Nature of polymerization and properties of silicate melts and glasses at high pressure

The structures of sodium silicate and aluminosilicate glasses quenched from melts at high pressure (6-10 GPa) with varying degrees of polymerization (fractions of nonbridging oxygen) were explored using solid-state NMR [¹⁷O and ²⁷Al triple-quantum magic-angle spinning (3QMAS) NMR]. The bond connectivity in melts among four and highly coordinated network polyhedra, such as ^[4]Al, ^[5,6]Al, ^[4]Si, and ^[5,6]Si, at high pressure is shown to be significantly different from that at ambient pressure. In particular, in the silicate and aluminosilicate melts, the proportion of nonbridging oxygen (NBO) generally decreases with increasing pressure, leading to the formation of new oxygen clusters that include 5- and 6-coordinated Si and Al in addition to 4-coordinated Al and Si, such as ^[4]Si-O-^[5,6]Si, ^[4]Si-O-^[5,6]Al and Na-O-^[5,6]Si. While the fractions of ^[5,6]Al increase with pressure, the magnitude of this increase diminishes with increasing degrees of ambient-pressure polymerization under isobaric conditions. Incorporating the above structural information into models of melt properties reproduces the anomalous pressure-dependence of O²⁻ diffusivity and viscosity often observed in silicate melts.     

A computer simulation study of natural silicate melts. Part II: High pressure properties

The thermodynamic, structural and transport properties of natural silicate melts under pressure are investigated by molecular dynamics simulation with the help of a force field recently introduced by us [Guillot B. and Sator N. (2007) A computer simulation study of natural silicate melts. Part I: low pressure properties. Geochim. Cosmochim. Acta71, 1249-1265]. It is shown that the simulation reproduces accurately the bulk moduli of a large variety of silicate liquids as evaluated from ultrasonic studies. The equations of state (EOS) of the simulated melts are in good agreement with the density data on mid-ocean ridge basalt, komatiite, peridotite and fayalite as obtained either by sink/float experiments or by shock-wave compression. From the structural point of view it is shown that the population of ^[5]Al and ^[6]Al species increases rapidly upon initial compression (0-50 kbar) whereas for Si these highly coordinated species are found in a significant abundance (>5%) only above ∼50 kbar for ^[5]Si and ∼100-150 kbar for ^[6]Si. This increase of the coordination of network formers is not the only response of the melt structure to the densification: there is also a large redistribution of the T-O-T (T = Si, Al) bond angles with the pressure and noticeably upon initial compression in rhyolitic and basaltic liquids. Furthermore, a detailed analysis of the population of bridging oxygens (BO) and nonbridging oxygens (NBO) points out that the polymerization of the melt generally increases when the pressure increases, the magnitude of this polymerization enhancement being all the more important that the melt is depolymerized at low pressure. The role of triclusters (threefold coordinated oxygens to network former cations) is particularly emphasized in acidic and basaltic liquids. The pressure-induced redistribution of the oxygen atoms through the melt structure is also stressed. Finally, the simulation predicts a nonmonotonic behavior of the diffusivity of network former ions when the pressure increases, a feature with depends on the melt composition. This could have a counterpart in the electrical conductivity at sufficiently high temperature when the viscosity of the liquid is low.     

The crystal and melt structure of spinel and alumina at high temperature: An in-situ XANES study at the Al and Mg K-edge

We present structural information obtained on spinel and alumina at high temperature (298-2400 K) using in-situ XANES at the Mg and Al K-edges. For spinel, ^[4](Al_x,Mg_1−x)^[6](Al_2−x,Mg_x)O_4, with increasing temperature, a substitution of Mg by Al and Al by Mg in their respective sites is observed. This substitution corresponds to an inversion of the Mg and Al sites. There is a significant change in the Al K-edge spectra between crystal and liquid, which can be attributed to a change of the ^[6]Al normally observed in corundum at room temperature, to a mixture of ^[6]Al-^[4]Al in the liquid state. This conclusion is in good agreement with previous ²⁷Al NMR experiments. Furthermore, both experiments at the Al and Mg K-edges are in good agreement with XANES calculation made using FDMNES code.     

