Daniel R. Neuville

Viscosity and mixing in molten (Ca, Mg) pyroxenes and garnets

Abstract A creep apparatus has been built to measure, with inaccuracies of less than 0.04 log poise, viscosities of supercooled silicate melts in the range 109–1014 poises. Measurements on seven pyroxene and five garnet supercooled liquid compositions along the joins MgSiO3-CaSiO3 and Mg3Al2Si3O12Ca3 Al2Si3O12 made between 1000 and 1150 K show deep minima in the viscosity-composition relationship for both joins. These minima reduce when the temperature increases and disappear eventually. Within the framework of the configurational entropy theory of relaxation processes, these observations can be accounted for quantitatively in terms of the contribution of ideal (Ca, Mg) mixing to the total configurational entropy of the melts. The configurational entropies determined from the viscosity measurements agree with the values determined by calorimetry for liquid CaSiO3, CaMgSi2O6, MgSiO3, and Mg3Al2Si3O12. The heat capacities of Ca3Al2Si3O12 glass and liquid have also been obtained from dropcalorimetry measurements.

Amorphous materials: Properties, structure, and durability† Structure of Mg- and Mg/Ca aluminosilicate glasses: 27Al NMR and Raman spectroscopy investigations

Abstract The structure and properties of glasses and melts in the MgO-Al2O3-SiO2 (MAS) and CaO-MgOAl2O3- SiO2 (CMAS) systems play an important role in Earth and material sciences. Aluminum has a crucial influence in these systems, and its environment is still questioned. In this paper, we present new results using Raman spectroscopy and 27Al nuclear magnetic resonance on MAS and CMAS glasses. We propose an Al/Si tetrahedral distribution in the glass network in different Qn species for silicon and essentially in Q4 and VAl for aluminum. For the CMAS glasses, an increase of VAl and VIAl is clearly visible as a function of the increase of Mg/Ca ratio in the (Ca,Mg)3Al2Si3O12 (garnet) and (Ca,Mg)AlSi2O8 (anorthite) glass compositions. In the MAS system, the proportion of VAl and VIAl increases with decreasing SiO2 and, similarly with calcium aluminosilicate glasses, the maximum of VAl is located in the center of the ternary system.

Advances in understanding the structure of borosilicate glasses: A Raman spectroscopy study

Abstract This study is focused on the behavior of ternary SiO2-Na2O-B2O3 borosilicate glasses at temperatures between 298 and 1800 K. Unpolarized Raman spectra were measured up to high temperature. SiO2-Na2O-B2O3 glass samples were prepared with different values of the ratio R = [Na2O]/[B2O3], while the ratio K = [SiO2]/[B2O3] was kept constant and equal to 2.12. Spectra were measured at room temperature in samples with 0.43 ≤ R ≤ 1.68, and the effect of the modifier content was clearly observed in these glasses, only in partial agreement with previous literature results. In particular, the formation in the glass of sodium-danburite units Na2O·B2O3·2SiO2 was postulated. This feature led to a new assessment of R*, the critical value of R above which every new alkali atom added to the system breaks a Fo-O-Fo (Fo = glass former) bridge causing depolymerization of the glass. A revised formula is proposed to obtain the value of R* as a function of K. Raman spectra measured at high temperature yielded important information about the temperaturedependent evolution of the borosilicate system. In particular, borate and borosilicate units including tetra-coordinated boron seem to be unstable at high temperature, where the formation of metaborate chains or rings is fostered. Above 1500 °C, evaporation of borate compounds is clearly observed, stemming from the small sample size.

Metamictization and chemical durability of detrital zircon.

