The temperature dependence of the speciation of water in NaAlSi3O8-KAlSi3O8 melts: an application of fictive temperatures derived from synthetic fluid-inclusions

 The speciation of water dissolved in glasses along the join NaAlSi₃O₈-KAlSi₃O₈ has been investigated using infrared spectroscopy. Hydrous melts have been hydrothermally synthesized by chemical equilibration of cylinders of bubble-free anhydrous start glasses with water at 1040^° C and 2 kbar. These melts have been isobarically and rapidly (200^° C/s) “drop”-quenched to room temperature and then subsequently depressurized. The speciation of water in the quenched glasses reflects the state of water speciation at a temperature (the so-called fictive temperature) where the quenched-in structure of the glasses closely corresponds to the melt structure at equilibrium. This fictive temperature is detectable as the macroscopically measureable glass transition temperature of these melt compositions. A separate set of experiments using vesicular samples of the same chemistry has precisely defined the glass transition temperature of these melts (±5^° C) on the basis of homogenization temperatures for water-filled fluid inclusions (Romano et al. 1994). The spectroscopic data on the speciation of water in these quenched glasses has been quantified using experimentally determined absorptivities for OH and H₂O for each individual melt composition. The knowledge of glass transition temperatures, together with quantitative speciation data permits an analysis of the temperature dependence of the water speciation over the 113^° C range of fictive temperatures obtained for these water-saturated melts. The variation of water speciation, cast as the equilibrium constant K where K = [H₂O] [O _m ]/[OH]² is plotted versus the fictive temperature of the melt to obtain the temperature dependence of speciation. Such a plot describes a single linear trend of the logarithm of the equilibrium constant versus reciprocal temperature, implying that the exchange of K for Na has little influence on melt speciation of water. The enthalpy derived from temperature dependence is 36.5(±5) kJ/mol. The results indicate a large variation in speciation with temperature and an insensitivity of the speciation to the K–Na exchange. Received: 8 March 1995/Accepted: 6 June 1995     

Coordination of sodium ions in NaAlO2–SiO2 melts: a high temperature 23Na NMR study

The coordination environment of the sodium ion in the melts of several simple ionic liquids and an Na₂O–Al₂O₃–SiO₂ mixture has been investigated by high temperature ²³Na NMR measurements. A new high temperature NMR probe was utilized for the measurements of the compositional and temperature dependence of the ²³Na NMR chemical shift at temperatures up to 1600?°C. ²³Na NMR spectra of ionic liquids, NaCl, NaBr and NaNO₃, show two peaks at their solid to liquid transition, corresponding to the solid and liquid state, respectively. The ²³Na NMR peak shift in passing from the liquid to the solid is positive. This suggests a decrease in the coordination number for the molten state compared to the crystalline state. The ²³Na peak position for the Na₂O–Al₂O₃–SiO₂ melts of the composition range Na/Al≥1 shifted almost linearly in the positive direction as a function of both the increased degree of depolymerization, NBO/T, and [Al]/([Al]+[Si]). ²³Na MAS-NMR measurement for crystalline silicate compounds of known structure provided a revised relationship between the mean Na–O distances and ²³Na chemical shifts. Comparison of the ²³Na chemical shift of the melts with that of crystalline silicate compounds suggests that the coordination number of Na in those melts is around 6–8 with little compositional dependence. The ²³Na peak position shifted in the negative direction with increasing temperature for sodium silicates, whereas that of aluminosilicates did not show any temperature dependence. The activation energy from the temperature dependence of the ²³Na line width shows little compositional dependence, and the value (51～58?kJ/mol) was close to that of the trace Na ion diffusion in NaAlSi₃O₈ glass.     

