of host and lamellar domains formed through exsolution. Based on ternary feldspar solvus models, re-integrated compositions of feldspars from a variety of rock types yield the minimum equilibrium temperatures ranging from 1000 to 1110 ° C (0.8 GPa). These data confirm the suggestion that the regional thermal conditions of the Napier Complex reached or exceeded 1100 ° C. As feldspar is one of the common constituents of the crustal rocks, this approach could be applicable to a wide variety of rocks in which feldspar represents exsolution textures.
TL;DR: The single-crystal elastic moduli of natural samples of both calcite and rhodochrosite (MnCO 3 ) have been measured by Brillouin spectroscopy under ambient condition as mentioned in this paper.
Abstract: The single-crystal elastic moduli of natural samples of both calcite (CaCO 3 ) and rhodochrosite (MnCO 3 ) have been measured by Brillouin spectroscopy under ambient condition. Based on the trigonal unit cell, the elastic constants C 11 , C 33 , C 44 , C 12 , C 13 , and C 14 are 149.4(7), 85.2(18), 34.1(5), 57.9(11), 53.5(9), −20.0(2), and 223.9(15), 132.6(41), 44.5(9), 93.4(21), 76.0(23), −17.3(6) GPa for CaCO 3 and MnCO 3 , respectively. Our data for calcite are in good agreement with earlier data obtained by ultrasonic experiments. The off-diagonal elastic constants ( C 12 , C 13 , and C 14 ) for rhodochrosite have systematically larger values than the trend defined by other isostructural carbonates, in all of which the divalent cations are alkaline-earth metals. This is a distinctive signature of transition–metal-bearing oxides, which is present in silicates and simple oxides as well.
TL;DR: In this paper, the most As-rich natural hydr ous ferric oxide (HFO) material was characterized using powder X-ray diffraction (pXRD), transmission electron microscopy (TEM), XRF, light element analysis using gas chromatography (GC), visible-infrared (vis-IR) diffuse reflectivity, 57 Fe Mossbauer spectroscopy, and superconducting quantum interference device (SQUID) magnetometry.
Abstract: We characterized the most As-rich natural hydr ous ferric oxide (HFO) material ever reported using powder X-ray diffraction (pXRD), transmission electron microscopy (TEM), X-ray fluorescence spectroscopy (XRF), light element analysis using gas chromatography (GC), visible-infrared (vis-IR) diffuse reflectivity, 57 Fe Mossbauer spectroscopy, and superconducting quantum interference device (SQUID) magnetometry. We find that the natural As-HFO material is very similar to synthetic coprecipitated As-HFO materials, but is significantly different from all known natural and synthetic As-free HFO materials and ferrihydrite samples. The pXRD patterns show systematic differences with patterns for 2-line ferrihydrite, that are interpreted as evidence for significant populations of oxygen-coordinated Fe-As pairs. Observations by TEM, combined with energy dispersive spectroscopy (EDS) microanalysis, show agglomerations of nanophase primary particles and no evidence for other Fe- or As-bearing phases. Mossbauer spectroscopy shows octahedrally coordinated Fe 3+ , with a large fraction (~20%) of the octahedral Fe environments that are significantly distorted by the presence of As, compared to the Fe local environments in As-free ferrihydrite and HFO samples. The loss on ignition (LOI) is quantitatively consistent with OH + H 2O, measured by GC, which, in turn, is consistent with ~1 nm diameter primary particles having all their surface cations (Fe 3+ , As 5+ , Si 4+ , C 4+ ) coordinated on the free surface side by OH ‐ and OH2. The banding into adjacent yellowish and reddish layers that occurs in the As-HFO deposits was studied by performing mineralogical analyses of the separated adjacent layers of two couplets of yellowish and reddish material. The yellowish samples were found not to contain secondary crystalline phases (as did the reddish samples, in small amounts) and to be relatively As-rich, C- and Si-poor. The observed anticorrelations between As and Si and between As and inorganic C suggest that natural HFOs, which usually contain significant molar amounts of Si, may not be as efficient at surface complexing As (and P) as their Si and C-free synthetic counterparts, unless formed by co-precipitation with the As (or P). The yellowish and reddish layers were also clearly resolved by both Mossbauer spectroscopy and magnetometry. Complexation of arsenate onto the HFO core was found to significantly increase the average quadrupole splitting (QS) obtained from Mossbauer spectroscopy by an amount that could not be explained by other chemical differences and that is consistent with an ~1 nm diameter particle size and somewhat smaller HFO core. The Munsell hue YR index (5‐10 YR) was found to be strongly correlated to the average QS, thereb y establishing that the color differences, corresponding to the measured shifts of the main visible band edge, are due to the local distortions in the
TL;DR: In this article, the authors present 17 O MAS NMR data for crystalline calcium dialuminate (grossite), CaAl4O7 and monoaluminate, CaAl2O4.