Abstract We have investigated the effect that metamictization has on the weathering of zircon in detrital continental sediments and tropical soils of the Amazon basin, Brazil. The degree of radiation damage in the near-surface region of the zircon grains was determined by Raman microprobe. In each of the four series investigated (i.e., sediment, podzol, topsoil, and subsoil horizons of lateritic soil), the degree of radiation damage ranges from less than 1014 to ~3.5 × 1015 α−decay/mg. The maximum degree of radiation damage coincides with the first percolation threshold of the metamictization process at ~3.5 × 1015 α-decay/mg. Below this threshold, amorphous volumes in the structure of damaged zircon are not connected to each other. The ranges of U, Th, and Pb contents (in ppm) measured by proton induced X-ray emission (PIXE) microanalysis are 100 < U < 7000, 100 < Th < 18000, and 100 < Pb < 1300. Chemical ages, assessed from U, Th, and total-Pb, range between 0.15 Ga and 2.8 Ga. This range is roughly consistent with the ages reported for the Precambrian shields of the Amazon basin (0.45-3.5 Ga). Corresponding radiation doses range between <2 × 1015 and 3 × 1016 α-decay/mg. Comparison of calculated doses with the degree of structural damage indicates that most of the zircon grains have experienced significant annealing. However, the degree of annealing differs from one grain to another. Thus, the acute maximum limit observed for the degree of radiation damage of the whole zircon series is better explained by low-temperature alteration or weathering processes than by thermal resetting. Following this interpretation, our results provide evidence for a dramatic decrease in the chemical durability of zircon in natural weathering environments when the radiation dose exceeds 3.5 × 1015 α-decay/mg. Below the first percolation threshold, the zircon population survives the soil formation intact, but more damaged zircons are dissolved during weathering/ alteration processes.

Thermodynamic and rheological properties of rhyolite and andesite melts

The heat capacities of a rhyolite and an andesite glass and liquid have been investigated from relative-enthalpy measurements made between 400 and 1800 K. For the glass phases, the experimental data agree with empirical models of calculation of the heat capacity. For the liquid phases, the agreement is less good owing to strong interactions between alkali metals and aluminum, which are not currently accounted for by empirical heat capacity models. The viscosity of both liquids has been measured from the glass transition to 1800 K. The temperature dependence of the viscosity is quantitatively related to the configurational heat capacity (determined calorimetrically) through the configurational entropy theory of relaxation processes. For both rhyolite and andesite melts, the heat capacity and viscosity do not differ markedly from those obtained by additive modeling from components with mineral compositions.

Effect of temperature and TiO2 content on the structure of Na2Si2O5Na2Ti2O5 melts and glasses

Abstract The interaction of Ti4+ with the anionic structure of glasses and melts along the join Na2Si2O5Na2Ti2O5 has been examined in situ at temperatures between 25 and 1316°C with microRaman spectroscopy. In the Ti-free endmember system the expression, (1) 2Q3 ⇔ Q2 + Q4 describes the anionic equilibria adequately. Solution of Ti4+ stabilizes an additional unit, Q1. This structural change requires an additional expression, (2) 2Q2 ⇔ Q3 + Q1, to represent the equilibria. The ΔHx1 for reaction 1 is ∼24 kJ/mol, whereas that for reaction 2 is ∼ −40 kJ/mol. These values differ from those in the analogous Na2Si2O5Na2(NaAl)2O5 system (Mysen and Frantz, 1994a), where ΔHx1 for Al-bearing melts is −11–−14 kJ/mol, and that of reaction 2 is 17–33 kJ/mol, depending on the Al (Al + Si) . Increasing TiO2 concentration is positively correlated with the abundance of Q2 and Q1 structural units, whereas those of Q3 and Q4 are negatively correlated. In the equivalent Al-system (Mysen and Frantz, 1994a), the Q2, Q1, and Q4 are positively correlated and the Q3 abundance is negatively correlated with increasing Al2O3. The Raman spectra of Ti-bearing glasses and melts are consistent with Ti4+ in at least three different structural positions (Si4+ ⇔ Ti4+ substitution, clusters perhaps of TiO2 type, and Ti4+ as a network-modifying cation). This behavior depends on both temperature and TiO2 concentration. The fraction of tetrahedrally coordinated Ti4+ increases from near 0% for ≤3.5 mol% TiO2 to nearly 80% in glasses and melts with 20 mol% TiO2. Increasing temperature above that of the glass transformation interval (400–500°C) results in a 10–30% decrease in the fraction Ti4+ in tetrahedral coordination. As the temperature is raised beyond 800–900°C, this trend reverses. It is suggested that the temperature- and composition-dependent structural behavior of Ti4+ in the melts might be reflected in unusual behavior of melt properties such as viscosity, thermal expansion, and thermodynamic properties.

