Local structural heterogeneity,mixing behaviour and saturation effects in the grossular–spessartine solid solution

Local structural heterogeneities in crystals of the binary grossular–spessartine solid solution have been analyzed using powder IR absorption spectroscopy. Wavenumber shifts of the highest energy Si–O stretching mode in spectra collected at room temperature are consistent with variations in Si–O bond length from structural data. They show a smaller positive deviation from linearity across the join than is seen for the grossular–pyrope and grossular–almandine binaries. The effective line widths, corr, of three selected wavenumber regions all deviate positively from linear behaviour. An empirical calibration of this excess spectroscopic property, obtained by comparison with calorimetric enthalpy of mixing data, gives an estimate for the symmetric Margules parameter of W^H_spec = 14.4(7) kJ mol^–1 in H^mix = W^H_specX_GrX_Sp. W^H_spec values derived on the same basis for four aluminosilicate garnet solid solutions analyzed by IR spectroscopy vary with V², where V represents the difference in molar volume between the end members of each binary system. Measurements of lattice parameters and IR spectra were made over a range of temperatures for seven samples with different compositions. Positive excess molar volumes of mixing at low temperature (30 K) may be larger than the excess molar volumes at room temperature. The saturation temperatures of the molar volumes show no correlation with composition, however, in contrast with what had been expected on the basis of data for the grossular–pyrope binary. Saturation temperatures for spectroscopic parameters and lattice parameters of samples with compositions Gr15Sp85 and Gr60Sp40 seem to be outliers in all experiments. It is concluded that the data hint at systematic changes in saturation temperatures across the solid solution, with implications for both the excess entropy of mixing and the excess volume of mixing, but more precise data or further sample characterization are needed to prove that this composition dependence is real in garnet solid solutions.     

Thermodynamic mixing behavior of synthetic Ca-Tschermak–diopside pyroxene solid solutions: III. An analysis of IR line broadening and heat of mixing behavior

A series of synthetic Ca-Tschermak–diopside (CaAlAlSiO₆–CaMgSi₂O₆) clinopyroxenes were investigated by powder infrared spectroscopy at room temperature in the wavenumber range 80–2,000 cm⁻¹. Measurable local structural heterogeneities in the crystals are suggested by the line broadening parameter, Δcorr that are observed for intermediate solid-solution compositions. The broadening is most pronounced in the high wavenumber region of the IR spectra that contains stretching modes involving the TO₄ polyhedra. The effective line widths for three selected wavenumber regions deviate positively from linear behavior. This is also observed for the enthalpy of mixing of this solid solution. The relationship between “excess Δcorr”, δΔcorr, and heat of mixing, ΔH _mix, behavior was investigated for this clinopyroxene series and for several other binary silicate solid solutions. The ΔH _mix versus δΔcorr slope values show a linear relationship with respect to the integrated excess volume of the various solid solutions.     

Synthesis and stability of the garnet calderite in the system Fe-Mn-Si-O

The rare garnet end member calderite, Mn ₃ ²⁺ Fe ₂ ³⁺ Si₃O₁₂, has been synthesized, under the oxygen fugacity of the hematite/magnetite buffer, at pressures not lower than 22 kbar. The synthetic crystals are generally zoned and may contain up to 3 mol% of the Fe²⁺-(=skiagite-) or 10 mol% of the Mn³⁺-(=blythite-) end members, but, under equilibrium conditions, the stable garnets have a restricted compositional range with about 1–4 mol% skiagite. Mn³⁺ represents only a residue of the Mn₂O₃ used in the starting material. At temperatures above 720° C (at 24 kbar) to 850° C (at 30 kbar) these garnets break down into coexisting pairs of magnetite-jacobsite and pyroxmangite-FeSiO₃ solid solutions. No indication for divariancy of this breakdown reaction could be established so that the observed coexistence of garnet and breakdown products over a PT interval must be due to disequilibrium. Although extrapolations of the high-pressure stability data towards lower pressures are hazardous, it is clear that nearly pure calderite garnets can only form in metamorphic environments characterized by geothermal gradients not exceeding some 10°–15° C/km, that is in subduction zone metamorphism. A low-pressure end of the calderite stability is likely because, at temperatures below 250°–300° C, pyroxmangite probably becomes unstable and hydrous Mn²⁺-silicates appear among the low-temperature breakdown products of calderite. Since the upper temperature stability limits of the common garnet end members spessartine and andradite lie some 800° C above that for calderite, solid solutions with these components will drastically stabilize the garnet phase towards both higher temperatures and lower pressures. This explains why garnets containing around 70 mol% calderite can be formed in amphibolitefacies metamorphism.     

