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Xiaoming Sun - One of the best experts on this subject based on the ideXlab platform.
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onset and duration of zn pb mineralization in the talate pb zn fe skarn deposit nw china constraints from Spessartine u pb dating
Gondwana Research, 2018Co-Authors: Xiaoming SunAbstract:Abstract Distal Pb—Zn skarn commonly displays ambiguous relationships with the nearby intrusions, leading to poorly constrained and often controversial skarn metallogenic models. In this study, we discuss the potential usage of Spessartine, a common U-bearing gangue mineral in many Pb—Zn skarn deposits, in U—Pb dating. Spessartine from the Talate skarn Pb—Zn deposit (Xinjiang, NW China) can be divided into two types (GI and GII) based on mineral assemblage. GI Spessartine (Sp56.5 Al28.3 Gr12.4 to Sp66.0 Al22.3 Gr8.5) is coarse-grained with core-rim texture occasionally well-preserved, disseminated magnetite and magmatic TI titanite grains are hosted within the Spessartine GI core, indicating that the Spessartine GI is slightly postdate the magmatic titanite TI and disseminated magnetite. GII massive Spessartine (Sp64.2 Al24.8 Gr2.7 to Sp69.6 Al20.1 Gr1.2) occurs with hydrothermal titanite TII and shows a close relationship with coarse-grained hydrothermal magnetite, and is cut by quartz or fine sulfide (e.g., pyrite, pyrrhotite and chalcopyrite) veins. The flat time-resolved signals obtained from the depth profile analyses and inclusion-free trace element mapping of U in Spessartine indicate that the U is structurally bounded in the lattice. Both GI and GII Spessartine show clear positively correlation between U, Al and Mn, but no apparent correlation is present between U and Fe, which indicates that Al and Mn are more important in the U incorporation in Spessartine. Integrating with the positive correlations with U and LREEs in Spessartine, which suggest U is incorporated in the lattice by a substitution of [U4+]VIII + 2[Al3+]IV − [Ca2+]VIII + 2[Si4+]IV. In-situ U—Pb dating of the two Spessartine types yielded a weighted average 206Pb/238U age of 231.7 ± 7.2 Ma (GI; MSWD = 0.56; n = 35) and 211.5 ± 5.8 Ma (GII; MSWD = 1.1; n = 47). Considering the errors caused by common Pb and matrix mismatch, the corrected GI and GII Spessartine ages are consistent with the TI (228.0 ± 4.6 Ma; MSWD = 2; n = 28) and TII (209.8 ± 3.6 Ma; MSWD = 1.03; n = 39) titanite ages, suggesting that Spessartine U—Pb dating is robust and reliable. The least 8.1 My gap between the formation of the two titanite and spessartite types reflects two mineralization events, suggesting an influx of magmatic fluids during the retrograde alteration/mineralization. Our study represents a new way of directly constraining the timing of skarn alteration/mineralization.
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Onset and duration of Zn—Pb mineralization in the Talate Pb—Zn (—Fe) skarn deposit, NW China: Constraints from Spessartine U—Pb dating
Gondwana Research, 2018Co-Authors: Li Dengfeng, Xiaoming SunAbstract:Abstract Distal Pb—Zn skarn commonly displays ambiguous relationships with the nearby intrusions, leading to poorly constrained and often controversial skarn metallogenic models. In this study, we discuss the potential usage of Spessartine, a common U-bearing gangue mineral in many Pb—Zn skarn deposits, in U—Pb dating. Spessartine from the Talate skarn Pb—Zn deposit (Xinjiang, NW China) can be divided into two types (GI and GII) based on mineral assemblage. GI Spessartine (Sp56.5 Al28.3 Gr12.4 to Sp66.0 Al22.3 Gr8.5) is coarse-grained with core-rim texture occasionally well-preserved, disseminated magnetite and magmatic TI titanite grains are hosted within the Spessartine GI core, indicating that the Spessartine GI is slightly postdate the magmatic titanite TI and disseminated magnetite. GII massive Spessartine (Sp64.2 Al24.8 Gr2.7 to Sp69.6 Al20.1 Gr1.2) occurs with hydrothermal titanite TII and shows a close relationship with