Thermoelastic behavior and dehydration process of cancrinite

The high-temperature thermoelastic behavior of a natural cancrinite has been investigated by in situ single-crystal X-ray diffraction. The unit-cell volume variation as a function of temperature (T) exhibits a continuous trend up to 748 K (hydrous expansion regime). The unit-cell edges expansion clearly shows an anisotropic expansion scheme (α _a  < α _c ). At 748 K, a dehydration process takes place, and a series of unit-cell parameter measurements at constant temperature (748 K) for a period of 12 days indicate that the dehydration process continued for the entire period of time, until the cell parameters were found to be constant. After the dehydration process is completed, the structure expands almost linearly with increasing temperature up to 823 K, where a sudden broadening of the diffraction peaks, likely due to the impending decomposition, did not allow the collection of further data points. Even with a very limited temperature range for the anhydrous regime, we observed that the behavior of the two (i.e., hydrous and anhydrous) high-temperature structures is similar in terms of (1) volume thermal expansion coefficient and (2) thermoelastic anisotropy. The structure refinements based on the data collected at 303, 478 and 748 K (after the dehydration), respectively, showed a change in the mechanism of tilting of the quasi-rigid (Si,Al)O₄ tetrahedra, following the loss of H₂O molecules, ascribable to the high-temperature Na⁺ coordination environment within the cages.     

Thermal expansion and phase transitions in åkermanite and gehlenite

Thermal expansion has been measured by laboratory and synchrotron X-ray powder diffraction for end-member åkermanite (ak, Ca₂MgSi₂O₇) and gehlenite (ge, Ca₂Al₂SiO₇) in the range 20–1,500 K. In ak in the range 340–390 K, there is a negative linear thermal expansion in [001] direction. This is related to the phase transition from an incommensurate modulated structure (IC) to a normal one (N). The volumetric mean thermal expansion coefficients for ak and ge, obtained with a linear fit of the experimental data in the temperature range 298–1,400 K, are respectively 32.1×10^–6 and 28.3×10^–6 K^–1 . The variation of the c/a ratio with temperature, due to different thermal expansion along the crystallographic axes, can be related to the different behaviour of the tetrahedral layers in the N and IC phases. Analysis of the variation of the superstructure peaks intensity across the phase transition confirms the tricritical behaviour of the IC/N transition in ak.     

The partial molar volume, thermal expansivity, and compressibility of H2O in NaAlSi3O8 liquid: new measurements and an internally consistent model

The molar volumes of 19 hydrous albitic liquids (1.9 to 6.1 wt% H₂O^total) were determined at one bar and 505–765 K. These volume data were derived from density measurements on hydrous glasses at 298 K, followed by measurements of the thermal expansion of each glass from 298 K to its respective glass transition temperature. The technique exploits the fact that the volume of a glass is equal to that of the corresponding liquid at the limiting fictive temperature (T _f′), and that T _f′ can be approximated as the temperature near the onset of the rapid increase in thermal expansion that occurs in the glass transition interval. The volume data of this study were combined with available volume data for anhydrous, Na₂O-Al₂O₃-SiO₂ liquids to derive the partial molar volume (±1) of the H₂O component in an albitic melt at ∼565 K and one bar. To extend the determination of to higher temperatures and pressures, the molar volumes of the hydrous albitic liquids determined in this study were combined with those measured by previous authors at 1023–1223 K and 480–840 MPa, leading to the following fitted values (±1) at 1673 K and one bar: (±0.46)×10⁻³ cm⁻³/mol-K, and dVˉ ^H ₂ ^{O total} /dP=−3.82 (±0.36)×10⁻⁴ cm³/mol-bar. The measured molar volumes of this study and those of previous authors can be recovered with a standard deviation of 0.5%, which is within the respective experimental errors. There is a significant difference between the values for derived in this study as a function of temperature and pressure and those obtained from an existing polynomial, primarily caused by the previous absence of accurate density measurements on anhydrous silicate liquids. The coefficients of thermal expansion (=4.72×10⁻⁴/K) and isothermal compressibility ( _T =1.66×10⁻⁵/bar) for the H₂O component at 1273 K and 100 MPa, indicate that H₂O^total is the single most expansive and compressible component in silicate liquids. For example, at 1473 K and 70 MPa (conditions of a mid-ocean ridge crustal magma chamber), the presence of just 0.4 wt% H₂O will decrease the density of a basaltic liquid by more than one percent. An equivalent decrease in melt density could be achieved by increasing the temperature by 175 degrees or the decreasing pressure by 230 MPa. Therefore, even minor quantities of dissolved water will have a marked effect on the dynamic properties of silicate liquids in the crustal environment. Received: 20 August 1996 / Accepted: 15 March 1997     

A revised model for the density and thermal expansivity of K2O-Na2O-CaO-MgO-Al2O3-SiO2 liquids from 700 to 1900 K: extension to crustal magmatic temperatures

