Charles A. Geiger

Segregation Vesicles, Gas Filter-Pressing, and Igneous Differentiation

Some vesicles in certain subaerial flows of basalt and basaltic andesite are partially filled with dark, partly-glassy segregation material. The segregation material has about three times the concentration of K₂O, TiO₂, and P₂O₅ as the associated host rock. Therefore, the segregation materials are inferred to be frozen residual liquids that migrated into the vesicles. Several possible processes for filling the vesicles are examined within the context of the cooling time implied by heat conduction. The preferred process is one in which the residual liquid migrates through a porous and permeable, but rigid, network of interlocking crystals in response to a pressure gradient generated by vapor saturated crystallization. As crystallization proceeds H₂O is concentrated in the residual liquid and gas. This produces a higher pressure in the gas-poor matrix than in the vesicles and forces liquid to migrate into the vesicles. Textural features suggestive of or consistent with the process include: (1) vesiculation of segregation material, (2) convex perforations of the segregation lining of vesicles, (3) vuggy voids in the matrix, and (4) bubbles in residual glass. We suggest that filter pressing by gas effervescence may petrologically be important to crustal depths as great as several kilometers.

Mn 3 Al 2 Si 3 O 12 spessartine and Ca 3 Al 2 Si 3 O 12 grossular garnet; structural dynamic and thermodynamic properties

Abstract The structures of synthetic Mn3Al2Si3O12 spessartine and Ca3Al2Si3O12 grossular garnet have been refined using single-crystal X-ray diffraction methods at 100 K, 293 K, and 500-550 K. The divalent X-site cations, located in large dodecahedral sites, show measurable anisotropic dynamic disorder in contrast to the rigid vibrational behavior of the SiO4 tetrahedra and AlO6 octahedra. The amplitudes of vibration of Mn2+ in spessartine are similar to those of Fe2+ of almandine, in the plane of the longer X-O(4) bonds, and both are about twice that of Ca2+ in grossular, despite the lighter mass of the latter. Heat capacities measured between 300 and 1000 K on synthetic poly crystalline spessartine and two natural nearly end-member spessartine crystals are similar to those of almandine. In addition, the IR active modes of spessartine at low frequencies are very similar to those of almandine suggesting that their heat capacities are also similar at lower temperatures. The low-energy phonon spectra of pyrope and grossular are probably considerably distinct from the two transition metal-containing garnets as suggested by their different low frequency IR active modes, reflecting the different bonding properties for Mg and Ca in garnet. The large pressure-temperature stability field of spessartine, relative to the other aluminosilicate garnets, does not appear to be due to any sort of intrinsic entropy stabilization.

Heat capacities and entropies of mixing of pyrope-grossular (Mg3Al2Si3O12-Ca3Al2Si3O12) garnet solid solutions: A low-temperature calorimetric and a thermodynamic investigation

Abstract The low-temperature heat capacities for a series of synthetic garnets along the pyrope-grossular (Py-Gr) join were measured with the heat capacity option of the Physical Properties Measurement System (PPMS) produced by Quantum Design. The measurements were performed between 5 and 300 K on milligram-sized polycrystalline garnets that have been well characterized in previous studies. The CP measurements indicate positive excess heat capacities (ΔCPxs) for all solid-solution compositions at temperatures <50 K with a maximum value of 2.31 ± 0.18 J/(mol·K) for the composition Py50Gr50 at about 35 K. Pyrope-rich garnets (i.e., Py90Gr10 and Py75Gr25) have no or slightly positive ΔCPxs at higher temperatures, whereas grossular-rich garnets (i.e., Py10Gr90 and Py25Gr75) show negative ΔCPxs values in the temperature range between 50 and 150 K. At T > 150 K, ΔCPxs values scatter around zero for all compositions and the experimental error is too large to permit a clear determination of whether ΔCP xs is different from zero within 2σ uncertainty. Excess entropies (ΔSxs) at 298.15 K, calculated from the CP data of the various solid-solution members, are asymmetric in nature with the largest positive deviations in pyrope-rich compositions. An asymmetric Margules mixing model was found to be inappropriate for modeling the ΔSxs-X data and, thus, a two-parameter Redlich-Kister model was used to describe the excess entropy-composition relationships. Using this macroscopic mixing model for the excess entropy, a T-X diagram for Py-Gr garnets was calculated using different published values for the excess enthalpies of mixing. The effect of short range Ca-Mg order in the solid solution also was considered in the calculations. The calculations give a solvus for the pyrope-grossular join with a higher critical temperature in the range 850-1330 °C at XGr = 0.35 compared to previous thermodynamic models (Tcrit < 600 °C) that use symmetric mixing models to describe the excess entropy. Unmixing of garnets in nature, as documented from occurrences in ultramafic diatremes may, therefore, have occurred at higher temperatures than previously thought. The atomistic and lattice-dynamic properties of Py-Gr garnets are reviewed and compared to the macroscopic CP data. Published IR and Raman spectra are consistent with the occurrence of positive ΔCPxs values at low temperatures.