coarse-grained hydrothermal magnetite, and is cut by quartz or fine sulfide (e.g., pyrite, pyrrhotite and chalcopyrite) veins. The flat time-resolved signals obtained from the depth profile analyses and inclusion-free trace element mapping of U in Spessartine indicate that the U is structurally bounded in the lattice. Both GI and GII Spessartine show clear positively correlation between U, Al and Mn, but no apparent correlation is present between U and Fe, which indicates that Al and Mn are more important in the U incorporation in Spessartine. Integrating with the positive correlations with U and LREEs in Spessartine, which suggest U is incorporated in the lattice by a substitution of [U4+]VIII + 2[Al3+]IV − [Ca2+]VIII + 2[Si4+]IV. In-situ U—Pb dating of the two Spessartine types yielded a weighted average 206Pb/238U age of 231.7 ± 7.2 Ma (GI; MSWD = 0.56; n = 35) and 211.5 ± 5.8 Ma (GII; MSWD = 1.1; n = 47). Considering the errors caused by common Pb and matrix mismatch, the corrected GI and GII Spessartine ages are consistent with the TI (228.0 ± 4.6 Ma; MSWD = 2; n = 28) and TII (209.8 ± 3.6 Ma; MSWD = 1.03; n = 39) titanite ages, suggesting that Spessartine U—Pb dating is robust and reliable. The least 8.1 My gap between the formation of the two titanite and spessartite types reflects two mineralization events, suggesting an influx of magmatic fluids during the retrograde alteration/mineralization. Our study represents a new way of directly constraining the timing of skarn alteration/mineralization.
Charles A Geiger - One of the best experts on this subject based on the ideXlab platform.
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An analysis of the magnetic behavior of olivine and garnet substitutional solid solutions
American Mineralogist, 2019Co-Authors: Charles A Geiger, Michael Grodzicki, Edgar DachsAbstract:Abstract The low-temperature magnetic properties and Néel temperature, TN, behavior of four silicate substitutional solid solutions containing paramagnetic ions are analyzed. The four systems are: fayaliteforsterite olivine [Fe22+SiO4-Mg2SiO4], and the garnet series, grossular-andradite [Ca3(Alx,Fe1−x3+)2Si3O12], grossular-Spessartine [(Cax,Mn1−x2+)3Al2Si3O12], and almandine-Spessartine [(Fex2+,Mn1−x2+)3Al2Si3O12]. Local magnetic behavior of the transition-metal-bearing end-members is taken from published neutron diffraction results and computational studies. TN values are from calorimetric heat capacity, CP, and magnetic susceptibility measurements. These end-members, along with more transition-metal-rich solid solutions, show a paramagnetic to antiferromagnetic phase transition. It is marked by a CP λ-anomaly that decreases in temperature and magnitude with increasing substitution of the diamagnetic component. For olivines, TN varies between 65 and 18 K and TN for the various garnets is less than 12 K. Local magnetic behavior can involve one or more superexchange interactions mediated through oxygen atoms. TN behavior shows a quasi-plateau-like effect for the systems fayalite-forsterite, grossular-andradite, and grossular-Spessartine. More transition-metal-rich crystals show a stronger TN dependence compared to transition-metal-poor ones. The latter may possibly show superparamagnetic behavior. (Fex2+,Mn1−x2+)3Al2Si3O12 garnets show fundamentally different magnetic behavior. End-member almandine and Spessartine have different and complex interacting local superexchange mechanisms and intermediate compositions show a double-exchange magnetic mechanism. For the latter, TN values show negative deviations from linear interpolated TN values between the end-members. Double exchange seldom occurs in oxides, and this may be the first documentation of this magnetic mechanism in a silicate. TN behavior may possibly be used to better understand the nature of macroscopic thermodynamic functions, CP and S°, of both end-member and substitutional solid-solution phases.
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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.