A revised model for the volume and thermal expansivity of K₂O-Na₂O-CaO-MgO-Al₂O₃-SiO₂ liquids, which can be applied at crustal magmatic temperatures, has been derived from new low temperature (701–1092 K) density measurements on sixteen supercooled liquids, for which high temperature (1421–1896 K) liquid density data are available. These data were combined with similar measurements previously performed by the present author on eight sodium aluminosilicate samples, for which high temperature density measurements are also available. Compositions (in mol%) range from 37 to 75% SiO₂, 0 to 27% Al₂O₃, 0 to 38% MgO, 0 to 43% CaO, 0 to 33% Na₂O and 0 to 29% K₂O. The strategy employed for the low temperature density measurements is based on the assumption that the volume of a glass is equal to that of the liquid at the limiting fictive temperature, T _f ^′. The volume of the glass and liquid at T _f ^′ was obtained from the glass density at 298 K and the glass thermal expansion coefficient from 298 K to T _f ^′. The low temperature volume data were combined with the existing high temperature measurements to derive a constant thermal expansivity of each liquid over a wide temperature interval (767–1127 degrees) with a fitted 1 error of 0.5 to 5.7%. Calibration of a linear model equation leads to fitted values of Vˉ _i ±1 (cc/mol) at 1373 K for SiO₂ (26.86 ± 0.03), Al₂O₃ (37.42±0.09), MgO (10.71±0.08), CaO (15.41±0.06), Na₂O (26.57±0.06), K₂O (42.45 ± 0.09), and fitted values of dVˉ _i /dT (10⁻³ cc/mol-K) for MgO (3.27±0.17), CaO (3.74±0.12), Na₂O (7.68±0.10) and K₂O (12.08±0.20). The results indicate that neither SiO₂ nor Al₂O₃ contribute to the thermal expansivity of the liquids, and that dV/dT ^liq is independent of temperature between 701 and 1896 K over a wide range of composition. Between 59 and 78% of the thermal expansivity of the experimental liquids is derived from configurational (vs vibrational) contributions. Measured volumes and thermal expansivities can be recovered with this model with a standard deviation of 0.25% and 5.7%, respectively. Received: 2 August 1996 / Accepted: 12 June 1997     

Argon and CO2 on the race track in silicate melts: A tool for the development of a CO2 speciation and diffusion model

We have analysed the kinetics of Argon and CO₂ diffusion in simplified iron free rhyolitic to hawaiitic melts using the diffusion couple technique. The concentration distance profiles of Ar and CO₂ were measured with electron microprobe analysis and Fourier Transform Infrared Spectroscopy, respectively. Error functions were fitted to the symmetrical concentration distance profiles to extract the diffusion coefficients.In the temperature range 1373 to 1773 K the activation energies for Ar diffusion range from 169 ± 20 to 257 ± 62 kJ mol⁻¹. Ar diffusivity increases exponentially with the degree of depolymerisation. In contrast, the mobility of total CO₂, that is identical to Ar mobility in rhyolitic melt, keeps constant with changing bulk composition from rhyolite to hawaiite. CO₂ speciation at 1623 K and 500 MPa was modeled for the range of compositions studied using the diffusion data of Ar and total CO₂ in combination with network former diffusion calculated from viscosity data. Within error this model is in excellent agreement with CO₂ speciation data extrapolated from temperatures near the glass transition temperature for dacitic melt composition. This model shows that even in highly depolymerised hawaiitic and tholeiitic melts molecular CO₂ is a stable species and contributes 70 to 80% to the total CO₂ diffusion, respectively.     

The crystal and magnetic structure of Li-aegirine LiFe3+Si2O6: a temperature-dependent study