Evaluation of thermochemical data on Fe-Mg olivine,orthopyroxene, spinel and Ca-Fe-Mg-Al garnet

Thermochemical data on Fe-Mg olivine, orthopyroxene, spinel and Ca-Fe-Mg garnet have been tested and reevaluated in reproducing experimental equilibrium data. All data (except of spinel) adjusted in this process lie within the error limits of original calorimetric experiments. For spinel, an enthalpy of −2307.2 kJ/mol and an entropy of 81.5 J/mol-K has been recommended. Recommended interaction parameters for the spinel-hercynite and forsterite-fayalite solutions are as follows:Spinel: W_{spinel-hercynite} = 9124.0 J/mol. W_{hercynite-spinel} = 0.0 J/molOlivine: W = 4500.0 J/mol for 1 cation.Excess entropies (on 1 cation basis) necessary to reproduce phase equilibria for the pyrope-almandine and almandine-grossular solutions are as follows:Mg-Fegarnet: W^s_{pyrope-almandine} = 11.760 − 0.00167 J/mol-K. W^s_{almandine-pyrope} = −10.146 +0.0037T J/mol-K.Fe-Ca garnet: W^s = −16.07 + 0.0126T J/mol-K.     

Effects of cation substitutions in garnet and pyroxene on equilibrium oxygen isotope fractionations

High-grade metamorphic rocks were used to explore oxygen isotope fractionations between pyroxene and garnet, and to investigate the effects on fractionation factors of the cation substitutions Fe³⁺Al_?1 and Ca(Fe,Mg)_?1. Recrystallized, granulite facies (725 °C) wollastonite ores from the northern Adirondack highlands contain essentially only the minerals clinopyroxene (a Di–Hd solid solution)+garnet (a Grs–Adr solid solution)±wollastonite, and exhibit a systematic dependence of measured fractionations on the Fe³⁺ content of calcic garnet: Δ(Cpx–CaGrt)=(0.14±0.12)+(0.78±0.20)X_Adr and Δ(Wo–CaGrt)=(0.15±0.22)+(0.57±0.33)X_Adr. In eclogites formed at T ≤650 °C, measured compositions of Ca-poor garnet and omphacite combined with experimental data indicate that Ca-poor, Fe-rich garnet is enriched in ¹⁸O compared to both diopside and grossular: extrapolating to 1000 K, Δ(Alm–Di)≈c. 0.2 and Δ(Alm–Grs)≈c. 0.5. Orthopyroxene and clinopyroxene from Gore Mountain, New York, show a constant fractionation that is independent of rock type, as expected if they have the same closure temperature. These data imply Δ(Opx-Cpx)≈c. 0.7 at 1000 K. Measured fractionations among Ca-poor garnet, orthopyroxene, clinopyroxene and hornblende in the Gore Mountain rocks further indicate an ¹⁸O enrichment in Ca-poor garnet over Grs (≈c. 0.5 at 1000 K). The new measurements are indistinguishable from expected equilibrium values based on experiments for the minerals enstatite, diopside, grossular, wollastonite and feldspar, but consistently indicate a significant isotope effect for the simple octahedral cation substitutions Fe³⁺Al_?1 (Grs vs. Adr) and Ca(Fe,Mg)_?1 (Ca-poor garnet vs. Grs; Opx vs. Cpx). Neither cation substitution has been directly investigated for its effect on ¹⁸O/¹⁶O fractionation with experiments in silicates. Chemical characterization of minerals is required prior to petrological interpretation of oxygen isotope trends.     