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A calorimetric investigation of Spessartine: Vibrational and magnetic heat capacity
Geochimica et Cosmochimica Acta, 2009Co-Authors: Edgar Dachs, Charles A Geiger, Anthony C. Withers, Eric J. EsseneAbstract:Abstract The heat capacity (Cp) of two synthetic Spessartine samples (Sps) was measured on 20–30 mg-size samples in the temperature range 2–864 K by relaxation calorimetry (RC) and differential scanning calorimetry (DSC). The polycrystalline Spessartine samples were synthesized in two different laboratories at high pressures and temperatures from glass and oxide-mixture starting materials and characterized by X-ray powder diffraction and electron-microprobe analysis. The low-temperature heat capacity data show a prominent lambda transition with a peak at 6.2 K, which is interpreted to be the result of a paramagnetic–antiferromagnetic phase transition. The DSC data around ambient T agree excellently with the RC data and can be represented by the Cp polynomial for T > 250 K: C p Sps = 610 - 3060 · T - 0.5 - 1.45 · 10 7 · T - 2 + 1.82 · 10 9 · T - 3 . Integration of the low temperature Cp data yields a calorimetric standard entropy for the two different samples of So = 334.6 ± 2.7 J/mol · K and 336.0 ± 2.7 J/mol · K. The preferred standard third-law entropy for Spessartine is So = 335.3 ± 3.8 J/mol · K, which is the mean value from the two separate determinations. The lattice (vibrational) heat capacity of Spessartine was calculated using the single-parameter phonon dispersion model of Komada and Westrum. The lattice entropy at 298.15 K is S vib 298.15 = 297.7 J / mol · K , which represents 89% of the calorimetric entropy. The magnetic heat capacity and entropy of Spessartine, Smag, at 298.15 K were also calculated. The Smag of the two samples is 38.7 and 37.4 J/mol · K, which is 87% and 83% of the maximum possible magnetic entropy given by 3Rln6 = 44.7 J/mol · K. Published model-dependent lattice-dynamic calculations of S vib 298.15 are analyzed and compared to the experimental data. Using the calorimetrically determined So and the Cp polynomial for Spessartine, together with high P–T experimental phase-equilibrium data on Mn2+–Mg partitioning between garnet and olivine, allows calculation of the standard enthalpy of formation of Spessartine. This gives Δ H f , Sps o = - 5693.6 ± 1.4 kJ / mol , a value nearly 50 kJ more negative than some published values. The Gibbs free energy of Spessartine was also calculated and gives Δ G f , Sps o = - 5364.3 kJ / mol at 298.15 K. The new standard entropy and enthalpy of formation values for Spessartine lead to revised estimates for the enthalpies of formation of other Mn2+-silicates. Resulting Δ H f o values for Mn-biotite, Mn-chlorite, Mn-cordierite, Mn-staurolite and Mn-chloritoid are 7–34 kJ more negative than their values listed in the thermodynamic database “THERMOCALC”. As an example, the new standard entropy and enthalpy of formation for Spessartine have been applied to Mn–Fe partitioning between garnet and orthopyroxene from manganiferous iron formations. Excellent agreement between the predicted and observed distribution coefficient was obtained.
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Fe2+-O and Mn2+-O bonding and Fe2+- and Mn2+-vibrational properties in synthetic almandine-Spessartine solid solutions: an X-ray absorption fine structure study
European Journal of Mineralogy, 2004Co-Authors: Alessandra Sani, Simona Quartieri, Federico Boscherini, G. Antonioli, Anne Feenstra, Charles A GeigerAbstract:X-ray absorption fine structure (XAFS) measurements at the Fe and Mn K-edges were undertaken to investigate the Fe2+-O and Mn2+-O bonding, and Fe2+- and Mn2+-vibrational properties for a series of synthetic almandine-Spessartine, (Fe,Mn)(3)Al2Si3O12, garnet solid solutions. The two end members almandine, Fe3Al2Si3O12, and Spessartine, Mn3Al2Si3O12, and three solid solutions of composition Alm75Sps25, Alm50Sps50 and Alm25Sps75 were studied at different temperatures between 77 K and 423 K. The spectra show that the state of alternating bonds provides an appropriate description for the two crystallographic ally independent X-O(1) and X-O(2) bonds in the solid solution. An exchange of X-site cations in garnet is associated with measurable structural relaxation. The compositional and temperature dependence of the XAFS Debye-Waller factors for Fe2+ and Mn2+ in the plane of the X-O(1) and the X-O(2) bonds were also determined. Their values in the plane of the X-O(2) bonds are greater compared to those for X-O(1) and they do not vary greatly as a function of garnet composition. In the case of end-member Spessartine, the XAFS Debye-Waller factors for Mn2+ are compared to the Debye-Waller factors measured from single-crystal X-ray diffraction. An analysis shows correlation in the atomic displacements along the shorter Mn-O(1) bond.