Single crystals of Li-aegirine LiFe³⁺Si₂O₆ were synthesized at 1573?K and 3?GPa, and a polycrystalline sample suitable for neutron diffraction was produced by ceramic sintering at 1223?K. LiFe³⁺Si₂O₆ is monoclinic, space group C2/c, a=9.6641(2)?Å, b= 8.6612(3)?Å, c=5.2924(2)?Å, β=110.12(1)^° at 300?K as refined from powder neutron data. At 229?K Li-aegirine undergoes a phase transition from C2/c to P2₁ /c. This is indicated by strong discontinuities in the temperature variation of the lattice parameters, especially for the monoclinic angle β and by the appearance of Bragg reflections (hkl) with h+k≠2n. In the low-temperature form two non-equivalent Si-sites with 〈SiA–O〉=1.622?Å and 〈SiB–O〉=1.624?Å at 100?K are present. The bridging angles of the SiO₄ tetrahedra O3–O3–O3 are 192.55(8)° and 160.02(9)° at 100?K in the two independent tetrahedral chains in space group P2₁ /c, whereas it is 180.83(9)° at 300?K in the high-temperature C2/c phase, i.e. the chains are nearly fully expanded. Upon the phase transition the Li-coordination changes from six to five. At 100?K four Li–O bond lengths lie within 2.072(4)–2.172(3)?Å, the fifth Li–O bond length is 2.356(4)?Å, whereas the Li–O3?A bond lengths amount to 2.796(4)?Å. From ⁵⁷Fe Mössbauer spectroscopic measurements between 80 and 500?K the structural phase transition is characterized by a small discontinuity of the quadrupole splitting. Temperature-dependent neutron powder diffraction experiments show first occurrence of magnetic reflections at 16.5?K in good agreement with the point of inflection in the temperature-dependent magnetization of LiFe³⁺Si₂O₆. Distinct preordering phenomena can be observed up to 35?K. At the magnetic phase transition the unit cell parameters exhibit a pronounced magneto-striction of the lattice. Below T _N Li-aegirine shows a collinear antiferromagnetic structure. From our neutron powder diffraction experiments we extract a collinear antiferromagnetic spin arrangement within the a–c plane.     

Structure and Mössbauer spectroscopy of barbosalite Fe2+Fe3+ 2 (PO4)2(OH)2 between 80 K and 300 K

Natural barbosalite Fe²⁺Fe³⁺ ₂ (PO₄)₂(OH)₂ from Bull Moose Mine, South Dakota, U.S.A., having ideal composition, was investigated with single crystal X-ray diffraction techniques, Mössbauer spectroscopy and SQUID magnetometry to redetermine crystal structure, valence state of iron and evolution of ⁵⁷Fe Mössbauer parameter and to propose the magnetic structure at low temperatures. At 298?K the title compound is monoclinic, space group P2₁/n, a _o ?= 7.3294(16)?Å, b _o ?=?7.4921(17)?Å, c _o ?=?7.4148 (18)?Å, β?=?118.43(3)°, Z?=?2. No crystallographic phase transition was observed between 298?K and 110?K. Slight discontinuities in the temperature dependence of lattice parameters and bond angles in the range between 150?K and 180?K are ascribed to the magnetic phase transition of the title compound. At 298?K the Mössbauer spectrum of the barbosalite shows two paramagnetic components, typical for Fe²⁺ and Fe³⁺ in octahedral coordination; the area ratio Fe³⁺/Fe²⁺ is exactly two, corresponding to the ideal value. Both the Fe²⁺ and the Fe³⁺ sublattice order magnetically below 173?K and exhibit a fully developed magnetic pattern at 160?K. The electric field gradient at the Fe²⁺ site is distorted from axial symmetry with the direction of the magnetic field nearly perpendicular to V_zz, the main component of the electric field gradient. The temperature dependent magnetic susceptibility exhibits strong antiferromagnetic ordering within the corner-sharing Fe³⁺-chains parallel to [101], whereas ferromagnetic coupling is assumed within the face-sharing [1?1?0] and [?1?1?0] Fe³⁺-Fe²⁺-Fe³⁺ trimer, connecting the Fe³⁺-chains to each other.     