Short-range order in tourmaline: a vibrational spectroscopic approach to elbaite

Polarized Fourier-transform infrared and Raman spectra were acquired on an elbaite sample previously characterized by electron- and ion microprobe analysis, X-ray diffraction and structure refinement. Spectra from the two vibrational spectroscopy techniques reveal a close similarity in the OH-stretching region, with three main absorption bands strongly polarized in the c-axis direction. By means of bond-valence theory arguments, the observed OH bands are interpreted and assigned to specific local cation arrangements around the O1 (≡W) and O3 (≡V) anion sites. In combination with the relatively simple composition of the studied sample, bond-valence constraints are used to identify stable anion-cation arrangements, which permit the occurrence of short-range ordering to be assessed. Evidence for nearly complete short-range order at the O1 site, with the stable arrangements ^Y(LiAlAl)_0.6–^W(OH)_0.6 and ^Y(LiLiAl)_0.4–^W(F)_0.4, are presented. These two local arrangements can be further expanded to obtain the larger ordered clusters [^W(OH)–^Y(LiAl₂)–^V(OH)₃–^Z(Al)₆]_0.6 and [^W(F)–^Y(Li₂Al)–^V(OH)₃–^Z(Al)₆]_0.4.     

Chemical composition and evolution of the garnets in the Astamal Fe-LREE distal skarn deposit,Qara-Dagh–Sabalan metallogenic belt,Lesser Caucasus,NW Iran

The chemistry of garnet can provide clues to the formation of skarn deposits. The chemical analyses of garnets from the Astamal Fe-LREE distal skarn deposit were completed using an electron probe micro-analyzer. The three types of garnet were identified in the Astamal skarn are: (I) euhedral coarse-grained isotropic garnets (10–30 mm across), which are strongly altered to epidote, calcite and quartz in their rim and core, with intense pervasive retrograde alteration and little variation in the overall composition (Adr_94.3–84.4 Grs_8.5–2.7 Alm_1.9–0.2) (garnet I); (II) anhedral to subhedral brecciated isotropic garnets (5–10 mm across) with minor alteration, a narrow compositional range along the growth lines (Adr_82–65.4 Grs_21.9–11.7 Alm_11.1–2.4) and relatively high Cu (up to 1997 ppm) and Ni (up to 1283 ppm) (garnet II); and (III) subhedral coarser grained garnets (> 30 mm across) with moderate alteration, weak diffusion and irregular zoning of discrete grossular-almandine-rich domains (Adr_84.2–48.8 Grs_32.4–7.6 Alm_19.9–3.5) (garnet III). In the third type, the almandine content increases with increasing grossular/andradite ratio and increasing substitutions of Al for Fe^3 +.Almost all three garnet types have been replaced by fine-grained, dark-brown allanite that is typically disseminated and has the same relief as andradite. The Cu content increases while Ni content decreases slightly towards the rim of garnet II and garnet III. Copper in garnet II is positively correlated with increasing almandine content and decreasing andradite content, indicating that the almandine structure, containing relatively more Fe^2 +, is more suitable than andradite and grossular to host divalent cations such as Cu^2 +. Nickel in garnet II is positively correlated with increasing andradite content, total Fe, and decreasing almandine content. This is because Ni^2 + substitutes for Fe^3 + in the Y (octahedral) position. There are unusual discrete grossular-almandine rich domains within andraditic garnet III, indicating the low diffusivity of Ca compared to Fe at high temperatures.     

