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Charles A Geiger - One of the best experts on this subject based on the ideXlab platform.
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Micro- and nano-size hydrogrossular-like clusters in Pyrope crystals from ultra-high-pressure rocks of the Dora-Maira Massif, western Alps
Contributions to Mineralogy and Petrology, 2020Co-Authors: Charles A Geiger, George R. RossmanAbstract:The supracrustal metamorphic rocks of the Dora-Maira Massif, western Alps, have been intensively studied. Certain ultra-high-pressure lithologies contain coesite and nearly end-member composition Pyrope, Mg_3Al_2Si_3O_12, making this locality petrologically and mineralogically unique. Structural OH^-, loosely termed “water”, in Pyrope crystals of different composition has been investigated numerous times, using different experimental techniques, by various researchers. However, it is not clear where the minor OH^- is located in them. IR single-crystal spectra of two Pyropes of composition {Mg_2.79,Fe^2+_0.15,Ca_0.04}_Σ2.98[Al]_2.02(Si)_2.99O_12 and {Mg_2.90,Fe^2+_0.04,Ca_0.02}_Σ2.96[Al]_2.03(Si)_3O_12 were recorded at room temperature (RT) and 80 K. The spectra show five distinct OH^- bands located above 3600 cm^-1 at RT and seven narrow bands at 80 K and additional fine structure. The spectra were curve fit and the OH^- stretching modes analyzed and assigned. It is argued that OH^- is located in microscopic- and nano-size Ca_3Al_2H_12O_12-like clusters. The basic substitution mechanism is the hydrogarnet one, where (H_4O_4)^4- ⇔ (SiO_4)^4-, and various local configurations containing different numbers of (H_4O_4)^4- groups define the cluster type. The amounts of H_2O range between 5 and 100 ppm by weight, depending on the IR calibration adopted, and are variable among crystals. Hydrogrossular-like clusters are also present in a synthetic Pyrope with a minor Ca content grown hydrothermally at 900 °C and 20 kbar, as based on its IR spectra at RT and 80 K. Experiment and nature are in agreement, and OH^- groups are partitioned into various barely nano-size hydrogrossular-like clusters. This proposal is new and significant mineralogical, petrological, and geochemical implications result. Ca and proton ordering occur. Hypothetical “defect” and/or coupled-substitution mechanisms to account for structural OH^- are not needed to interpret experimental results. OH^- incorporation in Pyrope of different generations at Dora-Maira is discussed and OH^- could potentially be used as an indicator of changing $$ P_{{{\text{H}}_{{\text{2}}} {\text{O}}}} {\text{(}}a_{{{\text{H}}_{{\text{2}}} {\text{O}}}} {\text{) - }}T $$ P H 2 O ( a H 2 O ) - T conditions in a metamorphic cycle. Published experimental hydration, dehydroxylation, and hydrogen diffusion results on Dora-Maira Pyropes can now be interpreted atomistically.
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Low-temperature single-crystal Raman spectrum of Pyrope
Physics and Chemistry of Minerals, 2000Co-Authors: B. A. Kolesov, Charles A GeigerAbstract:The single-crystal polarized Raman spectra of synthetic Pyrope, Mg3Al2Si3O12, were measured at room temperature and 5 K, as were the room-temperature unpolarized spectra of two natural Pyrope-rich crystals. No major differences in the spectra between room temperature and 5 K are observed or are present between the synthetic and the natural crystals. The spectra are consistent with the proposal that the Mg cation is dynamically disordered and not statically distributed over subsites in the large triangular-dodecahedral E-site in Pyrope. A low-energy band at about 135 cm−1 softens and shows a large decrease in its line width with decreasing temperature. The presence of a weak, broad band at about 280 cm−1 may be due to anharmonic effects, as could the one at 135 cm−1. The latter is assigned to the rattling motion of Mg in Pyrope in the plane of the longer Mg-O(4) bonds (Kolesov and Geiger 1998). The successful modeling of the anisotropic motion of the Mg cation in Pyrope, which has an anharmonic character, provides a valuable test of the validity of empirical or semi-empirical lattice-dynamic calculations for silicates.