Partitioning of manganese between forsterite and silicate liquid

Partition coefficients for Mn between forsterite and liquid in the system MgO-CaO-Na₂O-Al₂O₃-SiO₂ (+ about 0.2% Mn) were measured by electron microprobe for a variety of melt compositions over the temperature range 1250–1450°C at one atm pressure. The forsterite-liquid partition coefficient of Mn (mole ratio, MnO in Fo/MnO in liquid, designated D_mn^fo?Liq) depends on liquid composition as well as temperature: at 1350°C, D_Mn^Fo?Liqranges from 0.60 (basic melt, SiO₂ = 47wt%) to 1.24 (acidic melt, SiO₂ = 65wt%). At lower temperatures, the partition coefficient is more strongly dependent on melt composition.The effects of melt composition and temperature on D_Mn^fo?Liq can be separately evaluated by use of the Si:O atomic ratio of the melts. A plot of D_mn^Fo?Liq measured at various temperatures vs melt Si:O for numerous liquid compositions reveals discrete, constant-temperature curves that are not well defined by plotting D_Mn^Fo?Liq against other melt composition parameters such as melt basicity or MgO content. For constant Si:O in the melt, In D_Mn^Fo?Liq vs reciprocal absolute temperature is linear; however, the slope of the plot becomes more positive for higher values of Si:O, indicating a higher energy state for Mn²⁺ ions in acidic melts than in basic melts.Comparison of Mn partitioning data for the iron-free system used in this study with data of other workers on iron-bearing compositions suggests that the effect of iron on Mn partitioning between olivine and melt is small over the range of basalt liquidus temperatures.     

The structure of sodium tetrasilicate glass from neutron diffraction, reverse Monte Carlo simulations and Raman spectroscopy

The reverse Monte Carlo (RMC) method was used for modelling the three-dimensional structure of sodium tetrasilicate glass (Na₂Si₄O₉, NS4). Neutron diffraction data over a large momentum transfer range and nuclear magnetic resonance results (Q-species distribution) as well as chemical bonding considerations have been used to constrain the RMC simulations. Very good agreement with the experimental structure factor was achieved. The three-dimensional model is analysed in terms of partial pair distribution functions, bond-angle distributions, short-range order (SRO) clusters and ring statistics. The average Si–O–Si bridging angle is different for the different Qⁱ–Q^j units. The effect of different Q species on the network connectivity is analysed by comparison with statistical SRO distributions. While the Q² species are randomly distributed, a high preference for the formation of Q⁴–Q⁴ units in three-membered rings is found. This explains why during the initial stages of hydration of NS4 glass, Si–OH groups are preferentially formed by breaking Q⁴–Q⁴ linkages. The RMC model also provides an independent test for the assignment of bands in the Si–O stretching region of the Raman spectrum. It is shown that the bands at about 1050 and 1100 cm⁻¹ are probably due to Q³ units surrounded by different numbers of Q⁴ units. Received: 7 April 1997 / Revised, accepted: 5 July 1997     

Thin intergranular melt films and melt pockets in spinel peridotite xenoliths from the Rhön area (Germany): early stage of melt generation by grain boundary melting

Optical microscopy and transmission electron microscopy (TEM) on a porphyroclastic high temperature spinel peridotite from the Rhön area reveal fine, irregular glass layers and pockets along mineral interfaces, cracks in olivine, inside olivine crystals and in spongy rims of clinopyroxene. The chemical composition of the glass deviates significantly from the composition of the host basanite. Electron diffraction technique confirms the amorphous nature of the glass, thus classifying it as a former melt. Every grain or phase boundary shows amorphous intergranular glass layers of variable thickness and characteristic chemical composition with distinct chemical inhomogeneities. Olivine grain boundaries, as the most common type of interfaces, exhibit two different types of melt glasses: (1) Type I melt at olivine grain boundaries, which is characterized by low contents of SiO₂ (～37?wt%) and Al₂O₃ (～5?wt%) and elevated contents of MgO (～31?wt%) and FeO (～22?wt%), is supposed to have formed prior to or during the thermal overprint and the dynamic recrystallisation of the xenolith in the mantle. Melt inclusions inside olivine grains with an average composition of type I melt are suggested to be earlier melt droplets at olivine interfaces, overgrown by migrating olivine grain boundaries during recrystallization in the mantle prior to the uplift of the xenolith. (2) Type II melt, the most common type of melt in the xenolith, shows higher contents of SiO₂ (～48?wt%) and Al₂O₃ (～17?wt%) but lower contents of MgO (～20?wt%) and FeO (～11?wt%). The observation of different types of glass within a single xenolith indicates the development of different chemical melt equilibria at interfaces or triple junctions in the xenolith. The absence of geochemical trends in bivariate plots excludes a unifying process for the genesis of these glasses. Melt inclusions in the spongy rims of clinopyroxene are interpreted to be the product of a potassium-rich metasomatism. The formation of most amorphous intergranular melt layers and pockets at the mineral interfaces including type II melt at olivine grain boundaries is suggested to result from decompression melting during the uplift with the basalt magma. We suggest that these glasses were produced by grain boundary melting due to lattice mismatch and impurity segregation. The observed intergranular amorphous layers or melts represent the very beginning of mineral melting by grain boundary melting.     