The crystal chemistry and Fe<Superscript>II</Superscript>–site properties of aluminosilicate garnet solid solutions as revealed by Mössbauer spectroscopy and electronic structure calculations

The three binary garnet solid solutions Fe^II₃Al₂Si₃O₁₂–X^II₃Al₂Si₃O₁₂ (X^II= Mg^II, Mn^II, Ca^II) have been investigated by ⁵⁷Fe Mössbauer spectroscopy at 298 and 77 K and by electronic structure calculations in the local spin density approximation. The spectra yield isomer shifts and quadrupole splittings that are typical for Fe^II in the dodecahedral X-site of 222 point symmetry and are similar for each of the three binaries recorded. Conversely, electronic structure calculations based on the experimental crystal structure of the different end-member garnets exhibit pronounced variations in some of the electronic properties of Fe^II that are not reflected in the spectroscopic data. These results are interpreted as indicating that the different X–O bonds in garnet solid solutions retain to a large degree the intrinsic lengths that they possess in their respective end members, and that the Fe–O bond does not change greatly as a function of composition. This is evidence for the state of alternating bonds and not for the virtual crystal approximation in describing the X–O bond types or lengths in aluminosilicate garnet solid solutions. The observed degree and behavior of the Fe^II doublet asymmetry in the Mössbauer spectra for the three solid solution series do not indicate major variations in the anisotropic recoil-free fraction of Fe^II. Variations in doublet asymmetry are more likely a result of complex next-nearest X-site neighbor interactions and/or some degree of short-range cation ordering, though doublets representing different local X-site cation configurations cannot be resolved or fitted to the experimental spectra.     

Anomalous birefringence in andradite–grossular solid solutions: a quantum-mechanical approach

The static linear optical properties (refractive indices, birefringence and axial angle) of andradite–grossular (Ca₃Fe₂Si₃O₁₂–Ca₃Al₂Si₃O₁₂) solid solutions have been computed at the ab initio quantum-mechanical level through the Coupled Perturbed Kohn–Sham scheme, using an all-electron Gaussian-type basis set. Geometry relaxation after substitution of 1–8 Al for Fe atoms in the primitive cell of andradite yields 23 non-equivalent configurations ranging from cubic to triclinic symmetry. Refractive indices vary quite regularly between the andradite (1.860) and grossular (1.671) end-members; the birefringence δ and the axial angle 2V at intermediate compositions can be as large as 0.02° and 89°, respectively. Comparison with experiments suffers from inhomogeneities and impurities of natural samples; however, semi-quantitative agreement is observed.     

Hydrogen in some natural garnets studied by nuclear reaction analysis and vibrational spectroscopy

A suite of 11 gem-quality, optically completely clear garnet crystals with a broad variety of compositions in the space of the end members pyrope–almandine–spessartine–grossular–andradite–goldmanite were analyzed for trace amounts of “water” by nuclear reaction analysis, NRA, based on the reaction ¹H(¹⁵N, αγ)¹²C, and by single-crystal absorption spectroscopy in the ν_OH vibrational range using microscope-FTIR-spectroscopic methods. The aim was to establish a calibration of the highly sensitive IR method with high areal resolution for “water” determination in garnets, by studying garnets of a wide compositional range, and to check for compositional dependencies of the integral molar absorptivities of the “water” component, ?_int[1mol_H2O^?1cm^?2], in the nominally “water”-free garnets. The results of NRA show a broad variation of water contents in the range (14 ± 3) to (950 ± 80) wt ppm_H2O, the values being low and very high for the garnet solid solutions (PyrAlm)_SS and close-to-end-member Gross_SS, respectively. There were no indications of inhomogeneities in the OH distribution, except possibly for one of the garnets (grossular, variety hessonite, from Tanzania). The quantitative evaluation of the complex ν_OH spectra, which showed similar shape only for members of the (PyrAlm)_SS, yielded integral absorption coefficients, α_int (cm^?2), which allowed the calculation of integral molar absorptivities, ?_int, using the “water” values of NRA. The ?_int values obtained varied in a wide range but with no obvious correlation with the composition of the garnet except for the extremely high values, in the 10⁴ range, of the two specimen with compositions close to end-member grossular. In all other garnets, ?_int was in the 10³ range with an average of ?_int=3630±1580[1mol_H2O^?1cm^?2]. Therefore, this value is proposed for the use in routine “water” determinations of compositionally different garnets by the micro-IR method, except for garnets near to end-member grossular.     