Al environment in tectosilicate and peraluminous glasses: A Al MQ-MAS NMR, Raman, and XANES investigation

Tecto-aluminosilicate and peraluminous glasses have been prepared by conventional and laser heating techniques, respectively, in the CaO-Al₂O₃-SiO₂ system. The structure of these glasses were studied using Raman spectroscopy, X-ray absorption at the Al K-edge and ²⁷Al NMR spectroscopy with two different high fields (400 and 750 MHz). Raman spectroscopy and X-ray absorption are techniques sensitive to the network polymerization and, in particular, show different signal as a function of silica content. However, these two techniques are less sensitive than NMR to describe the local aluminium environment. For tectosilicate glasses, aluminium in five-fold coordination, ^[5]Al, was found and a careful quantification allows the determination of a significant amount of ^[5]Al (7% in the anorthite glass). The proportion of ^[5]Al increases for the peraluminous glasses with small amounts (<2%) of six-fold coordination, ^[6]Al. The presence of ^[5]Al agrees with previous observations of the existence of nonbridging oxygens (NBOs) in tectosilicate compositions. However, the proportion of ^[5]Al in the present study indicates that no major proportion of triclusters (oxygen coordinated to three (Si,Al)O₄ tetrahedra) is required to explain these NBOS.     

Thermodynamics of multicomponent pyroxenes: III. Calibration of Fe2+(Mg)-1, TiAl2(MgSi2)-1, TiFe 2 3+ (MgSi2)-1, AlFe3+(MgSi)-1, NaAl(CaMg)-1, Al2(MgSi)-1 and Ca(Mg)-1 exchange reactions between pyroxenes and silicate melts

A thermodynamic model for the Gibbs free energy of igneous pyroxenes with the general formula [Na, Ca, Fe²⁺, Mg]^M2[Fe²⁺, Mg, Ti, Al, Fe³⁺]^M1[Al, Fe³⁺, Si]^TetSiO₆ is calibrated from experimentally determined compositions of coexisting pyroxene and silicate melt. The model is based upon the general formulation, and relies upon the calibration of the “quadrilateral” subsystem, previously published by the present authors. The calibration database of pyroxene-liquid equilibria spans a broad spectrum of temperature, pressure and oxygen fugacity conditions, ranging from 1000°–1600°C, 0.001–30 kbar and iron-wüstite to air. Chemical potentials of endmember pyroxene components as well as exchange potentials between pyroxenes and coexisting liquids are defined utilizing the present authors' thermodynamic melt model. Model parameters are extracted from these relations by regression analysis. The resulting model and derivative endmember properties are internally consistent with an existing standard state thermodynamic database. The success of the model and its applicability to igneous petrogenesis are demonstrated by comparing calculated and experimentally determined liquidus compositions, temperatures and symmetry states for pyroxenes crystallizing from a variety of silicate melts, ranging in composition from tholeiites and angrites through rhyolites to potash ankaratrites.     

Pressure-induced (Al,Si)[4]-ordering in dioctahedral micas?

Pure, synthetic 2M₁-muscovites (Ms) and 2M₁-paragonites (Pg) prepared at 7, 13.5, and 18 kbar have been subjected to careful X-ray powder diffractometry and IR powder spectrometry. The results of X-ray studies show that the lattice constants of these micas are independent of the pressure employed in synthesizing them. Similarly, the IR powder spectra of Ms and Pg remain unaffected by the synthesis pressure. Neither the Al^[4]-O-Al^[4] vibrations decrease in relative intensity nor do the Si-O-Al^[4] vibrations increase in intensity with increasing pressure of synthesis, as they should, if pressure-induced (Al, Si)^[4] ordering had been operative. Finally, the v _OH bands in Ms and Pg are also found to retain their fine structural details, regardless of the pressures employed in synthesizing these phases. In particular, the half widths of v _OH in these (Al,Si)^[4]-disordered micas remain constant at around 60 cm⁻¹, compared to ∼ 10 cm⁻¹ in (Al,Si)^[4]-ordered mica margarite. In the light of this study, shortening of the b lattice parameter and progressive (Al,Si)^[4] ordering with increasing pressure of synthesis of muscovites reported by Velde (1980) would appear to be anomalous. Consequently, any discussion of the effect of (Al,Si)^[4] ordering on the stability of muscovite is a moot point at present.     