X-ray absorption study of Ti-bearing silicate glasses

Ti K-edge XANES spectra have been collected on a series of Ti-bearing silicate glasses with metasilicate and tetrasilicate compositions. The intensity of the preedge feature in these spectra has been found to change with glass composition and varies from 29 to 58% (normalized intensity) suggesting a variation in structural environent around the absorbing atom. The pre-edge peak intensity increases for the alkali titanium tetrasilicate glasses from 35% to 58% in the order Li < Na < K < Rb, Cs whereas for the metasilicate compositions there is a maximum for the K-bearing glass. The pre-edge peak intensity remains constant for the alkaline earth titanium metasilicate glasses, Ca and Sr (34%) but increases slightly for Ba (41%). As the intensity of this feature is inversely correlated with coordination number, a comparison of the pre-edge intensity data for the investigated glasses with those of materials of known coordination number leads us to establish a regression equation and to infer that the average coordination number of Ti in these glasses ranges from 4.8 to 5.8. Large alkali cations appear to stabilize a relatively low average coordination number for Ti in silicate melts. The Ti structural environment results appear also to vary as a function of SiO₂ content within the K₂O-TiO₂-SiO₂ system. A number of physical properties of the melts from which these glasses were quenched and of other Ti-bearing silicate melts, have been determined in recent years. Clear evidence of a variable coordination number of Ti, consistent with the interpretation of the present XANES data is available from density measurements. These and other property determinations are compared with the present spectroscopic observations in an attempt to relate structure and properties in these melts which contain a major component with variable coordination number.     

Low-temperature heat capacities,entropies and high-pressure phase relations of MgSiO<Subscript>3</Subscript> ilmenite and perovskite

Low-temperature isobaric heat capacities (C _p ) of MgSiO₃ ilmenite and perovskite were measured in the temperature range of 1.9–302.4 K with a thermal relaxation method using the Physical Properties Measurement System. The measured C _p of perovskite was higher than that of ilmenite in the whole temperature range studied. From the measured C _p , standard entropies at 298.15 K of MgSiO₃ ilmenite and perovskite were determined to be 53.7 ± 0.4 and 57.9 ± 0.3 J/mol K, respectively. The positive entropy change (4.2 ± 0.5 J/mol K) of the ilmenite–perovskite transition in MgSiO₃ is compatible with structural change across the transition in which coordination of Mg atoms is changed from sixfold to eightfold. Calculation of the ilmenite–perovskite transition boundary using the measured entropies and published enthalpy data gives an equilibrium transition boundary at about 20–23 GPa at 1,000–2,000 K with a Clapeyron slope of −2.4 ± 0.4 MPa/K at 1,600 K. The calculated boundary is almost consistent within the errors with those determined by high-pressure high-temperature in situ X-ray diffraction experiments.     

Thermal expansion and crystal structure of FeSi between 4 and 1173 K determined by time-of-flight neutron powder diffraction

The thermal expansion and crystal structure of FeSi has been determined by neutron powder diffraction between 4 and 1173?K. No evidence was seen of any structural or magnetic transitions at low temperatures. The average volumetric thermal expansion coefficient above room temperature was found to be 4.85(5)?×?10^?5?K^?1. The cell volume was fitted over the complete temperature range using Grüneisen approximations to the zero pressure equation of state, with the internal energy calculated via a Debye model; a Grüneisen second-order approximation gave the following parameters: θ_D=445(11)?K, V ₀=89.596(8)?ų, K ₀′=4.4(4) and γ′=2.33(3), where θ_D is the Debye temperature, V ₀ is V at T=0?K, K ₀′ is the first derivative with respect to pressure of the incompressibility and γ′ is a Grüneisen parameter. The thermodynamic Grüneisen parameter, γ_th, has been calculated from experimental data in the range 4–400?K. The crystal structure was found to be almost invariant with temperature. The thermal vibrations of the Fe atoms are almost isotropic at all temperatures; those of the Si atoms become more anisotropic as the temperature increases.     

