The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Charles A Geiger - One of the best experts on this subject based on the ideXlab platform.
-
cation order disorder in fe bearing pyrope and grossular Garnets a 27al and 29si mas nmr and 57fe mossbauer spectroscopy study
American Mineralogist, 2015Co-Authors: Aaron C Palke, Charles A Geiger, Jonathan F Stebbins, Gerold TippeltAbstract:A suite of Fe-bearing natural and synthetic grossular-rich [(Ca,Fe)3(Al,Fe)2Si3O12] and pyrope-rich [(Mg,Fe)3Al2Si3O12] Garnets were investigated using 27Al and 29Si MAS NMR and 57Fe Mossbauer spectroscopy. This was done to study the state of cation order-disorder in Garnet solid solutions by analyzing paramagnetically shifted resonances in high-resolution NMR spectra. The Mossbauer spectra, along with electron microprobe results, give the concentrations of Fe2+ and Fe3+ and their site occupancies, even in grossular samples with very low concentrations of Fe. MAS NMR spectra were collected on Fe2+-bearing grossular- and pyrope-rich Garnets with up to 25 mol% almandine component and on other Fe3+-bearing grossular samples with up to 9 mol% andradite component. Despite peak broadening and signal loss, structural information was even obtained from Garnet with relatively high Fe contents (25 mol% almandine component). Paramagnetically shifted NMR peaks, related to the presence of Fe2+, were observed in grossular samples and are similar in nature to those reported previously for natural, relatively low-Fe2+ pyrope Garnets by Stebbins and Kelsey (2009). Additional NMR peaks appear as the concentration of Fe2+ increases, reflecting an increase in the average number of neighboring Fe2+ cations around AlO6 and SiO4 groups. These newly observed peaks hold potential to provide information concerning the presence or absence of short-range ordering in certain Fe-bearing silicate Garnets. The effect of Fe3+ on the MAS NMR spectra of Garnet appears to be less pronounced, because it does not produce any observable paramagnetically shifted resonances.
-
Thermodynamic mixing properties and behavior of almandine-spessartine solid solutions
Geochimica et Cosmochimica Acta, 2014Co-Authors: Edgar Dachs, Charles A Geiger, Artur Benisek, Michael GrodzickiAbstract:Abstract The heat capacity, Cp, of five solid-solution members of the almandine(Alm)–spessartine(Sps) binary, consisting of three synthetic polycrystalline and two natural single-crystal samples, was measured in the temperature range between 2 and 300 K using relaxation calorimetry and between 282 and 764 K using DSC methods. All Garnets exhibit a λ-type heat-capacity anomaly at low temperatures resulting from a paramagnetic to antiferromagnetic phase transition. The temperature of the magnetic transition in Fe-rich Garnets occurs between those of the two end-members (i.e. 9.2 K for almandine and 6.2 K for spessartine), but lies at lower values between 3.5 and 4.5 K for more Sps-rich compositions with X Mn grt > 0.5 . The calorimetric entropy at 298 K shows mechanical-mixture behavior for Sps-rich Garnets and a slight possible negative deviation from such behavior for Alm-rich compositions. At the 2σ level all data are, however, consistent with ideal mixing behavior and the Margules entropy interaction parameter, W S , FeMn grt , is zero for the Alm–Sps binary. Thermodynamic analysis of published high P and T phase-equilibrium Fe–Mn exchange experiments between Garnet and ilmenite shows that the excess Gibbs free energy of mixing, ΔGex, for Fe–Mn in Garnet is positive and asymmetric towards spessartine. Margules enthalpy interaction parameters of W H,FeMn grt = 4170 ± 518 J/cation⋅mol and W H, MnFe = 1221 ± 588 J/cation⋅mol are derived giving a maximum of Δ G ex ≈ 0.7 kJ/cation⋅mol at X Mn grt ≈ 0.6 . ΔHex obtained using autocorrelation analysis of published IR spectra of Alm–Sps solid solutions is in reasonable agreement with that derived from phase-equilibrium and calorimetry data. Previous diffraction and spectroscopic results on Alm–Sps Garnets and quantum mechanical calculations made on almandine are used to interpret the macroscopic thermodynamic behavior from a microscopic basis. The relevance of the new Garnet Fe–Mn mixing model for petrological calculations is demonstrated by incorporating it into the quaternary Garnet mixing model of Berman (1990) . Thus, better agreement for temperatures calculated using Fe–Mn Garnet-ilmenite and Fe–Mg Garnet-biotite geothermometry could be achieved. Temperatures calculated for Mn-poor and Mn-rich Garnet-bearing assemblages, applying Garnet-biotite thermometry, are in better agreement taking Fe–Mn mixing into account.