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properties in synthetic almandine-Spessartine solid solutions: an X-ray absorption fine structure study
2004Co-Authors: Alessandra Sani, Charles A Geiger, Simona Quartieri, Federico Boscherini, G. Antonioli, Anne Feenstra, Salita SperoneAbstract:X-ray absorption fine structure (XAFS) measurements at the Fe and Mn K-edges were undertaken to investigate the Fe 2+ -O and Mn 2+ -O bonding and Fe 2+ -a nd Mn 2+ -vibrational properties for a series of synthetic almandine-Spessartine, (Fe,Mn)3Al2Si3O12, garnet solid solutions. The two end members almandine, Fe3Al2Si3O12, and Spessartine, Mn3Al2Si3O12, and three solid solutions of composition Alm75Sps25, Alm50Sps50 and Alm25Sps75 were studied at different temperatures between 77 K and 423 K. The spectra show that the state of alternating bonds provides an appropriate description for the two crystallographically independent X-O(1) and X-O(2) bonds in the solid solution. An exchange of X-site cations in garnet is associated with measurable structural relaxation. The compositional and temperature dependence of the XAFS Debye-Waller factors for Fe 2+ and Mn 2+ in the plane of the X-O(1) and the X-O(2) bonds were also determined. Their values in the plane of the X-O(2) bonds are greater compared to those for X-O(1) and they do not vary greatly as a function of garnet composition. In the case of end-member Spessartine, the XAFS Debye-Waller factors for Mn 2+ are compared to the Debye-Waller factors measured from single-crystal X-ray diffraction. An analysis shows correlation in the atomic displacements along the shorter Mn-O(1) bond.
A. R. Cabral - One of the best experts on this subject based on the ideXlab platform.
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NEGATIVE CERIUM ANOMALY IN Spessartine GARNET FROM A BARITE-RICH ROCK, OTJOSONDU FERROMANGANESE DEPOSIT, NAMIBIA: THE FINGERPRINT OF OXIC SEA WATER
South African Journal of Geology, 2012Co-Authors: A. R. Cabral, J.m. MooreAbstract:Otjosondu, a Neoproterozoic ferromanganese deposit located about 130 km north-east of Okahandja, Namibia, has a garnet-braunite-barite rock containing ~32 weight % barium. The garnet is intergrown with barite and has end-member proportions as follows: Spessartine (48 to 55 mol %), grossular (25 to 35 mol %), andradite (7 to 13 mol %) and, notably, blythite (4 to 9 mol % Mn(II) 3 Mn(III) 2 [SiO 4 ] 3 ). Spot measurements for rare-earth elements (REE) and yttrium indicate that the garnet is characterised by a shale-normalised negative anomaly of cerium that is typical of oxic sea water. The garnet has Y/Ho ratios between 18 and 25, a range of values indicative of aqueous fractionation of yttrium from holmium, as observed in marine ferromanganese deposits formed in oxic sea water. The REE patterns of the Otjosondu garnet drastically differ from those found in garnet from Broken Hill-type deposits in Australia, particularly in the absence of cerium anomalies in the latter. The Otjosondu garnet is thought to be metamorphosed hydrothermally derived ferromanganese precipitates in oxic sea water. The abundance of barite and its coexistence with the Spessartine-rich garnet suggest that Otjosondu may represent an oxidised variety of Broken Hill-type, sediment-hosted exhalative mineralisation.