Abstract: We present 17 O MAS NMR data for crystalline calcium dialuminate (grossite), CaAl4O7 and monoaluminate, CaAl2O4. The first of these contains an oxygen tricluster site and serves as a model compound for sites of this type in aluminosilicate glasses. Tricluster site NMR parameters are distinct from those of bridging O atoms (Al-O-Al), allowing partial resolution in triple quantum MAS NMR spectra. Such spectra for calcium aluminosilicate glasses are consistent with the presence of a small fraction of tricluster sites. Observed chemical shifts for non-bridging oxygen (NBO) atoms in an impurity phase in the CaAl2O4 sample are distinct from those for NBO in Ca-aluminosilicate glasses, indicating that the latter are primarily bonded to Si, not Al.
TL;DR: In this article, single-crystal electron diffraction patterns of nanocrystals in two samples of synthetic six-line ferrihydrite (6LFh) were obtained using electron nanodiffraction to produce diffraction pattern from areas between 0.7 and 5 nm across.
Abstract: Single-crystal electron diffraction patterns of nanocrystals in two samples of synthetic six-line ferrihydrite (6LFh) were obtained using electron nanodiffraction to produce diffraction patterns from areas between ~0.7 and 5 nm across. One of the samples was synthesized at 75 °C by a conventional method; the other was synthesized by a recently developed room-temperature technique. Structures of individual nanocrystals were investigated by comparing experimental and simulated electron-diffraction patterns. The most common structure in each sample is based on double-hexagonal (ABAC) stacking of close-packed oxygen layers, and is similar to the “defect-free ferrihydrite” structure proposed by Drits and co-workers. Nanocrystals with maghemite- or magnetite-like structures and structures similar to those in two-line ferrihydrite (2LFh) occur less commonly in both samples. The sample synthesized at room temperature has a small amount of hematite, which was not found in nanodiffraction patterns from the 75 °C sample. The most common structure in 6LFh is distinct from those previously determined by nanodiffraction of 2LFh, indicating that 6LFh is not simply a more crystalline form of 2LFh.
TL;DR: In this paper, a pair of two hydroxyl groups oriented parallel to the layer is formed in structures of both dickite and kaolinite producing the high-frequency components of the OH-stretching frequencies.
Abstract: Ab-initio density-functional molecular dynamics simulations and full relaxation of all atomic positions are used to reconcile the crystal structures with IR spectra of dickite and kaolinite. The relaxation of atomic positions preserves the accepted space group symmetries. A pair of two hydroxyl groups oriented parallel to the layer is formed in structures of both dickite and kaolinite producing the high-frequency components of the OH-stretching frequencies. Other hydroxyls make relatively strong interlayer hydrogen bonds and produce down-shifted stretching bands. All hydroxyl groups are involved in effective hydrogen bonds. The OH-stretching frequencies depend linearly on the strength of the hydrogen bond.
TL;DR: In this article, the growth zones in quartz phenocrysts from the rhyolitic Bishop Tuff have been studied and three recognizable patterns of growth zones have been identified: weakly zoned cores and bright CL rims, weakly zone cores and rims; and no CL intensity difference from core to rim.