-
crystal chemistry and stability of li7la3zr2o12 Garnet a fast lithium ion conductor
Inorganic Chemistry, 2011Co-Authors: Charles A Geiger, Evgeny V Alekseev, Biljana Lazic, Martin Fisch, Thomas Armbruster, Ramona Langner, Michael Fechtelkord, Namjun Kim, Thomas Pettke, Werner WeppnerAbstract:Recent research has shown that certain Li-oxide Garnets with high mechanical, thermal, chemical, and electrochemical stability are excellent fast Li-ion conductors. However, the detailed crystal chemistry of Li-oxide Garnets is not well understood, nor is the relationship between crystal chemistry and conduction behavior. An investigation was undertaken to understand the crystal chemical and structural properties, as well as the stability relations, of Li7La3Zr2O12 Garnet, which is the best conducting Li-oxide Garnet discovered to date. Two different sintering methods produced Li-oxide Garnet but with slightly different compositions and different grain sizes. The first sintering method, involving ceramic crucibles in initial synthesis steps and later sealed Pt capsules, produced single crystals up to roughly 100 μm in size. Electron microprobe and laser ablation inductively coupled plasma mass spectrometry (ICP-MS) measurements show small amounts of Al in the Garnet, probably originating from the crucible...
-
Molar volumes of mixing of almandine-pyrope and almandine-spessartine Garnets and the crystal chemistry and thermodynamic-mixing properties of the aluminosilicate Garnets
American Mineralogist, 1997Co-Authors: Charles A Geiger, Anne FeenstraAbstract:The aluminosilicate Garnet binaries almandine-pyrope and almandine-spessartine were studied by powder X-ray and 57 Fe Mossbauer methods. Refinements of the unit-cell con- stants along the almandine-pyrope join show that the volumes of mixing are ideal. Those of the almandine-spessartine join show very small positive deviations from ideality, which can be fitted with a symmetric model having an interaction parameter ofW V 5 0.24 (60.05) cm 3 /mol. Mossbauer spectra recorded at 298 and 77 K show the presence of small amounts of (6) Fe 31 , which in the case of almandine-pyrope Garnets is also measurable from micro- probe analyses. The amount of Fe 31 is generally less than 3.5% of the total Fe for the almandine-pyrope Garnets and 1-2% for almandine-spessartine Garnets. The molar volumes of mixing of the aluminosilicate Garnet binaries are interpreted using a crystal-chemical model involving rigid tetrahedral rotation. The degree of tetrahedral rotation is not linear with increasing size of the divalent X-site cation for the four common aluminosilicate Garnet end-members or along the solid solution binary pyrope-grossular. The vibrational entropies of mixing should be positively correlated with the volumes of mixing in the case of Garnet, but the masses of the X-site cations must also be considered. The phonon density of states at low energies should show the vibrations of the weakly bonded divalent cations and rigid-unit modes related to tetrahedral rotation. Positive excess vibrational entropies of mixing along a binary could result from increased amplitudes and lower frequencies of vibration of the smaller of the two X-site cations substituting within larger and more distorted dodecahedral sites, as compared to the X site in the smaller volume end-member.
Michael Grodzicki - One of the best experts on this subject based on the ideXlab platform.
-
Thermodynamic mixing properties and behavior of almandine-spessartine solid solutions
Geochimica et Cosmochimica Acta, 2014Co-Authors: Edgar Dachs, Charles A Geiger, Artur Benisek, Michael GrodzickiAbstract:Abstract The heat capacity, Cp, of five solid-solution members of the almandine(Alm)–spessartine(Sps) binary, consisting of three synthetic polycrystalline and two natural single-crystal samples, was measured in the temperature range between 2 and 300 K using relaxation calorimetry and between 282 and 764 K using DSC methods. All Garnets exhibit a λ-type heat-capacity anomaly at low temperatures resulting from a paramagnetic to antiferromagnetic phase transition. The temperature of the magnetic transition in Fe-rich Garnets occurs between those of the two end-members (i.e. 9.2 K for almandine and 6.2 K for spessartine), but lies at lower values between 3.5 and 4.5 K for more Sps-rich compositions with X Mn grt > 0.5 . The calorimetric entropy at 298 K shows mechanical-mixture behavior for Sps-rich Garnets and a slight possible negative deviation from such behavior for Alm-rich compositions. At the 2σ level all data are, however, consistent with ideal mixing behavior and the Margules entropy interaction parameter, W S , FeMn grt , is zero for the Alm–Sps binary. Thermodynamic analysis of published high P and T phase-equilibrium Fe–Mn exchange experiments between Garnet and ilmenite shows that the excess Gibbs free energy of mixing, ΔGex, for Fe–Mn in Garnet is positive and asymmetric towards spessartine. Margules enthalpy interaction parameters of W H,FeMn grt = 4170 ± 518 J/cation⋅mol and W H, MnFe = 1221 ± 588 J/cation⋅mol are derived giving a maximum of Δ G ex ≈ 0.7 kJ/cation⋅mol at X Mn grt ≈ 0.6 . ΔHex obtained using autocorrelation analysis of published IR spectra of Alm–Sps solid solutions is in reasonable agreement with that derived from phase-equilibrium and calorimetry data. Previous diffraction and spectroscopic results on Alm–Sps Garnets and quantum mechanical calculations made on almandine are used to interpret the macroscopic thermodynamic behavior from a microscopic basis. The relevance of the new Garnet Fe–Mn mixing model for petrological calculations is demonstrated by incorporating it into the quaternary Garnet mixing model of Berman (1990) . Thus, better agreement for temperatures calculated using Fe–Mn Garnet-ilmenite and Fe–Mg Garnet-biotite geothermometry could be achieved. Temperatures calculated for Mn-poor and Mn-rich Garnet-bearing assemblages, applying Garnet-biotite thermometry, are in better agreement taking Fe–Mn mixing into account.