Redox state of deep off-craton lithospheric mantle: new data from garnet and spinel peridotites from Vitim, southern Siberia

Oxygen fugacity (fO₂) affects melting, metasomatism, speciation of C–O–H fluids and carbon-rich phases in the upper mantle. fO₂ of deep off-craton mantle is poorly known because garnet-peridotite xenoliths are rare in alkali basalts. We examine the redox and thermal state of the lithospheric mantle between the Siberian and North China cratons using new Fe³⁺/ΣFe ratios in garnet and spinel obtained by M?ssbauer spectroscopy, major element data and P–T estimates for 22 peridotite xenoliths as well as published data for 15 xenoliths from Vitim, Russia. Shallow spinel-facies mantle is more oxidized than deep garnet peridotites (average, ?0.1 vs. ?2.5 ΔlogfO₂(FMQ)). For intermediate garnet–spinel peridotites, fO₂ estimates from spinel-based oxybarometers are 1.5–3.2 ΔlogfO₂(FMQ) lower than those from garnet-based oxybarometers. These rocks may be out of phase and chemical inter-mineral equilibrium because the spinel–garnet reaction and concomitant changes in mineral chemistry do not keep up with P–T changes (e.g., lithospheric heating by recent volcanism) due to slow diffusion of trivalent cations and because gar-, gar-spl and spl-facies rocks may coexist on centimeter–meter scale. The spinel-based fO₂ estimates may not be correct while garnet-based fO₂ values provide conditions before the heating. The T (780–1,100?°C) and fO₂ ranges of the Vitim xenoliths overlap those of coarse garnet and spinel cratonic peridotites. However, because of a higher geothermal gradient, the deepest Vitim garnet peridotites are more reduced (by 0.5–2.0 ΔlogfO₂(FMQ)) than cratonic garnet peridotites at similar depths, and the “water maximum” conditions (>80?% H₂O) in the off-craton mantle exist in a more shallow and narrow depth range (60–85?km) than in cratonic roots (100–170?km). The base of the off-craton lithospheric mantle (≥90?km) at 2.5?GPa and 1,150?°C has fO₂ of ?3.0 ?logfO₂(FMQ), with dominant CH₄ and H₂O and minor H₂ in the fluid. Melting near the base of off-craton mantle lithosphere may be induced by increasing water share in migrating fluids due to oxidation of methane.     

Manganocummingtonite from the mesoproterozoic,Sausar fold belt,central India

Manganocummingtonite occurs with spessartine, quartz and pyrolusite in the Chikmara area, Sausar fold belt, central India. Its composition is [Ca_0.3–0.35(Mg_3.3–3.5Mn_1.6–1.8Fe²⁺ _1.4–1.5)(Si_{7.931–7.997}Al^iv _{0.003–0.069})O₂₂(OH_1.5–2.0F_0.0–0.5)] being fairly rich in Ca, which is indicative of metamorphic temperature in the amphibolite facies. The garnet contains 77.5% spessartine, 13% almandine and minor andradite, grossular and pyrope components. Unusually, there is no carbonate, pyroxene, pyroxmangite, rhodonite, magnetite or hematite. The available Al in the rock stabilized garnet and this mineral incorporated minor Fe³⁺ present in the rock as andradite component. The manganocummingtonite-garnet pairs developed at ～600°C during amphibolite facies metamorphism in low $X_{CO_2 } $ system, stabilized with $X_{Mn/(Mn + Fe^{2 + } + Mg)} $ = 0.25 to 0.28 in the amphibole and 0.85 in the garnet and formed under unusually low fO ₂ conditions for the Sausar region, near channelized fluids which deposited quartz may have controlled the fO ₂ .     