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GEOCHEMICAL AND MINERALOGICAL CONSTRAINTS ON THE GENESIS OF THE OTJOSONDU FERROMANGANESE DEPOSIT, NAMIBIA: HYDROTHERMAL EXHALATIVE VERSUS HYDROGENETIC (INCLUDING SNOWBALL-EARTH) ORIGINS
South African Journal of Geology, 2011Co-Authors: A. R. Cabral, J.m. Moore, Benjamin Mapani, M. Koubová, C.-d. SattlerAbstract:The stratigraphic position of the Otjosondu ferromanganese deposit in the Neoproterozoic Damara Supergroup in the Central Zone of the Pan-African Damara orogenic belt is uncertain. Comparison of the enclosing host rocks with regional profiles in the southern Central Zone would indicate that it is hosted by the Etusis Formation (Fe-rich feldspar-quartz rocks) of the Nosib Group, in a condensed stratigraphic section of the supergroup, and not the Chuos Formation (diamictite, turbidite and thin iron-formation layers) of the Swakop Group, as previously proposed. The Otjosondu ore comprises ferromanganese silicate-oxide assemblages that occur in a feldspathic-quartzite sequence lacking the mixtite facies typical of the Chuos Formation. Hematitic iron formation containing minor hyalophane, Spessartine and barite, and hyalophane-bearing Spessartine-quartz rock are present in close spatial association with the Mn silicate-oxide rocks. Barite is a widespread and locally abundant mineral in the Mn silicate-oxide ore. Bulk-rock chemical analyses indicate trace-element associations of Ag-As-Cu-Co-Bi-B-P with Mn and Sr-Mo-W with Ba in the Mn silicate-oxide and Spessartine-quartz rocks. These element associations are explained by adsorption/substitution processes in an oxygenated alkaline marine setting. Sulfur-isotope compositions of barite from the Mn silicate-oxide ore vary from +10.1 to +16.7 ‰, a range of δ34S values that shows negative correlation with bulk-rock Fe content and is interpreted as mixing of isotopically light hydrothermal S with heavier sea-water S. The protoliths of the ferromanganese rocks were precipitates deposited in a basin-margin setting as a result of mixing of an exhalative hydrothermal fluid with oxygenated, sulfate-bearing sea water, and varying degrees of detrital addition. These conclusions are in contradiction to the deep-water, anoxic, sulfate-depleted oceanic settings required for formation of glacially associated Rapitan-type iron and manganese formations. The formation of the Otjosondu ferromanganese deposit is therefore unrelated to any Chuos/Sturtian global glacial event. Origins as hydrogenetic deposits are mostly excluded due to the base-metal- and P-poor, but Ba-rich nature of the Otjosondu ores. The baritic ferromanganese, hematite-quartz and Spessartine-quartz rocks at Otjosondu may be compared to oxidised baritic portions of the Gamsberg (Broken Hill-type) base-metal deposit in South Africa and may represent the distal or shallow-water oxidised equivalents of sediment-hosted exhalative base-metal mineralisation such as the penecontemporaneous Tsongoari and Rosh Pinah deposits.
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Spessartine tourmaline bearing manganiferous itabirite at miguel congo petrographic evidence for a mn rich metaexhalite in the quadrilatero ferrifero of minas gerais brazil
Applied Earth Science, 2003Co-Authors: A. R. CabralAbstract:Abstract For the first time, petrographic and electron-microprobe data are provided to demonstrate the existence of a Spessartine–tourmaline-bearing itabirite in the Quadrilatero Ferrifero of Minas Gerais, Brazil. This mineral assemblage gives evidence for a metamorphosed B-bearing Fe–Mn-rich sediment, possibly of exhalative origin.
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Spessartine–tourmaline-bearing manganiferous itabirite at Miguel Congo: petrographic evidence for a Mn-rich metaexhalite in the Quadrilátero Ferrífero of Minas Gerais, Brazil
Applied Earth Science, 2003Co-Authors: A. R. CabralAbstract:Abstract For the first time, petrographic and electron-microprobe data are provided to demonstrate the existence of a Spessartine–tourmaline-bearing itabirite in the Quadrilatero Ferrifero of Minas Gerais, Brazil. This mineral assemblage gives evidence for a metamorphosed B-bearing Fe–Mn-rich sediment, possibly of exhalative origin.
Alok K. Gupta - One of the best experts on this subject based on the ideXlab platform.
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Experimental study of the system grossularite-Spessartine at P H 2 0 = P Total 1 kbar and variable temperatures
1993Co-Authors: Rajat Mazumder, T. L. Venkatesh, Alok K. GuptaAbstract:The system grossulatite-Spessartine hasbeen studied at 1 kbar and 700, 750 and 80°C. Based onX-ray diffraction study and refractive indexmeasurement it was observed that there is continuoussolid solution relationship between the two endmembers; there is thus no basis for including grossulariteand Spessartine into two separate groups on the basis ofthe assumption that there may be immiscible relation between these two garnets.