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Single-crystal IR- and UV/VIS-spectroscopic measurements on transition-metal-bearing Pyrope the incorporation of hydroxide in garnet
European Journal of Mineralogy, 2000Co-Authors: Charles A Geiger, Andreas Stahl, George R. RossmanAbstract:Pyrope single crystals doped with transition-metal ions (Co, Cr, Ni, Ti and V) were synthesised in a piston-cylinder device at 950–1050°C and 25 kbar. Stoichiometric oxide mixtures were used as starting materials and distilled water was used as a fluid flux. Crystals up to 2 mm in size were grown. Microprobe analysis and optical absorption spectroscopy were used to determine on which positions and in which oxidation states the transition-metal ions are incorporated in the Pyrope structure. Cr^(3+)-ions occupy the octahedral site and Co^(2+) and Ni^(2+) the dodecahedral site. Although extra metallic Ti was included in the synthesis of Ti-bearing Pyropes, only Ti^(4+) and no measurable Ti^(3+) could be stabilised on the octahedral site. The optical absorption spectra of V-bearing Pyropes show, in addition to the spin-allowed dd-transitions ^3T_(1g)(F) ^3T_(2g)(F) at ~ 17000 cm_(-1) and ^3T_(2g)(F) ^3T_(1g)(P) at ~ 20000 cm^(-1) corresponding to V^(3+) on the octahedral site, absorption bands which are thought to be caused by dd-transitions of V^(3+) in the tetrahedral site and V^(4+) on octahedral and tetrahedral sites. V^(4+) was not observed in silicate garnets before. IR spectra in the OH- stretching region between 4000 and 3000 cm^(-1), obtained on Pyrope single-crystals which only contain divalent and trivalent transition-metal ions like Ni^(2+), Co^(2+), and Cr^(3+), are similar to that normally shown by end-member Pyrope (Geiger et al., 1991). At room temperature the spectra show a single band at ≈ 3630 cm^(-1), which splits at ~ 79 K into two bands of smaller FWHM's at ∼ 3618 cm^(-1) and 3636 cm^(-1). These bands are assigned to OH–stretching modes resulting from the hydrogarnet substitution. The spectra of Ti^(4+)-bearing Pyrope measured at 298 K show four OH–stretching bands at approximately 3686, 3630, 3567 and 3527 cm^(-1). At ∼ 79 K the band at 3630 cm^(-1) splits into two narrow bands at 3636 cm^(-1) and 3614 cm^(-1). This suggests that additional OH- substitutional mechanisms occur in Ti-containing garnets. In the IR spectrum of a V^(4+)-bearing Pyrope the same number of OH-stretching bands is observed, suggesting that higher charged cations cause additional OH- substitutions and increased OH- concentrations in garnet. The IR spectra of most natural Pyrope-rich garnets appear to be different from those of the synthetics, which suggests that they are not characterised by the hydrogarnet substitution. However, the OH–substitution mechanism and concentrations in garnets from grospydite or similar parageneses are similar to those of the synthetics, which may reflect their formation in water-rich environments.
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A powder infrared spectroscopic investigation of garnet binaries in the system Mg 3 Al 2 Si 3 O 12 - Fe 3 Al 2 Si 3 O 12 - Mn 3 Al 2 Si 3 O 12 - Ca 3 Al 2 Si 3 O 12
European Journal of Mineralogy, 1998Co-Authors: Charles A GeigerAbstract:The powder IR phonon spectra of synthetic garnets of the six binaries in the system Mg3Al2Si30i2 (Pyrope - Py) - Fe3Al2Si30i2 (Almandine - Al) - Mn3Al2Si3Oi2 (Spessartine - Sp) - Ca3AbSi30i2 (Grossular - Gr) have been recorded and analyzed. Between 14 and 17 Fiu-symmetry infrared active modes were observed for the different garnet end members and can, to a first approximation, be assigned to internal SiU4 vibrations and external lattice vibrations of the S1O4 tetrahedron and the Al3+ and X2+-site cations. This description is limited by mode mixing which is most pronounced for the lowest frequency modes. Two mode behavior is observed for X2+-cation translations for most of the binaries. The site-group and factor-group splittings have been calculated for all six binaries. For the almandine/Pyrope-grossular binaries, the factor-group splittings suggest that Si04 group vibrational interactions do not change linearly between the two end-members. No evidence in the IR spectra is found for long range X2+-cation order and all synthetic solid solutions are long-range disordered. The IR spectra provide information on the possible lattice heat capacities and entropies of mixing for the six solid solutions. The frequency changes of the lowest frequency external modes of Pyrope-grossular and almandine-grossular garnets are consistent with the proposal of excess lattice entropies of mixing at low temperatures. The almandine-spessar tine binary should be thermodynamically ideal, while the almaridine-Pyrope, spessartine-p yrope and spessartine- grossular binaries could show some excess lattice properties. The higher frequency modes above 400 cm"1 of the solid solution compositions can be estimated well from a linear interpolation between the two end-members. This is consistent with the proposal that no excess lattice entropies of mixing should be present in garnet solid solutions above 300 K.