Viscosity of andesitic melts—new experimental data and a revised calculation model

The viscosity of a synthetic andesite-like melt was measured in the low viscosity range (10¹-10⁶ Pa s) using the falling sphere(s) method and in the high viscosity range (10⁸-10¹³ Pa s) using parallel-plate viscometry. Falling sphere experiments with melts containing 2.3 and 5.6 wt.% H₂O were carried out in an internally heated gas pressure vessel (IHPV) at 500 MPa confining pressure. The sinking velocity of Pt and Pd spheres and in one case of a corundum sphere was used to measure the melt viscosity. In addition, a creep experiment was performed at ambient pressure using a glass containing 2.73 wt.% H₂O . A more water-rich glass (5.6 wt.% H₂O ) was investigated with a high pressure parallel-plate viscometer at 400 MPa confining pressure in an IPHV. By combining our new data with previous results for a similar melt composition we derived the following expression to describe the viscosity η (in Pa s) as a function of temperature T (in K) and water content w (in wt.%)

     

Pressure-induced structural changes and densification of vitreous MgSiO3

Compression of MgSiO₃ glass in a 6/8 multianvil apparatus to 10.0 ± 0.5 GPa results in demonstrable changes in density and silicon coordination. Under high-pressure, samples were heated over a range of temperatures from 300 to 773 K, quenched to room temperature and decompressed at rates of 10.4 and 0.08 GPa/min. Recovered glasses have bulk densities that are 2.6-11.0% higher than the non-compressed glass. ²⁹Si MAS NMR spectra of compressed glasses show narrowing of the ^[4]Si peak resulting from a reduction in the spread of the Si-O-Si bond angle distribution. After heating and rapid decompression, ²⁹Si MAS NMR spectra of recovered glasses exhibit peaks assignable to ^[4]Si, ^[5]Si, and ^[6]Si with relative fractions of 0.945, 0.045, and 0.008, respectively. These changes in Si coordination and in Si-O-Si bond angle distribution with pressure only represent part of the structural changes associated with permanent densification of heated and unheated samples. The abundance of ^[6]Si is found to be insensitive to decompression rate, while ^[5]Si reverts to ^[4]Si on slow decompression at room temperature. These observations demonstrate that high-coordinated silicon species in MgSiO₃ glass are formed on compression below glass transition temperatures and that pressure-induced structural changes can be preserved with rapid decompression. The ease with which ^[5]Si reverts to ^[4]Si during decompression suggests that the conversion of ^[4]Si → ^[5]Si principally involves short-range atomic displacement. The reversible and irreversible features of densification of MgSiO₃ glass, provide insights into the fundamental structural and rheological properties of refractory silicate melts similar to those found in the Earth’s mantle.     

Divergent T–ƒO<Subscript>2</Subscript> paths during crystallisation of H<Subscript>2</Subscript>O-rich and H<Subscript>2</Subscript>O-poor magmas as recorded by Ce and U in zircon,with implications for TitaniQ and TitaniZ geothermometry