Thermodynamic parameters of CaMgSi2O6-Mg2Si2O6 pyroxenes based on regular solution and cooperative disordering models

Thermodynamic parameters for the reaction: $$\begin{gathered} Mg_2 Si_2 O_6 = Mg_2 Si_2 O_6 \hfill \\ enstatite clinopyroxene \hfill \\ \end{gathered} $$ in the system CaO-MgO-SiO₂ have been deduced from phase equilibrium and enthalpy of solution data. From the regular solution theory, the seventeen currently available reversed experimental compositions of coexisting enstatite and clinopyroxene, presumed to be ordered diopside, lead, by a statistical regression, to the following best fit parameters: ΔH ^o=6.80 kJ ΔS ^o=2.75 J/K W _H ^Cpx =24.47 kJ (regular solution enthalpy parameter) W _V ^Cpx =0.105 J/bar (regular solution volume parameter). The derived parameters are not significantly affected by the (necessary) choice of W ^Opx in the range 20–50 kJ. The above values are in very good agreement with deductions from the solution calorimetry on synthetic CaMgSi₂O₆-Mg₂Si₂O₆ clinopyroxenes of Newton et al. (1979), which also places bounds on possible departures from the optimal values of these parameters. The calorimetric data may also be interpreted in terms of a Bragg-Williams cooperative-disordering model (Navrotsky and Loucks, 1977), in which diopside-structure clinopyroxene and a ‘relaxed’ low-Ca clinopyroxene (‘Fe-free pigeonite’) approach each other in composition, structural state, and stability with increasing temperature. The ΔH ^o parameter deduced from the regular solution theory is reinterpreted as the enthalpy change of enstatite to Mg₂Si₂O₆ pigeonite; the ΔH ^o of the transformation of enstatite to the diopside structure would, in this case, be considerably larger than 6.8 kJ. The curvature of the enthalpy of solution data, explained by the regular solution theory in terms of M2-site energetics (involving W _H ^cpx ), is reinterpreted as due to disordering and ‘relaxation’ in the Navrotsky-Loucks model. Although the regular solution theory with the best-fit parameters accounts for all of the reversed enstatite and diopside compositions to within 18 ° C, and is a convenient representation of the phase equilibria for purposes of geothermometry, the disordering model is, at the present level of knowledge, equally valid and allows for a region of stability of two coexisting clinopyroxenes.     

Three cubic phases intergrown in a birefringent andradite-grossular garnet and their implications

The crystal chemistry across the garnet series is examined, and several systematic trends are reported. The crystal structure of three different cubic phases intergrown in a birefringent near end-member andradite from Namibia was refined by the Rietveld method, space group $ Ia\bar{3}d, $ Ia 3 ¯ d , and monochromatic synchrotron high-resolution powder X-ray diffraction data. Electron microprobe results indicate three phases with distinct compositions. The sample is birefringent, indicating that it is not cubic when observed optically. The reduced χ ² and overall R (F ²) Rietveld refinement values are 1.655 and 0.0284, respectively, so the multi-phase refinement is excellent. The composition, weight %, unit-cell parameter (Å), distances (Å), and site-occupancy factors (sofs) are as follows: phase-1, Adr₉₉, 88.5(1)  %, a = 12.06259(1), average 〈Ca–O〉 = 2.4310, Fe–O = 2.0189(4), Si–O = 1.6490(4) Å, Ca(sof) = 0.948(1), Fe(sof) = 0.934(1), and Si(sof) = 0.940(1). For phase-2: Adr₇₁Grs₂₈, 7.1(1) %, a = 12.00361(5), average 〈Ca–O〉 = 2.440, Fe–O = 1.979(3), Si–O = 1.641(3) Å, Ca(sof) = 0.913(5), Fe(sof) = 0.767(4), and Si(sof) = 0.932(5). For phase-3: Grs₇₉Adr₁₇, 4.4(1) %, a = 11.89719(4), average 〈Ca–O〉 = 2.404, Al–O = 1.935(4), Si–O = 1.667(3) Å, Ca(sof) = 0.944(6), Al(sof) = 1.069(7), and Si(sof) = 0.887(5). The dominant phase-1 (89 %; Adr₉₉) is nearly end-member andradite, Ca₃Fe ₂ ³⁺ Si₃O₁₂, which contains no cation order in the Ca(X) or Fe(Y) sites. The intergrowth of the three cubic phases causes considerable strain in the minor phases-2 and phases-3 that arise from different structural parameters and gives rise to strain-induced birefringence. For comparison, the results for an isotropic, single-phase, grossular–andradite garnet (Grs₇₆Adr₂₁) are also presented. The strain in the minor phases is about 3–5 times more than the unstrained dominant phase-1, or the unstrained single-phase grossular–andradite.     