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Raman spectra of silicate garnets
Physics and Chemistry of Minerals, 1998Co-Authors: B. A. Kolesov, Charles A GeigerAbstract:The single-crystal polarized Raman spectra of four natural silicate garnets with compositions close to end-members almandine, grossular, andradite, and uvarovite, and two synthetic end-members spessartine and Pyrope, were measured, along with the powder spectra of synthetic Pyrope-grossular and almandine-spessartine solid solutions. Mode assignments were made based on a comparison of the different end-member garnet spectra and, in the case of Pyrope, based on measurements made on additional crystals synthesized with 26Mg. A general order of mode frequencies, i.e. R(SiO4)>T(metal cation)>T(SiO4), is observed, which should also hold for most orthosilicates. The main factors controlling the changes in mode frequencies as a function of composition are intracrystalline pressure (i.e. oxygen-oxygen repulsion) for the internal SiO4-vibrational modes and kinematic coupling of vibrations for the external modes. Low frequency vibrations of the X-site cations reflect their weak bonding and dynamic disorder in the large dodecahedral site, especially in the case of Pyrope. Two mode behavior is observed for X-site cation vibrations along the Pyrope-grossular binary, but not along the almandine-spessartine join.
John P. Brodholt - One of the best experts on this subject based on the ideXlab platform.
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relative strength of the Pyrope majorite solid solution and the flow law of majorite containing garnets
Physics of the Earth and Planetary Interiors, 2010Co-Authors: Simon A. Hunt, Donald J. Weidner, David P. Dobson, John P. BrodholtAbstract:Even though the garnet phase is the second most abundant phase in the upper-mantle and transition-zone, no previous studies have directly measured the effect of majorite content on the strength of garnet under mantle conditions. Here we report the results of constant strain-rate and stress-relaxation experiments on garnets in the Pyrope-majorite solid solution which constrain the strength of majoritic containing garnets relative to Pyrope as a function of majorite content and temperature. We find that at temperatures below 650 degrees C both pure Pyrope and majoritic garnets have the same strength. Conversely, above 650 degrees C we find that majoritic garnets are initially stronger than pure Pyrope but weaken with increasing temperature and majorite content and with significant majorite contents are weaker than Pyrope above approximately 800 degrees C. We develop a flow law for the entire Pyrope majorite solid solution as a function of temperature and majorite content. (C) 2010 Elsevier B.V. All rights reserved.
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Relative strength of the Pyrope-majorite solid solution and the flow-law of majorite containing garnets
Physics of the Earth and Planetary Interiors, 2010Co-Authors: Simon A. Hunt, Donald J. Weidner, David P. Dobson, John P. BrodholtAbstract:Even though the garnet phase is the second most abundant phase in the upper-mantle and transition- zone, no previous studies have directly measured the effect of majorite content on the strength of garnet under mantle conditions. Here we report the results of constant strain-rate and stress-relaxation experiments on garnets in the Pyrope-majorite solid solution which constrain the strength of majoritic containing garnets relative to Pyrope as a function of majorite content and temperature. We find that at temperatures below 650 ∘C both pure Pyrope and majoritic garnets have the same strength. Conversely, above 650 ∘C we find that majoritic garnets are initially stronger than pure Pyrope but weaken with increasing temperature and majorite content and with significant majorite contents are weaker than Pyrope above approximately 800 ∘C. We develop a flow law for the entire Pyrope-majorite solid solution as a function of temperature and majorite content.