Heat capacity measurements of synthetic pyrope-grossular garnets between 320 and 1000 K by differential scanning calorimetry

The heat capacity of synthetic periclase (MgO), pyrope (M93Al2Si3O12), grossular (Ca3Al2Si3O12), and four pyrope-grossular solid solutions (PY89.8Gr10.2, PY73.6Gr26.4, PY48.8Gr51.2, PY24.0Gr76.0) has been measured between 320 and 1000 K by differential scanning calorimetry. Measurements were made in both interval-scanning and step-scanning modes; with the latter method, Cp was determined to a precision of better than ± 1%. The results, together with previous calorimetric measurements on synthetic grossular and pyrope, now permit a complete description of the heat capacities for both phases between 10-1200 K and 10-1350 K, respectively. The following best-fit equations for the temperature range from 298 to 1200 K were obtained for pyrope and grossular, respectively (T in Kelvin and Cp in J/ mol*K): Cp(pyrope) = 542.01 - 1387.07·T-0.5 - 20.492×106·T-2+2500.67×106·T-3; Cp(grossular) = 607.81 - 3214.49·T-0.5 - 12.141×106·T-2+1269.92×106·T-3. For the solid solution garnets a model assuming ideal mixing (linear combination) of molar heat capacities of the endmember phases reproduces the experimental heat capacities within errors. This indicates that the vibrational entropies of mixing and enthalpies of mixing are temperature-independent between room temperature and 1000 K. It is expected that excess heat capacities of mixing are not present in most silicate solid solutions at temperatures between 320 and 1000 K.

A 29 Si MAS NMR and IR spectroscopic investigation of synthetic pyrope-grossular garnet solid solutions

Maghemite in a tuffite deposit from Patos de Minas, Brazil, has been isolated by highgradient magnetic separation and analyzed by X-ray diffraction, electron microprobe analysis, Mossbauer spectroscopy, and magnetization measurements. The maghemite constitutes 5 wt% of the tuffite, where it is present as separate grains in the 1-400-~m-size range, some with associated anatase inclusions or intergrowths. The lattice parameter is ao = 0.8380(2) nm. No Fe2+ is detected, but the composition of the grains shows Fe ~ Mg 2:: Ti. The averaged composition and proposed cation distribution is [FeO.88Sio.OlM~.1l]{FeO.96M~.30 Tio.32Alo.o7Cro.03Mno.o2Do.30}04, where [ ] and { } denote A and B sites, respectively, of the spinel lattice, and D denotes cation vacancies. Magnetization, Os,is 18-31 J/(T. kg) at room temperature, and the Curie temperature, Tc, is 320-360 °C. The Mgrich maghemite is inherited by magnetic soils forming on the tuffite.

Cordierite I: The coordination of Fe 2+

Abstract The incorporation of Fe2+ was investigated in four natural cordierite samples. 57Fe Mössbauer, single-crystal UV-VIS optical absorption, and X-ray absorption spectroscopies, as well as X-ray single-crystal diffraction were used. Mössbauer, optical, and XAS spectroscopy show that Fe2+ is incorporated on two different structural sites in two Mg-rich samples. Mössbauer measurements give the best quantitative measure of the amounts of Fe2+, but the optical spectra are the most sensitive for determinations at low concentrations and at high-bulk Fe2+ concentrations in cordierite. The spectroscopic data are most consistent with small amounts of Fe2+ (i.e., 0.02 of Fe2+ per formula unit) being located on a tetrahedral site rather than in the center (or off center) of the six-membered tetrahedral rings or in channel cavities. X-ray single-crystal refinements on two Mg-rich cordierites show a very small excess electron density on T11 and not in the channels. A third refinement on a slightly more iron-rich sample shows, in contrast, no excess electron density on T11. We interpret these data as indicating that small amounts of Fe2+ (0.01 to 0.02 atoms per formula unit) replace tetrahedral Al11 in cordierite, where charge balance is achieved by placing Na in the center of the six-membered rings. This substitution is consistent with the known chemistry of natural cordierites and with simple structural energetics. The identification and assignment of small amounts of Fe2+ on T11 requires spectroscopic determination or careful X-ray single-crystal refinements and cannot be achieved from composition data and structural formula calculations.

Molar volumes of mixing of almandine-pyrope and almandine-spessartine garnets and the crystal chemistry and thermodynamic-mixing properties of the aluminosilicate garnets

Abstract The aluminosilicate garnet binaries almandine-pyrope and almandine-spessartine were studied by powder X-ray and 57Fe Mössbauer methods. Refinements of the unit-cell constants 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 of Wv = 0.24 (±0.05) cm3/mol. Mössbauer spectra recorded at 298 and 77 K show the presence of small amounts| of [6]Fe3+. which in the case of almandine-pyrope garnets is also measurable from microprobe analyses. The amount of Fe3+ 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.