During solidification of magma chambers as systems closed to chemical exchange with environs, the residual siliceous melt may follow a trend of rising, constant, or decreasing oxidation state, relative to reference buffers such as nickel?+?nickel oxide (NNO) or fayalite?+?magnetite?+?quartz. Titanomagnetite–hemoilmenite thermometry and oxybarometry on quenched volcanic suites yield temperature versus oxygen fugacity arrays of varied positive and negative slopes, the validity of which has been disputed for several years. We resolve the controversy by introducing a new recorder of magmatic redox evolution employing temperature- and redox-sensitive trace-element abundances in zircon. The zircon/melt partition coefficients of cerium and uranium vary oppositely in response to variation of magma redox state, but vary in tandem as temperature varies. Plots of U/Pr versus Ce⁴⁺/Ce³⁺ in zircon provide a robust test for change in oxidation state of the melt during zircon crystallisation from cooling magma, and the plots discriminate thermally induced from redox-induced variation of Ce⁴⁺/Ce³⁺ in zircon. Temperature-dependent lattice strain causes Ce⁴⁺/Ce³⁺ in zircon to increase strongly as zircon crystallises from cooling magma at constant Ce⁴⁺/Ce³⁺ ratio in the melt. We examine 19 zircon populations from igneous complexes in varied tectonic settings. Variation of zircon Ce⁴⁺/Ce³⁺ due to minor variation in melt oxidation state during crystallisation is resolvable in 11 cases but very subordinate to temperature dependence. In many zircon populations described in published literature, there is no resolvable change in redox state of the melt during tenfold variation of Ce⁴⁺/Ce³⁺ in zircons. Varied magmatic redox trends indicated by different slopes on plots of zircon U/Pr versus Ce⁴⁺/Ce³⁺ are corroborated by Fe–Ti-oxide-based T–?O₂ trends of correspondingly varied slopes. Zircon and Fe–Ti-oxide compositions agree that exceptionally, H₂O-rich arc magmas tend to follow a trend of rising oxidation state of the melt during late stages of fluid-saturated magmatic differentiation at upper-crustal pressures. We suggest that H₂ and/or SO₃ and/or Fe²⁺ loss from the melt to segregating fluid is largely responsible. Conversely, zircon and Fe–Ti-oxide compositions agree in indicating that H₂O-poor magmas tend to follow a T–?O₂ trend of decreasing oxidation state of the melt during late stages of magmatic differentiation at upper-crustal pressures, because the precipitating mineral assemblage has higher Fe³⁺/Fe²⁺ than coexisting rhyolitic melt. We present new evidence showing that the Fe–Ti-oxide oxybarometer calibration by Ghiorso and Evans (Am J Sci 308(9):957–1039, 2008) retrieves experimentally imposed values of ?O₂ in laboratory syntheses of Fe–Ti-oxide pairs to a precision of ±?0.2 log unit, over a large experimental temperature range, without systematic bias up to at least log ?O₂?≈?NNO?+?4.4. Their titanomagnetite–hemoilmenite geothermometer calibration has large systematic errors in application to Ti-poor oxides that precipitate from very oxidised magmas. A key outcome is validation of Fe–Ti-oxide-based values of melt TiO₂ activity for use in Ti-in-zircon thermometry and Ti-in-quartz thermobarometry.     

A calorimetric investigation of spessartine: Vibrational and magnetic heat capacity

The heat capacity (C_p) 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 C_p polynomial for T > 250 K:

     

Phase transitions in ilvaite,a mixed-valence iron silicate

Ilvaite, Ca(Fe²⁺, Fe³⁺) Fe²⁺Si₂O₇O(OH), a mixed-valence iron silicate shows an insulator-semimetal transition with a band gap of 0.13 eV due to thermally induced charge delocalization between Fe²⁺ and Fe³⁺ ions (A sites) in double octahedral chains. The charge delocalization induces a second order crystallographic phase transition on heating from monoclinic (P2₁/a) to orthorhombic (Pnam) symmetry at 346 K. The unit cell dimensions within the 295–420 K range and the crystal structures at 295, 320, 340, 360, 380 and 400 K have been determined by high temperature single crystal X-ray diffraction. The degree of charge delocalization determined from the sizes of the Fe(Ao) and Fe(Am) octahedra is the primary order parameter, Q which couples linearly with the spontaneous strain component, ₁₃. The order parameter coupling and the associated free energy expression is given. The calculated normal modes of the space group symmetry change are consistent with the experimentally observed atomic displacements, which are parallel and antiparallel to c. Formation of antiphase lamellar twin domains parallel to (001) in the monoclinic phase is predicted to occur as a result of the phase transition. Above T_c (= 346 K), the slow asymptotic decrease of ₁₃ attaining a zero value at 380 K indicates the presence of fluctuating precursor clusters with considerable short-range order above T_c. A peak in the specific heat (C_p) measurements coincides with the onset of longrange order at 380 K, whereas ⁵⁷Fe Mössbauer measurements indicate the onset of charge localization at a considerbly higher temperature (470 K). The coupling of the d⁶ electron of the Fe²⁺ (A) ion with a longitudinal optic phonon with the polarization vector along c ^* is the likely mechanism to drive the phase transition. The electronphonon coupling also provides a charge conduction mechanism through electron hopping, whereby the short-bonded Fe²⁺-Fe³⁺ pair containing the d⁶ electron (intermediate polaron) will break up and re-form, thereby propagating the electron one step along the c axis.