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Relative strength of the Pyrope–majorite solid solution and the flow-law of majorite containing garnets
Physics of the Earth and Planetary Interiors, 2010Co-Authors: Simon A. Hunt, Donald J. Weidner, David P. Dobson, John P. BrodholtAbstract:International audienceEven though the garnet phase is the second most abundant phase in the upper-mantle and transition- zone, no previous studies have directly measured the effect of majorite content on the strength of garnet under mantle conditions. Here we report the results of constant strain-rate and stress-relaxation experiments on garnets in the Pyrope-majorite solid solution which constrain the strength of majoritic containing garnets relative to Pyrope as a function of majorite content and temperature. We find that at temperatures below 650 ∘C both pure Pyrope and majoritic garnets have the same strength. Conversely, above 650 ∘C we find that majoritic garnets are initially stronger than pure Pyrope but weaken with increasing temperature and majorite content and with significant majorite contents are weaker than Pyrope above approximately 800 ∘C. We develop a flow law for the entire Pyrope-majorite solid solution as a function of temperature and majorite content
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Relative strength of the Pyrope–majorite solid solution and the flow-law of majorite containing garnets
Physics of the Earth and Planetary Interiors, 2009Co-Authors: Simon A. Hunt, Donald J. Weidner, David P. Dobson, John P. BrodholtAbstract:Even though the garnet phase is the second most abundant phase in the upper-mantle and transition-zone, no previous studies have directly measured the effect of majorite content on the strength of garnet under mantle conditions. Here we report the results of constant strain-rate and stress-relaxation experiments on garnets in the Pyrope-majorite solid solution which constrain the strength of majoritic containing garnets relative to Pyrope as a function of majorite content and temperature. We find that at temperatures below 650 degrees C both pure Pyrope and majoritic garnets have the same strength. Conversely, above 650 degrees C we find that majoritic garnets are initially stronger than pure Pyrope but weaken with increasing temperature and majorite content and with significant majorite contents are weaker than Pyrope above approximately 800 degrees C. We develop a flow law for the entire Pyrope majorite solid solution as a function of temperature and majorite content. (C) 2010 Elsevier B.V. All rights reserved.
Donald J. Weidner - One of the best experts on this subject based on the ideXlab platform.
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relative strength of the Pyrope majorite solid solution and the flow law of majorite containing garnets
Physics of the Earth and Planetary Interiors, 2010Co-Authors: Simon A. Hunt, Donald J. Weidner, David P. Dobson, John P. BrodholtAbstract:Even though the garnet phase is the second most abundant phase in the upper-mantle and transition-zone, no previous studies have directly measured the effect of majorite content on the strength of garnet under mantle conditions. Here we report the results of constant strain-rate and stress-relaxation experiments on garnets in the Pyrope-majorite solid solution which constrain the strength of majoritic containing garnets relative to Pyrope as a function of majorite content and temperature. We find that at temperatures below 650 degrees C both pure Pyrope and majoritic garnets have the same strength. Conversely, above 650 degrees C we find that majoritic garnets are initially stronger than pure Pyrope but weaken with increasing temperature and majorite content and with significant majorite contents are weaker than Pyrope above approximately 800 degrees C. We develop a flow law for the entire Pyrope majorite solid solution as a function of temperature and majorite content. (C) 2010 Elsevier B.V. All rights reserved.
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Relative strength of the Pyrope-majorite solid solution and the flow-law of majorite containing garnets
Physics of the Earth and Planetary Interiors, 2010Co-Authors: Simon A. Hunt, Donald J. Weidner, David P. Dobson, John P. BrodholtAbstract:Even though the garnet phase is the second most abundant phase in the upper-mantle and transition- zone, no previous studies have directly measured the effect of majorite content on the strength of garnet under mantle conditions. Here we report the results of constant strain-rate and stress-relaxation experiments on garnets in the Pyrope-majorite solid solution which constrain the strength of majoritic containing garnets relative to Pyrope as a function of majorite content and temperature. We find that at temperatures below 650 ∘C both pure Pyrope and majoritic garnets have the same strength. Conversely, above 650 ∘C we find that majoritic garnets are initially stronger than pure Pyrope but weaken with increasing temperature and majorite content and with significant majorite contents are weaker than Pyrope above approximately 800 ∘C. We develop a flow law for the entire Pyrope-majorite solid solution as a function of temperature and majorite content.