Dorrit E Jacob - One of the best experts on this subject based on the ideXlab platform.
-
combined thermodynamic and rare earth element modelling of Garnet growth during subduction examples from ultrahigh pressure eclogite of the western gneiss region norway
Earth and Planetary Science Letters, 2008Co-Authors: Matthias Konradschmolke, Thomas Zack, Patrick J Obrien, Dorrit E JacobAbstract:Abstract Major and trace element zonation patterns were determined in ultrahigh-pressure eclogite Garnets from the Western Gneiss Region (Norway). All investigated Garnets show multiple growth zones and preserve complex growth zonation patterns with respect to both major and rare earth elements (REE). Due to chemical differences of the host rocks two types of major element compositional zonation patterns occur: (1) abrupt, step-like compositional changes corresponding with the growth zones and (2) compositionally homogeneous interiors, independent of growth zones, followed by abrupt chemical changes towards the rims. Despite differences in major element zonation, the REE patterns are almost identical in all Garnets and can be divided into four distinct zones with characteristic patterns. In order to interpret the major and trace element distribution and zoning patterns in terms of the subduction history of the rocks, we combined thermodynamic forward models for appropriate bulk rock compositions to yield molar proportions and major element compositions of stable phases along the inferred pressure-temperature path with a mass balance distribution of REEs among the calculated stable phases during high pressure metamorphism. Our thermodynamic forward models reproduce the complex major element zonation patterns and growth zones in the natural Garnets, with Garnet growth predicted during four different reaction stages: (1) chlorite breakdown, (2) epidote breakdown, (3) amphibole breakdown and (4) reduction in molar clinopyroxene at ultrahigh-pressure conditions. Mass-balance of the rare earth element distribution among the modelled stable phases yielded characteristic zonation patterns in Garnet that closely resemble those in the natural samples. Garnet growth and trace element incorporation occurred in near thermodynamic equilibrium with matrix phases during subduction. The rare earth element patterns in Garnet exhibit distinct enrichment zones that fingerprint the minerals involved in the Garnet-forming reactions as well as local peaks that can be explained by fractionation effects and changes in the mineral assemblage.
Anne Feenstra - One of the best experts on this subject based on the ideXlab platform.
-
Molar volumes of mixing of almandine-pyrope and almandine-spessartine Garnets and the crystal chemistry and thermodynamic-mixing properties of the aluminosilicate Garnets
American Mineralogist, 1997Co-Authors: Charles A Geiger, Anne FeenstraAbstract:The aluminosilicate Garnet binaries almandine-pyrope and almandine-spessartine were studied by powder X-ray and 57 Fe Mossbauer methods. Refinements of the unit-cell con- stants along the almandine-pyrope join show that the volumes of mixing are ideal. Those of the almandine-spessartine join show very small positive deviations from ideality, which can be fitted with a symmetric model having an interaction parameter ofW V 5 0.24 (60.05) cm 3 /mol. Mossbauer spectra recorded at 298 and 77 K show the presence of small amounts of (6) Fe 31 , which in the case of almandine-pyrope Garnets is also measurable from micro- probe analyses. The amount of Fe 31 is generally less than 3.5% of the total Fe for the almandine-pyrope Garnets and 1-2% for almandine-spessartine Garnets. The molar volumes of mixing of the aluminosilicate Garnet binaries are interpreted using a crystal-chemical model involving rigid tetrahedral rotation. The degree of tetrahedral rotation is not linear with increasing size of the divalent X-site cation for the four common aluminosilicate Garnet end-members or along the solid solution binary pyrope-grossular. The vibrational entropies of mixing should be positively correlated with the volumes of mixing in the case of Garnet, but the masses of the X-site cations must also be considered. The phonon density of states at low energies should show the vibrations of the weakly bonded divalent cations and rigid-unit modes related to tetrahedral rotation. Positive excess vibrational entropies of mixing along a binary could result from increased amplitudes and lower frequencies of vibration of the smaller of the two X-site cations substituting within larger and more distorted dodecahedral sites, as compared to the X site in the smaller volume end-member.