Abstract: Cathodoluminescence (CL) reveals growth zones in quartz phenocrysts from the rhyolitic Bishop Tuff. Melt inclusions occur in various zones and record the evolving melt composition during zonal growth. The zones form an oscillatory pattern between bright and dark CL quartz. There are three recognizable patterns of CL zoning in these crystals: (1) weakly zoned cores and bright CL rims; (2) weakly zoned cores and dark CL rims; and (3) no CL intensity difference from core to rim. Dark CL quartz generally occurs at crystal edges, contains most of the melt inclusions and is interpreted as fast-growing. Zones that occur along recognizable crystal edges (edge zones) are thicker than the same zone on adjacent faces, consistent with relatively fast growth of these zones. In each successive zone, these edge zones decrease in size toward the rim, while the zones along the crystal faces increase. Some of the melt inclusions have bright CL quartz locally associated with them. This is interpreted as the postentrapment crystallization of slow-growing quartz in the melt inclusions. Many crystals display zone discordance from the weakly zoned interiors to the rims. Most of the discordant surfaces are rational and probably are primary growth features. Pumice clasts from the southern vents are largely compositionally and texturally distinct from those from the northern vents, and this distinction is also evident in the quartz CL. The crystals that have bright CL rims are all associated with the late-erupted northern part of the Bishop Tuff. Melt inclusion compositions and CL zoning patterns suggest a common origin for early and middle-erupted quartz and the interior zones of late-erupted quartz; however, the bright CL rim on the late-erupted quartz indicates an additional stage of crystallization in late-erupted magma. Melt inclusions in individual early erupted crystals have small variations in Ba whereas inclusions in late-erupted crystals markedly increase in Ba toward the rim, which is opposite to the normal zoning of sequentially trapped melts expected during closed system crystallization differentiation. The quartz zoning features are consistent with the hypothesis of crystal settling in evolving magma that erupted late from northern vents.
TL;DR: In this paper, a single Landau free energy expansion is used to describe phase transitions in perovskites, from a cubic parent structure to tetragonal and orthorhombic structures with space groups related to the M 3 and R 25 points of the Pm 3 m reciprocal lattice.
Abstract: A single Landau free energy expansion is used to describe phase transitions in perovskites, from a cubic parent structure to tetragonal and orthorhombic structures with space groups related to the M 3 and R 25 points of the Pm 3 m reciprocal lattice. This expansion permits relationships between symmetry-adapted forms of the spontaneous strain and individual order parameter components to be predicted. Data from the literature for (Ca,Sr)TiO 3 perovskites are analyzed in the light of these predictions. Shear strains for I 4/ mcm , Pnma, and Cmcm structures tend to conform to the predicted pattern. The Pm 3 m ↔ I 4/ mcm transition has nearly tricritical character as a function of temperature in CaTiO 3 and more nearly second-order character as a function of composition at the Sr-rich end of the solid solution. Coupling with the volume strain appears to be both temperature and composition dependent, which may be a general feature of phase transitions in perovskites. Renormalization of fourth-order terms by changing the volume coupling coefficients could be responsible for the unusual order parameter evolution shown by CaTiO 3 and for changes in thermodynamic character of the phase transitions as a function of composition. The pattern of strain variations also correlates closely with patterns of variations in heat capacity from the literature, suggesting revisions to the subsolidus phase diagram for the (Ca,Sr)TiO 3 solid solution above room temperature.
TL;DR: Fluoro-edenite as discussed by the authors was found both as prismatic or acicular crystals of millimetric size and as fibers in the rock cavities in gray-red altered benmoreitic lavas occurring at Biancavilla.
Abstract: Fluoro-edenite, ideally NaCa 2 Mg 5 (Si 7 Al)O 22 F 2 , was found both as prismatic or acicular crystals of millimetric size and as fibers in the rock cavities in gray-red altered benmoreitic lavas occurring at Biancavilla (Etnean Volcanic Complex, Catania, Italy). It is associated with feldspars, quartz, clino- and orthopyroxene, fluoro-apatite, ilmenite, and hematite, and probably crystallized from late-stage hydrothermal fluids. Fluoro-edenite is transparent, intense yellow, non-fluorescent, has vitreous to resinous luster, and gives a yellow streak parallel to the c axis; Mohs’ hardness 5–6, D calc = 3.09 g/cm 3 , perfect cleavage on {110}, and conchoidal fracture. In plane-polarized light, fluoro-edenite is birefringent (1 st order), biaxial negative, α = 1.6058(5), β = 1.6170(5), γ = 1.6245(5), 2 V calc = 78.09° , Y ≡ β ⊥ (010), and γ : Z = 26° . No pleochroism is observed. Fluoro-edenite is monoclinic, space group C 2/ m, a = 9.847(2) A, b = 18.017(3) A, c = 5.268(2) A, β = 104.84(2)° , V = 903.45 A 3 , Z = 2; the ten strongest X-ray diffraction lines in the powder pattern are [ d ( I, hkl )]: 3.125(10, 310), 8.403(6,110), 3.271(5,240), 2.807(4,330), 2.703(3,151), 1.894(2,510), 2.938(2,221), 1.649(2, 461), 3.376(2,131), 1.438(2, 661). IR analysis showed absorption bands at 1066, 991, 791, 738, 667, 517, 475 cm −1 , and no bands in the OH-stretching region. Structure refinement allowed determination of cation site-preference and ordering. Microprobe analysis of the refined crystal gave SiO 2 52.92, TiO 2 0.29, Al 2 O 3 3.53, FeO t 2.50, MnO 0.46, MgO 22.65, CaO 10.83, Na 2 O 3.20, K 2 O 0.84, F 4.35, Cl 0.07 wt%, and the crystal-chemical formula obtained by combining all the available data is: A (Na 0.56 K 0.15 ) B (Na 0.30 Ca 1.62 Mg 0.03 Mn 0.05 ) C (Mg 4.68 Fe 2+ 0.19 Fe 3+ 0.10 Ti 4+ 0.03 ) T (Si 7.42 Al 0.58 ) O 22 O3 (F 1.98 Cl 0.02 ) 2 .