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Relative strength of the Pyrope–majorite solid solution and the flow-law of majorite containing garnets
Physics of the Earth and Planetary Interiors, 2010Co-Authors: Simon A. Hunt, Donald J. Weidner, David P. Dobson, John P. BrodholtAbstract:International audienceEven though the garnet phase is the second most abundant phase in the upper-mantle and transition- zone, no previous studies have directly measured the effect of majorite content on the strength of garnet under mantle conditions. Here we report the results of constant strain-rate and stress-relaxation experiments on garnets in the Pyrope-majorite solid solution which constrain the strength of majoritic containing garnets relative to Pyrope as a function of majorite content and temperature. We find that at temperatures below 650 ∘C both pure Pyrope and majoritic garnets have the same strength. Conversely, above 650 ∘C we find that majoritic garnets are initially stronger than pure Pyrope but weaken with increasing temperature and majorite content and with significant majorite contents are weaker than Pyrope above approximately 800 ∘C. We develop a flow law for the entire Pyrope-majorite solid solution as a function of temperature and majorite content
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Relative strength of the Pyrope–majorite solid solution and the flow-law of majorite containing garnets
Physics of the Earth and Planetary Interiors, 2009Co-Authors: Simon A. Hunt, Donald J. Weidner, David P. Dobson, John P. BrodholtAbstract:Even though the garnet phase is the second most abundant phase in the upper-mantle and transition-zone, no previous studies have directly measured the effect of majorite content on the strength of garnet under mantle conditions. Here we report the results of constant strain-rate and stress-relaxation experiments on garnets in the Pyrope-majorite solid solution which constrain the strength of majoritic containing garnets relative to Pyrope as a function of majorite content and temperature. We find that at temperatures below 650 degrees C both pure Pyrope and majoritic garnets have the same strength. Conversely, above 650 degrees C we find that majoritic garnets are initially stronger than pure Pyrope but weaken with increasing temperature and majorite content and with significant majorite contents are weaker than Pyrope above approximately 800 degrees C. We develop a flow law for the entire Pyrope majorite solid solution as a function of temperature and majorite content. (C) 2010 Elsevier B.V. All rights reserved.
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Plastic flow of Pyrope at mantle pressure and temperature
American Mineralogist, 2006Co-Authors: Hongbo Long, Paul Raterron, Donald J. WeidnerAbstract:Despite the abundance of garnet in deforming regions of the Earth, such as subduction zones, its rheological properties are not well defined by laboratory measurements. Here we report measurements of steady-state plastic properties of Pyrope in its stability field (temperature up to 1573 K, pressure up to 6.8 GPa, strain rate {approx}10-5 s-1) using a Deformation-DIA apparatus (D-DIA) coupled with synchrotron radiation. Synthetic Pyrope (Py100) and natural Pyrope (Py70Alm16Gr14) are both studied in a dry environment. Transmission electron microscopy (TEM) investigation of the run products indicates that dislocation glide, assisted by climb within grains and dynamic recrystallization for grain-boundary strain accommodation, is the dominant deformation process in Pyrope. Both synthetic-and natural-Pyropes' stress and strain-rate data, as measured in situ by X-ray diffraction and imaging, are best fitted with the single flow law:
George R. Rossman - One of the best experts on this subject based on the ideXlab platform.
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Micro- and nano-size hydrogrossular-like clusters in Pyrope crystals from ultra-high-pressure rocks of the Dora-Maira Massif, western Alps
Contributions to Mineralogy and Petrology, 2020Co-Authors: Charles A Geiger, George R. RossmanAbstract:The supracrustal metamorphic rocks of the Dora-Maira Massif, western Alps, have been intensively studied. Certain ultra-high-pressure lithologies contain coesite and nearly end-member composition Pyrope, Mg_3Al_2Si_3O_12, making this locality petrologically and mineralogically unique. Structural OH^-, loosely termed “water”, in Pyrope crystals of different composition has been investigated numerous times, using different experimental techniques, by various researchers. However, it is not clear where the minor OH^- is located in them. IR single-crystal spectra of two Pyropes of composition {Mg_2.79,Fe^2+_0.15,Ca_0.04}_Σ2.98[Al]_2.02(Si)_2.99O_12 and {Mg_2.90,Fe^2+_0.04,Ca_0.02}_Σ2.96[Al]_2.03(Si)_3O_12 were recorded at room temperature (RT) and 80 K. The spectra show five distinct OH^- bands located above 3600 cm^-1 at RT and seven narrow bands at 80 K and additional fine structure. The spectra were curve fit and the OH^- stretching modes analyzed and assigned. It is argued that OH^- is located in microscopic- and nano-size Ca_3Al_2H_12O_12-like clusters. The basic substitution mechanism is the hydrogarnet one, where (H_4O_4)^4- ⇔ (SiO_4)^4-, and various local configurations containing different numbers of (H_4O_4)^4- groups define the cluster type. The amounts of H_2O range between 5 and 100 ppm by weight, depending on the IR calibration adopted, and are variable among crystals. Hydrogrossular-like clusters are also present in a synthetic Pyrope with a minor Ca content grown hydrothermally at 900 °C and 20 kbar, as based on its IR spectra at RT and 80 K. Experiment and nature are in agreement, and OH^- groups are partitioned into various barely nano-size hydrogrossular-like clusters. This proposal is new and significant mineralogical, petrological, and geochemical implications result. Ca and proton ordering occur. Hypothetical “defect” and/or coupled-substitution mechanisms to account for structural OH^- are not needed to interpret experimental results. OH^- incorporation in Pyrope of different generations at Dora-Maira is discussed and OH^- could potentially be used as an indicator of changing $$ P_{{{\text{H}}_{{\text{2}}} {\text{O}}}} {\text{(}}a_{{{\text{H}}_{{\text{2}}} {\text{O}}}} {\text{) - }}T $$ P H 2 O ( a H 2 O ) - T conditions in a metamorphic cycle. Published experimental hydration, dehydroxylation, and hydrogen diffusion results on Dora-Maira Pyropes can now be interpreted atomistically.