REE in skarn systems: A LA-ICP-MS study of garnets from the Crown Jewel gold deposit

Metamorphic and magmatic garnets are known to fractionate REE, with generally HREE-enriched patterns, and high Lu/Hf and Sm/Nd ratios, making them very useful as geochemical tracers and in geochronological studies. However, these garnets are typically Al-rich (pyrope, almandine, spessartine, and grossular) and little is known about garnets with a more andraditic (Fe³⁺) composition, as frequently found in skarn systems. This paper presents LA-ICP-MS data for garnets from the Crown Jewel Au-skarn deposit (USA), discusses the factors controlling incorporation of REE into garnets, and strengthens the potential of garnet REE geochemistry as a tool to help understand the evolution of metasomatic fluids.Garnets from the Crown Jewel deposit range from Adr₃₀Grs₇₀ to almost pure andradite (Adr_>99). Fe-rich garnets (Adr_>90) are isotropic, whereas Al-rich garnets deviate from cubic symmetry and are anisotropic, often showing sectorial dodecahedral twinning. All garnets are extremely LILE-depleted, Ta, Hf, and Th and reveal a positive correlation of ΣREE³⁺ with Al content. The Al-rich garnets are relatively enriched in Y, Zr, and Sc and show “typical” HREE-enriched and LREE-depleted patterns with small Eu anomalies. Fe-rich garnets (Adr_>90) have much lower ΣREE and exhibit LREE-enriched and HREE-depleted patterns, with a strong positive Eu anomaly. Incorporation of REE into garnet is in part controlled by its crystal chemistry, with REE³⁺ following a coupled, YAG-type substitution mechanism , whereas Eu²⁺ substitutes for X²⁺ cations. Thermodynamic data (e.g., H_mixing) in grossular-andradite mixtures suggest preferential incorporation of HREE in grossular and LREE in more andraditic compositions.Variations in textural and optical features and in garnet geochemistry are largely controlled by external factors, such as fluid composition, W/R ratios, mineral growth kinetics, and metasomatism dynamics, suggesting an overall system that shifts dynamically between internally and externally buffered fluid chemistry driven by fracturing. Al-rich garnets formed by diffusive metasomatism, at low W/R ratios, from host-rock buffered metasomatic fluids. Fe-rich garnets grow rapidly by advective metasomatism, at higher W/R ratios, from magmatic-derived fluids, consistent with an increase in porosity by fracturing.     

Local structural heterogeneity in garnet solid solutions

Local heterogeneities in pyrope-almandine, almandine-grossular and pyrope-grossular solid solutions have been investigated using IR-powder absorption spectroscopy. Correlations of the wavenumber shifts and line broadening systematics with the thermodynamic mixing properties were found. Wavenumber shifts of the highest energy modes correlate closely with the Si-O bond distances and give an indirect view of the average distortions across the three solid solutions. They have a linear behaviour for Py-Alm, but show positive variations from linearity for Alm-Gr and Py-Gr systems. An effective line width (Δ_corr) of the absorption bands over a given wavenumber interval was obtained using the autocorrelation function. Line broadening is associated with local heterogeneities arising from cation substitution in the structure of samples at intermediate compositions. Non-linearities of the line broadening were found for Alm-Gr and Py-Gr and have a shape similar to the enthalpy of mixing, ΔH_mix. An empirical analysis was therefore carried out to compare ΔH_mix and Δ_corr quantitatively. Low-temperature far-IR spectra were recorded for the end-members pyrope, almandine and grossular and far-IR and mid-IR low-temperature spectra for Py₆₀Gr₄₀ in the temperature range 292–44 K. Softening of the lowest energy band with decreasing temperature was observed in the spectrum of pyrope and more enhanced in the spectrum of Py₆₀Gr₄₀. The same softening occurs by substitution of grossular component into pyrope. High energy modes of Py₆₀Gr₄₀ show the effect of saturation below 110–130 K, which correlates with the volume saturation at low temperature. This could provide an alternative explanation for the heat capacity anomaly found for Py-Gr solid solution at low-temperatures.     