Chris Ryan - One of the best experts on this subject based on the ideXlab platform.
-
cr pyrope Garnets in the lithospheric mantle i compositional systematics and relations to tectonic setting
Journal of Petrology, 1999Co-Authors: W L Griffin, J H Friedman, N I Fisher, Chris Ryan, Suzanne Y OreillyAbstract:Chrome-pyrope Garnet is a minor but widespread phase in ultramafic association with Mg. The position and slope of the lherzolite trend vary with temperature and tectonic setting, suggesting that the P/ rocks of the continental lithosphere; its complex chemistry preserves a record of events related to fluid movements in the mantle, including T ratio exerts a control on Ca/Cr in lherzolite Garnets. Garnets with less Ca than the lherzolite trend (‘subcalcic Garnets’) are largely melt extraction and metasomatism. We have examined the majorelement and trace-element composition of >12 600 Cr-pyrope confined to Archon suites, where they typically are concentrated in the 130–180 km depth range. The few subcalcic Garnets from (Cr2O3 > 1 wt %) xenocrysts in volcanic rocks to evaluate their compositional ranges and interelement relationships. Samples have Proton suites typically are lower in Cr and occur at shallower depths (100–120 km). Subcalcic Garnets are absent in Tecton been divided into three major groups (Archon, [2·5 Ga; Proton, 2·5–1 Ga; Tecton, <1 Ga) depending on the age of the last major suites analysed in this work. The complexity of the geochemical relationships illustrated here, and their variation with temperature tectonothermal event in the crust penetrated by the host volcanic rock. Relative depths of Garnets within each sample have been and tectonic setting, suggests that it is possible to define meaningful compositional populations of Garnets, which can be used to map determined by measurement of Nickel Temperature ( TNi). Mn, Ni and Zn contents of Cr-pyrope Garnets are controlled by T-dependent the stratigraphy and structure of the lithospheric mantle. partitioning between Garnet and mantle olivine. The expected correlation of mg-number with T is largely masked by effects of bulk composition and crystal chemistry. The Cr content of Garnet is a primary indicator of the degree of depletion of the host rock; Fe, Y,
-
Garnet geotherms pressure temperature data from cr pyrope Garnet xenocrysts in volcanic rocks
Journal of Geophysical Research, 1996Co-Authors: Chris Ryan, W L Griffin, Norman J PearsonAbstract:The temperatures and pressures of equilibration of single peridotitic Garnet xenocrysts are estimated using a combination of major- and trace-element data, determined using electron microprobe (EMP) and proton-induced X ray emission (PIXE). This new method enables the use of xenocrysts found in kimberlites and other volcanic rocks to determine the local paleogeotherm at the time of eruption of the magma which sampled and transported the xenocrysts. The “Ni thermometer” of Griffin et al. [1989], based on the strong temperature dependence of the partitioning of Ni between Garnet and olivine, is refined using an expanded database. Pressure is calculated from Garnet composition using an algorithm that combines a modification of the geobarometer of Nickel [1989], based on Cr solubility in coexisting Garnet and orthopyroxene, with the composition of a hypothetical coexisting orthopyroxene. The orthopyroxene composition is estimated by inverting the geothermometry equations of Gasparik [1987], Brey and Kohler [1990], and Harley [1984], and combining these with empirical relationships describing Cr in orthopyroxene in Cr-saturated peridotite (chromite present). The derived pressure (PCr) gives the equilibration pressure of peridotic Garnets provided they were in equilibrium with chromite; Garnets from Cr-undersaturated rocks will produce underestimates of pressure. Therefore, the locus of maximum PCr at a given TNi defines the “Garnet geotherm”, and provides a method for the determination of paleogeotherms based solely on PIXE and EMP analyses of Garnet grains in concentrates. The assumption of coexisting chromite is tested by comparing the temperature distributions of Garnets and chromites from the same concentrate. Chromite equilibration temperature is estimated using the “Zn thermometer”, based on the strong temperature dependence of the partitioning of Zn between chromite and olivine. This thermometer is calibrated against the new Ni thermometer using a suite of Garnet-chromite intergrowths. The Garnet geotherm technique provides an estimate of the geotherm with an accuracy comparable to xenolith-derived geotherms and provides a means of mapping the thermal state of the lithosphere where xenoliths are rare or absent.