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Single-crystal IR- and UV/VIS-spectroscopic measurements on transition-metal-bearing Pyrope the incorporation of hydroxide in garnet
European Journal of Mineralogy, 2000Co-Authors: Charles A Geiger, Andreas Stahl, George R. RossmanAbstract:Pyrope single crystals doped with transition-metal ions (Co, Cr, Ni, Ti and V) were synthesised in a piston-cylinder device at 950–1050°C and 25 kbar. Stoichiometric oxide mixtures were used as starting materials and distilled water was used as a fluid flux. Crystals up to 2 mm in size were grown. Microprobe analysis and optical absorption spectroscopy were used to determine on which positions and in which oxidation states the transition-metal ions are incorporated in the Pyrope structure. Cr^(3+)-ions occupy the octahedral site and Co^(2+) and Ni^(2+) the dodecahedral site. Although extra metallic Ti was included in the synthesis of Ti-bearing Pyropes, only Ti^(4+) and no measurable Ti^(3+) could be stabilised on the octahedral site. The optical absorption spectra of V-bearing Pyropes show, in addition to the spin-allowed dd-transitions ^3T_(1g)(F) ^3T_(2g)(F) at ~ 17000 cm_(-1) and ^3T_(2g)(F) ^3T_(1g)(P) at ~ 20000 cm^(-1) corresponding to V^(3+) on the octahedral site, absorption bands which are thought to be caused by dd-transitions of V^(3+) in the tetrahedral site and V^(4+) on octahedral and tetrahedral sites. V^(4+) was not observed in silicate garnets before. IR spectra in the OH- stretching region between 4000 and 3000 cm^(-1), obtained on Pyrope single-crystals which only contain divalent and trivalent transition-metal ions like Ni^(2+), Co^(2+), and Cr^(3+), are similar to that normally shown by end-member Pyrope (Geiger et al., 1991). At room temperature the spectra show a single band at ≈ 3630 cm^(-1), which splits at ~ 79 K into two bands of smaller FWHM's at ∼ 3618 cm^(-1) and 3636 cm^(-1). These bands are assigned to OH–stretching modes resulting from the hydrogarnet substitution. The spectra of Ti^(4+)-bearing Pyrope measured at 298 K show four OH–stretching bands at approximately 3686, 3630, 3567 and 3527 cm^(-1). At ∼ 79 K the band at 3630 cm^(-1) splits into two narrow bands at 3636 cm^(-1) and 3614 cm^(-1). This suggests that additional OH- substitutional mechanisms occur in Ti-containing garnets. In the IR spectrum of a V^(4+)-bearing Pyrope the same number of OH-stretching bands is observed, suggesting that higher charged cations cause additional OH- substitutions and increased OH- concentrations in garnet. The IR spectra of most natural Pyrope-rich garnets appear to be different from those of the synthetics, which suggests that they are not characterised by the hydrogarnet substitution. However, the OH–substitution mechanism and concentrations in garnets from grospydite or similar parageneses are similar to those of the synthetics, which may reflect their formation in water-rich environments.