TL;DR: The phase boundaries limiting the stability fields of the three different Mg(Fe)SiO 3 phases [orthoenstatite (Oen), low-temperature clinoenstatite, and high-pressure clinoENstite (HCen)] are inferred from Raman spectroscopy on quenched samples from high P -T experiments.
Abstract: The phase boundaries limiting the stability fields of the three different Mg(Fe)SiO 3 phases [orthoenstatite (Oen), low-temperature clinoenstatite (LCen), and high-pressure clinoenstatite (HCen)] that occur at the pressure-temperature conditions prevailing in the Earth’s upper mantle are inferred from Raman spectroscopy on quenched samples from high P - T experiments. There are subtle but significant differences between the spectra of Oen and the spectra of samples quenched within the stability field of HCen or LCen. The most prominent differences are additional peaks at 369 and 431 cm −1 in the spectrum of Cen and a systematic shift of the peak at 236 cm −1 in the Oen spectrum to 243 cm −1 in the Cen spectrum. However, no distinction can be made between samples quenched from the HCen and LCen stability fields. This is consistent with the fact that the HCen phase is non-quenchable and that the pyroxene phase observed in the experimental products is LCen as verified by powder X-ray diffraction. Experiments performed in the pressure-temperature range 1.2–14 GPa and 750–1900 K have been used to constrain the Mg(Fe)SiO 3 phase diagram. Pyroxene compositions cover the range from pure MgSiO 3 to Mg 0.9 Fe 0.1 SiO 3 with minor amounts of Al, Ca, Na, and Cr. The results are similar to previous determinations from X-ray and optical studies and tightly constrain the HCen–Oen phase boundary, which can be expressed by the equation P (GPa) = 0.00454 T (K) + 1.673. The LCen–Oen boundary is not as well constrained, but the data are sufficient to locate the invariant point where all three MgSiO 3 phases coexist at 6.6 GPa and 820 °C.
TL;DR: Aeromagnetic maps of the Egersund mid-proterozoic igneous province show a spectacular range of positive and negative magnetic anomalies with a contrast up to 15 600 nT.
Abstract: Aeromagnetic maps of the Egersund Mid-Proterozoic igneous province show a spectacular range of positive and negative magnetic anomalies with a contrast up to 15 600 nT. The positive magnetic anomalies are over magnetite norites and overlying mangerites and quartz mangerites of the Bjerkreim-Sokndal layered intrusion. These rocks are dominated by multi-domain (MD) magnetite. The negative magnetic anomalies are over ilmenite-rich norites of the Bjerkreim-Sokndal layered intrusion, the Tellnes ilmenite norite ore deposit, and massif anorthosites. These rocks are dominated by hemo-ilmenite and/or by silicates containing fine-grained oxide exsolution lamellae. Electron microprobe analyzes of coexisting Fe-Ti oxides in the layered intrusion confirm earlier observations that oxides in early magmatic rocks are dominated by hemo-ilmenite with minor end-member magnetite, followed by more reduced oxides dominated by titanomagnetite with minor near end-member ilmenite. What is not fully understood is the property of ilmenite with hematite exsolution lamellae, or, even more striking, hematite with ilmenite lamellae, to produce strong remanent magnetization of high coercivity and with a Neel temperature equal to or above the Curie temperature of magnetite. This property makes the rhombohedral oxides an important candidate to explain some high-amplitude deep-crustal anomalies on earth, or strong remanent magnetization on other planets. A remarkable feature in the Egersund province is that primitive magmas produced rocks rich in hemo-ilmenite causing negative magnetic anomalies related to magnetic remanence, and more evolved magmas produced rocks rich in magnetite related to positive induced magnetic anomalies, all in the course of crystallization-differentiation.