Raman spectra of silicate garnets

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. Received: 3 December 1996 / Revised, accepted: 13 April 1997     

Calorimetric study of high-pressure polymorphs of MnSiO3

With increasing pressure, MnSiO₃ rhodonite stable at atmospheric pressure transforms to pyroxmangite, then to clinopyroxene and further to tetragonal garnet, which finally decomposes into MnO (rocksalt) plus SiO₂ (stishovite). High temperature solution calorimetry of synthetic rhodonite, clinopyroxene and garnet forms of MnSiO₃ was used to measure the enthalpies of these transitions. ΔH ₉₇₄ ⁰ for the rhodonite-clinopyroxene and ΔH ₂₉₈ ⁰ for the clinopyroxene-garnet transition are 520±490 and 8,270±590 cal/mol, respectively. The published data on the enthalpy of the rhodonite-pyroxmangite transition, phase equilibrium boundaries, compressibility and thermal expansion data are used to calculate entropy changes for the transitions. The enthalpy, entropy and volume changes are very small for all the transitions among rhodonite, pyroxmangite and clinopyroxene. The calculated boundary for the clinopyroxene-garnet transition is consistent with the published experimental results. The pyroxene-garnet transition in several materials, including MnSiO₃, is characterized by a relatively small negative entropy change and large volume decrease, resulting in a small positiveP – T slope. The disproportionation of MnSiO₃ garnet to MnO plus stishovite and of Mn₂SiO₄ olivine to garnet plus MnO are calculated to occur at about 17–18 and 14–15 GPa, respectively, at 1,000–1,500 K.     

Calorimetric study of olivine solid solutions in the system Mg2SiO4-Fe2SiO4

Solution enthalpies of synthetic olivine solid solutions in the system Mg₂SiO₄-Fe₂SiO₄ have been measured in molten 2PbO·B₂O₃ at 979 K. The enthalpy data show that olivine solid solutions have a positive enthalpy of mixing and the deviation from ideality is approximated as symmetric with respect to composition, in contrast to the previous study. Applying the symmetric regular solution model to the present enthalpy data, the interaction parameter of ethalpy (W_H) is estimated to be 5.3±1.7 kJ/mol (one cation site basis). Using this W_h and the published data on excess free energy of mixing, the nonideal parameter of entropy (W_s) of olivine solid solutions is estimated as 0.6±1.5 J/mol·K.     

Andradite–Morimotoite Garnets as Promising U–Pb Geochronometers for Dating Ultrabasic Alkaline Rocks

U–Pb geochronological studies of garnet of the andradite–morimotoite series and Sm–Nd geochronological studies of this garnet and apatite from the Chikskii Massif (Tuva-Mongolia microcontinent) were carried out. The garnet studied is characterized by relatively high concentrations of U (14–16 ppm) and by a low level of common Pb (Pb_с/Pb_t = 0.07–0.1). The concordia age of garnet is 492 ± 2 Ma (MSWD = 0.01, probability 92%) and matches within the error with the Sm–Nd age determined by the isochrone for apatite, garnet, and bulk rock (489 ± 9 Ma, MSWD = 0.86). This allows us to consider calcic garnets of the andradite–morimotoite series as promising mineral geochronometers for U–Pb dating of ultrabasic alkaline rocks.