Aqueous alteration in the Northwest Africa 817 NWA 817 Martian meteorite

Samples of a new Martian meteorite of the nakhlite family (NWA 817) contain traces of an iron-rich alteration product. Textural arguments indicate that this alteration product has been formed on the parent body of the meteorite (Mars). The chemical composition and structural data (X-ray diffraction, transmission electron microscopy and vibrational spectroscopy) show that the alteration mineral is a hydrous phase from the smectite family. Major elements and rare earth elements suggest that the formation of the alteration phase is related to the circulation of an aqueous fluid which composition is controlled by the dissolution of feldspars to account for a positive Eu anomaly, olivine and possibly apatite. Hydrogen isotope data display negative deltaD values ranging from -60 to -280parts per thousand in olivine and pyroxenes and from -140 to -181parts per thousand in the alteration phase. The values of deltaD for the alteration product show a small scatter with a mean value of -170 +/- 14parts per thousand. These values are lower than those previously obtained on other Martian meteorites, which give mainly positive deltaD values. These positive values have been interpreted as resulting from the interaction of the Martian meteorites with water from the Martian atmosphere. Ruling out the effect of terrestrial alteration, it is suggested that alteration in the NWA 817 meteorite was likely produced on Mars by the circulation of an aqueous fluid originating from a chemical reservoir, such as the Martian mantle, which has not equilibrated with a fractionated Martian atmosphere

Structure and properties of low-silica calcium aluminosilicate glasses

Abstract The structure of three calcium aluminosilicate glasses with low-silica content (61CaO · 39Al 2 O 3 (Ca0.39), 55CaO · 35Al 2 O 3 · 10SiO 2 (Ca10.35) and 49CaO · 31Al 2 O 3 · 20SiO 2 (Ca20.31)), synthesized by a standard quenching technique, were studied by X-ray and neutron diffraction. The data are consistent with the hypothesis that Si and Al are present in tetrahedral sites and Ca in octahedral sites for all compositions. Si–T (T=Si or Al) and Si–O2 contributions (where O2 means the second nearest oxygen) appear at 10 mol% silica content, which indicate that silicon enters the aluminate network.

Cation sites in Al-rich MgSiO 3 perovskites

Abstract Local structure analysis of Al-containing magnesium silicate perovskite has been carried out with X-ray absorption spectra recorded at the Mg, Al, and Si K-edges using the SA32 beam-line of SuperAco (Orsay, France). The Al-XAFS spectrum of (MgSi)0.85Al0.3O3 perovskite (synthesized in a multi-anvil apparatus) cannot be explained by assuming that Al3+ occurs in octahedral or dodecahedral sites only. This conclusion is based on comparison between Al-spectrum and those recorded at the Mg and Si K-edges for the same structure, general trends found for Al-spectra in various atomic sites, and ab-initio calculations using the FEFF-6 code. Thus, Al appears to be partitioned between both octahedral and dodecahedral perovskite sites. However, the structural accommodations needed to stabilize Al3+ cation in such different sites are not straightforward. Also, Mg K-edge spectra in enstatite and perovskite were compared with those previously reported at the Fe K-edge for the same structures, confirming that these two elements are located in the same polyhedra in both structures, and thus that Fe2+ enters the dodecahedra of the silicate perovskite.

Viscosity, configurational entropy and relaxation kinetics of borosilicate melts

Abstract The viscosities of one silicate and four borosilicate liquids have been measured from about 1 up to more than 1014 Pa s when liquid unmixing did not take place. Equilibrium values at the highest viscosities do not show evidence for a change of rheology in the glass transition range. Using calorimetrically determined heat capacities, we account quantitatively for the non-Arrhenian variations of the viscosities over the whole viscosity interval with the Adam-Gibbs theory. The configurational entropies determined from the liquid viscosities depend markedly on composition and are the highest for Na and Ca-rich compositions. The temperatures, T1, of the empirical Tamman-Vogel-Fulcher viscosity equation are consistent with the temperatures at which configurational entropies vanish. Comparisons between a borosilicate and a window glass having similar viscosity-temperature relationships indicate a slight difference in the kinetics of structural relaxation. This difference agrees with a correlation between increasing fragility and shorter relaxation times and also suggests that the shear modulus at infinite frequency of borosilicates can vary by a factor of 100 as a function of composition.