TL;DR: In this article, the authors use constraints from experiments, thermodynamic modeling, and natural mineral assemblages to argue that Al 2 SiO 5 "triple-point" assemblage is not possible in common muscovite(Ms)+biotite(Bt)+quartz(Qtz)-bearing metapelitic rocks because the reactions that first introduce an Al 2SiO 5 mineral to these bulk compositions occur at higher temperature than the triple point.
Abstract: This paper uses constraints from experiments, thermodynamic modeling, and natural mineral assemblages to argue that Al 2 SiO 5 “triple-point” assemblages, in which all three Al 2 SiO 5 minerals are in stable equilibrium, are not possible in common muscovite(Ms)+biotite(Bt)+quartz(Qtz)-bearing metapelitic rocks because the reactions that first introduce an Al 2 SiO 5 mineral to these bulk compositions occur at higher temperature than the triple point. Less-common, highly aluminous bulk compositions may develop Al 2 SiO 5 minerals at temperatures below the triple point such that stable triple-point assemblages are theoretically possible. The “invisibility” of the triple-point to common Ms+Bt+Qtz-bearing metapelites calls into question most metapelitic triple-point localities reported in the literature, and carries implications for the topology of the metapelitic petrogenetic grid, the bathozone/bathograd scheme of Carmichael (1978), and the possibility of prograde kyanite → andalusite → sillimanite sequences. Re-examination of reported triple-point localities suggests that in most if not all cases, the Al 2 SiO 5 minerals grew at different times in the metamorphic history of the rock.
TL;DR: In this paper, a double hydroxysalt green rusts (GR) was extracted from hydromorphic soils in Fougeres (France) and studied by X-ray absorption spectroscopy (XAS) and transmission Mossbauer spectroglobalization (TMS) and the spectrum at the Fe K absorption edge of this mineral proved to be very similar to that of synthetic GRs.
Abstract: Layered double hydroxysalt green rusts, GRs, are very reactive compounds and their general formula, [Fe (1− x ) 2+ Fe x 3+ (OH) 2 ] x + [ x / n A n − · m H 2 O] x − , where x is the ratio Fe 3+ /Fe tot , reflects the structure in which brucite-like layers alternate with interlayers of anions A n − and water molecules. A GR mineral was extracted from hydromorphic soils in Fougeres (France) and studied by X-ray absorption spectroscopy (XAS) and transmission Mossbauer spectroscopy (TMS). The XAS spectrum at the Fe K absorption edge of this mineral proved to be very similar to that of synthetic GRs. However, the radial distribution function obtained for the GR mineral proved to be intermediate between those of GR(CO 3 2− ) and pyroaurite, that is between the Fe 2+ -Fe 3+ and Mg 2+ -Fe 3+ hydroxycarbonates. Consequently, a partial substitution of Fe 2+ by Mg 2+ occurs, leading to the general formula of [Fe (1− x ) 2+ Mg y 2+ Fe x 3+ (OH) (2+2y) ] x+ [ x/n A n− · m H 2 O] x − where A n− is the interlayer anion. Unfortunately, the XAS spectra of various GR proved to be independent of the interlayer anion, and the nature of the anions present in the mineral GR could not be determined. The Mossbauer spectrum of the mineral, measured at 77 K, is composed of four quadrupole doublets: D 1 and D 2 due to Fe 2+ [δ ≅ 1.26 mm/s and Δ E Q ≅ 2.5 and 2.9 mm/s, respectively] and D 3 and D 4 due to Fe 3+ [δ ≅ 0.46 mm/s and Δ E Q ≅ 0.5 and 1.0 mm/s, respectively]. Finally, synthetic Mg 2+ -Fe 2+ -Fe 3+ hydroxycarbonates could be prepared by coprecipitation from Mg and Fe salts and lead to Mossbauer spectra similar to that of the mineral. In particular, the partial substitution of Fe 2+ by Mg 2+ proved to be consistent with the existence of the unusual doublet D 4 .