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Crystal Field Stabilization Energies of Almandine-Pyrope and Almandine-Spessartine Garnets Determined by FTIR Near Infrared Measurements
Physics and Chemistry of Minerals, 1994Co-Authors: Charles A Geiger, George R. RossmanAbstract:The band positions of three partially overlapping Fe2+ spin-allowed transitions located between 4000 and 9000 cm−1 in almandine-Pyrope and almandinespessartine garnets solid solutions were measured using near-infrared (NIR) spectroscopy. The crystal field stabilization energies (CFSE) along both binaries were calculated assuming a splitting of 1100 cm−1 for the lower orbitals. The CFSE show a slight increase along the almandine-spessartine binary from 3730 to 3810 cm−1 and a larger increase from 3730 to 3970 cm−1 for the almandine-Pyrope binary. Dodecahedral Fe2+-site distortion increases with an increase in spessartine component and decreases with increasing Pyrope component, in agreement with average dodecahedral site distortions determined from diffraction experiments. The excess CFSE's along both joins are negative. For the almandinespessartine binary they are small, but are about 3.5 times larger in magnitude along the join almandine-Pyrope, where an interaction parameter of W= -2.9 KJ/mole has been derived from a symmetric mixing model. The excess CFSE are relatively small compared to the magnitudes of the excess enthalpies of mixing that have been assigned to garnet solid solutions. Moreover, they give no indication which could support the positive and asymmetric excess enthalpies of mixing that have been proposed for almandine-Pyrope solid solutions.
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The hydroxide component in synthetic Pyrope
American Mineralogist, 1991Co-Authors: Charles A Geiger, George R. Rossman, Klaus Langer, David R. Bell, Bjoern WinklerAbstract:A series of Pyrope single crystals up to 2 mm in size was synthesized over a range of hydrothermal pressures of 20.0 to 50.0 kbar and temperatures of 800 to 1200 °C using different starting materials (oxides, glass, gel) and fluid fluxes (H_2O, NaOH, HCl). The crystals were characterized by optical, SEM, microprobe, and X-ray techniques. Single crystal Fourier-transform infrared (FTIR) spectroscopy was used to measure the incorporated structural OH^-. Spectra measured in the region of 4000-3000 cm^(-1) wavenumbers were different for all samples grown from oxides or glass vs. those grown from the gel at temperatures less than 1000 °C. In spectra obtained at room temperature the former are characterized by a single OH^- stretching vibration at 3629 cm^(-1), full widths at half-height (FWHH) = 60 cm^(-1), which is present regardless of the synthesis conditions (P, T or fluid flux). At 78 K, the single band splits into two narrow bands of FWHH of 11 cm^(-1) each. The unit-cell dimension of Pyrope increases up to 0.004 A with the incorporation of OH^-. The best interpretation of these data is that OH^- defects are introduced into the Pyrope structure as a hydrogarnet component where (O_4H_4)^(4-) = SiO^(4-)_4, i.e., by the substitution Si^(4+) + 4O^2 - = ^[4]□ + 40H^-. The amount of OH^- substitution into Pyrope ranges from 0.02 to 0.07 wt% expressed as H_2O. The infrared (IR) spectra of Pyropes grown from a gel starting material, at temperatures less than 1000 °C, display four band spectra, which indicate that OH^- substitution is not governed solely by the hydrogarnet substitution. Natural Pyrope-rich garnets generally have lower OH^- concentrations and more complicated IR spectra than the synthetic Pyrope crystals grown from oxides. This is assumed to be caused by crystal chemistry differences and probably different mechanisms of OH^- incorporation.
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Elastic Properties of Pyrope
Physics and Chemistry of Minerals, 1991Co-Authors: Bridget F. O'neill, Charles A Geiger, George R. Rossman, Jay D. Bass, Klaus LangerAbstract:Brillouin spectroscopy was used to measure the single crystal elastic properties of a pure synthetic Pyrope and a natural garnet containing 89.9 mol% of the Pyrope end member (Mg_3Al_2Si_3O_(12)). The elastic moduli, c_(ij) , of the two samples are entirely consistent and agree with previous estimates of the elastic properties of Pyrope based upon the moduli of solid solutions. Our results indicate that the elastic moduli of Pyrope end-member are c_(11)=296.2±0.5, c_(12)=111.1±0.6, c_(44)=91.6±0.3, K_s=172.8±0.3, μ=92.0±0.2, all in units of GPa. These results differ by several percent from those reported previously for synthetic Pyrope, but are based upon a much larger data set. Although the hydrous components of the two samples from the present study are substantially different, representing both ‘dry’ and ‘saturated’ samples, we find no discernable effect of structurally bound water on the elastic properties. This is due to the small absolute solubility of water in Pyrope, as compared with other garnets such as grossular.