DFT Study of the Role of Al3+ in the Fast Ion-Conductor Li7–3xAl3+xLa3Zr2O12 Garnet

We investigate theoretically the site occupancy of Al3+ in the fast-ion-conducting cubic-garnet Li7–3xAl3+xLa3Zr2O12 (Ia-3d) using density functional theory. By comparing calculated and measured 27Al NMR chemical shifts an analysis shows that Al3+ prefers the tetrahedrally coordinated 24d site and a distorted 4-fold coordinated 96h site. The site energies for Al3+ ions, which are slightly displaced from the exact crystallographic sites (i.e., 24d and 96h), are similar leading to a distribution of slightly different local oxygen coordination environments. Thus, broad 27Al NMR resonances result reflecting the distribution of different isotropic chemical shifts and quadrupole coupling constants. From an energetic point of view, there is evidence that Al3+ could also occupy the 48g site with its almost regular octahedral coordination sphere. Although this has been reported by neutron powder diffraction, the NMR chemical shift calculated for such an Al3+ site has not been observed experimentally.

Silicate garnet: A micro to macroscopic (re)view

Abstract Silicate garnets, general formula E3G2Si3O12, form an important class of rock-forming minerals and, in nature, most are solid solutions. Their crystal-chemical and solid-solution properties are sometimes interpreted in terms of the widely used Pyralspite-Ugrandite classification scheme, and this can lead to erroneous conclusions. In this study, published data are reviewed and analyzed to achieve a synthesis of relevant experimental and computational results and to construct a working “crystal-chemical model” for describing aluminosilicate garnet, E3Al2Si3O12, over different length scales. The pyrope-grossular (Py-Gr) solid solution is given special attention, because it has received a great deal of study. It also shows interesting crystal-chemical and thermodynamic mixing behavior. Computational and experimental investigations made on Py-Gr garnets indicate that the shorter Ca/Mg-O2 bond lengths appear to remain roughly constant in length across the binary and can be described as showing “Pauling limit-type” behavior. The longer Ca/Mg-O4 bonds behave differently, because they lengthen with increasing Gr component in the solid solution. Bond behavior in almandine-spessartine (Al-Sp) garnets appears to be partly different, because both Fe/Mn-O2 and Fe/ Mn-O4 bonds show “Pauling limit-type” behavior. E-O bond-length variations are continuous. The bonding type in all aluminosilicate garnet end-members is similar. An analysis shows that various computational simulations on Py-Gr solid solutions are consistent with each other with respect to E-O bond behavior and also with experimental IR, Raman, NMR spectroscopic, and X-ray diffraction results, but not completely with XAS studies made at the Ca edge. Ca/Mg-O4 bond behavior can be used to explain, partly, the nature of various micro/nanoscopic crystal-chemical and strain properties and macroscopic excess thermodynamic mixing behavior of Py-Gr garnets. Micro/nanostrain for the Py-Gr binary is asymmetric in nature, as are the various thermodynamic mixing functions ΔHex, ΔSex, and ΔVex. The widely cited Pyralspite-Ugrandite classification scheme has limited use in terms of explaining many physical and chemical properties of garnet and it should not be used to predict or describe, for example, solid-solution behavior.

A Synthesis and Crystal Chemical Study of the Fast Ion Conductor Li7–3xGaxLa3 Zr2O12 with x = 0.08 to 0.84

Fast-conducting phase-pure cubic Ga-bearing Li7La3Zr2O12 was obtained using solid-state synthesis methods with 0.08 to 0.52 Ga3+ pfu in the garnet. An upper limit of 0.72 Ga3+ pfu in garnet was obtained, but the synthesis was accompanied by small amounts of La2Zr2O12 and LiGaO3. The synthetic products were characterized by X-ray powder diffraction, electron microprobe and SEM analyses, ICP-OES measurements, and 71Ga MAS NMR spectroscopy. The unit-cell parameter, a0, of the various garnets does not vary significantly as a function of Ga3+ content, with a value of about 12.984(4) Å. Full chemical analyses for the solid solutions were obtained giving: Li7.08Ga0.06La2.93Zr2.02O12, Li6.50Ga0.15La2.96Zr2.05O12, Li6.48Ga0.23La2.93Zr2.04O12, Li5.93Ga0.36La2.94Zr2.01O12, Li5.38Ga0.53La2.96Zr1.99O12, Li4.82Ga0.60La2.96Zr2.00O12, and Li4.53Ga0.72La2.94Zr1.98O12. The NMR spectra are interpreted as indicating that Ga3+ mainly occurs in a distorted 4-fold coordinated environment that probably corresponds to the general 96h crystallographic site of garnet.