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experimental study of the system grossularite Spessartine at p h 2 0 p total 1 kbar and variable temperatures
1993Co-Authors: Rajat Mazumder, T. L. Venkatesh, Alok K. GuptaAbstract:The system grossulatite-Spessartine hasbeen studied at 1 kbar and 700, 750 and 80°C. Based onX-ray diffraction study and refractive indexmeasurement it was observed that there is continuoussolid solution relationship between the two endmembers; there is thus no basis for including grossulariteand Spessartine into two separate groups on the basis ofthe assumption that there may be immiscible relation between these two garnets.
Anne Feenstra - One of the best experts on this subject based on the ideXlab platform.
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Fe2+-O and Mn2+-O bonding and Fe2+- and Mn2+-vibrational properties in synthetic almandine-Spessartine solid solutions: an X-ray absorption fine structure study
European Journal of Mineralogy, 2004Co-Authors: Alessandra Sani, Simona Quartieri, Federico Boscherini, G. Antonioli, Anne Feenstra, Charles A GeigerAbstract:X-ray absorption fine structure (XAFS) measurements at the Fe and Mn K-edges were undertaken to investigate the Fe2+-O and Mn2+-O bonding, and Fe2+- and Mn2+-vibrational properties for a series of synthetic almandine-Spessartine, (Fe,Mn)(3)Al2Si3O12, garnet solid solutions. The two end members almandine, Fe3Al2Si3O12, and Spessartine, Mn3Al2Si3O12, and three solid solutions of composition Alm75Sps25, Alm50Sps50 and Alm25Sps75 were studied at different temperatures between 77 K and 423 K. The spectra show that the state of alternating bonds provides an appropriate description for the two crystallographic ally independent X-O(1) and X-O(2) bonds in the solid solution. An exchange of X-site cations in garnet is associated with measurable structural relaxation. The compositional and temperature dependence of the XAFS Debye-Waller factors for Fe2+ and Mn2+ in the plane of the X-O(1) and the X-O(2) bonds were also determined. Their values in the plane of the X-O(2) bonds are greater compared to those for X-O(1) and they do not vary greatly as a function of garnet composition. In the case of end-member Spessartine, the XAFS Debye-Waller factors for Mn2+ are compared to the Debye-Waller factors measured from single-crystal X-ray diffraction. An analysis shows correlation in the atomic displacements along the shorter Mn-O(1) bond.
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properties in synthetic almandine-Spessartine solid solutions: an X-ray absorption fine structure study
2004Co-Authors: Alessandra Sani, Charles A Geiger, Simona Quartieri, Federico Boscherini, G. Antonioli, Anne Feenstra, Salita SperoneAbstract:X-ray absorption fine structure (XAFS) measurements at the Fe and Mn K-edges were undertaken to investigate the Fe 2+ -O and Mn 2+ -O bonding and Fe 2+ -a nd Mn 2+ -vibrational properties for a series of synthetic almandine-Spessartine, (Fe,Mn)3Al2Si3O12, garnet solid solutions. The two end members almandine, Fe3Al2Si3O12, and Spessartine, Mn3Al2Si3O12, and three solid solutions of composition Alm75Sps25, Alm50Sps50 and Alm25Sps75 were studied at different temperatures between 77 K and 423 K. The spectra show that the state of alternating bonds provides an appropriate description for the two crystallographically independent X-O(1) and X-O(2) bonds in the solid solution. An exchange of X-site cations in garnet is associated with measurable structural relaxation. The compositional and temperature dependence of the XAFS Debye-Waller factors for Fe 2+ and Mn 2+ in the plane of the X-O(1) and the X-O(2) bonds were also determined. Their values in the plane of the X-O(2) bonds are greater compared to those for X-O(1) and they do not vary greatly as a function of garnet composition. In the case of end-member Spessartine, the XAFS Debye-Waller factors for Mn 2+ are compared to the Debye-Waller factors measured from single-crystal X-ray diffraction. An analysis shows correlation in the atomic displacements along the shorter Mn-O(1) bond.
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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.