Roberta Oberti - One of the best experts on this subject based on the ideXlab platform.
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Magnesium K-edge EXAFS study of bond-length behavior in synthetic Pyrope-grossular garnet solid solutions
American Mineralogist, 2008Co-Authors: Simona Quartieri, Federico Boscherini, Chiara Dalconi, Gianluca Iezzi, Carlo Meneghini, Roberta ObertiAbstract:Direct structural characterization of the changes in the local environment of Mg occurring in the garnet structure as a function of the Ca content are determined by Mg K -edge X-ray absorption fine structure on synthetic samples along the Pyrope-grossular join. With increasing Ca content, the short Mg-O2 distance of the dodecahedron slightly decreases, while the long Mg-O4 distance tends to increase, so that the dodecahedron is more distorted in grossular-rich garnets than in end-member Pyrope. This quantitative direct description of the changes in the local environment of Mg in the Pyrope-grossular solid solution confirms and better defines previous experimental and recent computational results.
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scandium 45 nmr of Pyrope grossular garnets resolution of multiple scandium sites and comparison with x ray diffraction and x ray absorption spectroscopy
American Mineralogist, 2007Co-Authors: N Kim, Jonathan F Stebbins, Simona Quartieri, Roberta ObertiAbstract:Here we present 45 Sc and 27 Al NMR results on Sc-doped Pyrope (Mg 3 Al 2 Si 3 O 12 ), grossular (Ca 3 Al 2 Si 3 O 12 ), and an 80% grossular-20% Pyrope garnet (grs80) that have recently been well-studied by X-ray diffraction and X-ray spectroscopies. Clearly distinct NMR peaks are observed for Sc in the eight-coordinated X site (Pyrope and grs80) and in the six-coordinated Y site (grossular and grs80). X-ray and NMR data agree that only eight-coordinated Sc is present in Pyrope and that six-coordinated Sc is predominant in grossular; however, the XRD results also indicated significant X and Z site (four-coordinated) Sc in the Ca-rich garnet. Possible reasons for this apparent discrepancy are discussed. We demonstrate that 45 Sc NMR is potentially a useful new method for studies of the site occupancies of Sc 3+ in oxides and silicates, at least in experimental systems where its concentration is a few percent or greater.
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distinct local environments for ca along the non ideal Pyrope grossular solid solution a new model based on crystallographic and exafs analysis
Chemical Geology, 2006Co-Authors: Roberta Oberti, Simona Quartieri, Gianluca Iezzi, Maria Chiara Dalconi, F Boscherini, Massimo BoiocchiAbstract:Abstract A multi-technique approach (based on electron microprobe analysis, structure refinement, and EXAFS analysis at the Ca K-edge) was used to characterise the local geometry of Ca in synthetic and natural garnet compositions referable to the Pyrope–grossular solid solution. Multi-shell fits of the EXAFS data indicate that Ca assumes the standard [4 + 4]-fold coordination (the polyhedral shape being a triangular dodecahedron with Ca1–O = 2.30–2.31(1) and Ca2–O = 2.45–2.46(1) A) when Ca > 1.50 atoms per formula unit (apfu), but assumes a nearly regular [8]-fold coordination with Ca–O = 2.35–2.36 (1) A when (Mg, Fe2+, Mn2+) > 1.50 apfu. Therefore, in the Pyrope-dominant structure the Ca1–O distance lengthens and the Ca2–O distance shortens to converge towards the value observed for the Mg2–O bond in Pyrope. This finding is consistent with many distinct structural features observed in solid solution terms with (Mg, Fe2+, Mn2+) > 1.50 apfu or Ca > 1.50 apfu, as well as with the anomalous properties of the intermediate terms observed both in the short-range and in the long-range perspective. The presence of two distinct Ca coordinations in the Pyrope (almandine, spessartine)-like and in the grossular-like structure, and thus of an isosymmetric transition at the intermediate composition, can help to explain both the strong and asymmetric non-ideality of the solid solution between Pyrope (almandine) and grossular, as well as the differences in the ability to incorporate some trace elements (such as REE and actinides) which are commonly used as process-specific indicators. This feature must be taken into account when building theoretical models of the garnet solid solutions, which are at the moment the most promising approach for calculating thermodynamic properties or for interpreting and predicting trace-element behaviour in this crucial mineral phase.