Theoretical and Al CPMAS NMR investigation of aluminum coordination changes during aluminosilicate dissolution

Ab initio molecular orbital calculations were performed, and ²⁷Al CP MAS-NMR spectra were evaluated in order to investigate the possible tetrahedral to octahedral coordination change of Al at the feldspar-water interface under acidic conditions. Aluminum coordination is octahedral in solution, and tetrahedral in feldspar crystals. Whether this change in coordination can occur on feldspar surfaces as part of the dissolution mechanism has been debated. Molecular orbital calculations were performed on aluminosilicate clusters with a few surrounding water molecules to partially account for solvation effects at the feldspar-water interface. The calculations on both fully-relaxed and partially-constrained clusters suggest that the energy difference between ^[4]Al and ^[6]Al where both are linked to three Si-tetrahedra (i.e., Q³ Al) in the feldspar structure, is small enough to allow for the conversion of Q^3[4]Al to Q^3[6]Al in a hydrated layer of feldspar, prior to the release of Al ions to the aqueous solution. The introduction of a few water molecules to the clusters introduced the possibility of multiple optimized geometries for each Al coordination, with energy differences on the order of several hydrogen bonds. The calculation of activation energies and transition states between Q^3[4]Al, Q^3[5]Al, and Q^3[6]Al was complicated by the introduction of water molecules and the use of fully-relaxed aluminosilicate clusters. Calculated isotropic shifts for Q^1[6]Al, Q^2[6]Al, and Q^3[6]Al suggest that the ^[6]Al observed on aluminosilicate glass surfaces using ²⁷Al CP MAS-NMR is Q^1[6]Al and therefore formed as part of the dissolution process. The formation of ^[6]Al in situ on a feldspar surface (as opposed to re-precipitation from solution) has significant implications for the dissolution mechanism and surface chemistry of feldspars.     

固体核磁光谱法研究镁铝复合氧化物的水热转化机制

复合金属氧化物广泛分布于自然界,其中镁铝复合氧化物(简称MMO)则大量分布于基性-超基性岩浆岩、蒸发河湖相和海相沉积岩等之中。本研究以镁铝水滑石(HT)煅烧产物MMO为研究对象,采用水热合成方法,通过改变反应时长,考察并揭示MMO在富硅、碱性和160℃水热条件下的结构复原特性及其转化机制。X射线衍射结果显示,该条件下MMO首先复水形成水滑石相,进而水滑石相逐渐向皂石转化,并伴有钠霞石等杂相的形成与溶解。29Si和27Al魔角旋转核磁共振谱(MAS NMR)的结果证明,在皂石结晶过程中,Al3+优先占据四面体片层的四配位位置。该研究对揭示水滑石稳定性、水滑石与硅酸盐矿物异相成核与生长、热液类型的天然皂石矿床的形成过程等具有重要指示意义。     

Synthetic dumortierite: its PTX-dependent compositional variations in the system Al2O3-B2O3-SiO2-H2O

Dumortierite, generally simplified as Al₇BSi₃O₁₈, was synthesized in the pure system Al₂O₃–B₂O₃–SiO₂–H₂O (ABSH) using gels with variable Al/Si ratios mixed with H₃BO₃ and H₂O in known proportions as starting materials. Synthesis conditions ranged from 3 to 5 and 15 to 20 kbar fluid pressure at 650° to 880°C. On the basis of analyses, synthetic dumortierite shows relatively narrow homogeneity ranges with regard to Al/Si which, however, vary as a function of pressure: at low pressures (3–5 kbar) Al/Si is 2.77–2.94 versus 2.33–2.55 at high pressures (15–20 kbar). Outside of these homogeneity limits, dumortierite was found to coexist with quartz or corundum, depending on the starting composition. Whereas synthetic dumortierite invaribly contains 1.0 boron atom per formula unit (p.f.u.) based on 18 oxygens, the water contents vary drastically as a function of pressure and temperature (1.32–2.30 wt.% H₂O or 0.85–1.47 H p.f.u.). H₂O is an essential component in dumortierite. Structural formulae based on complete chemical analyses of the dumortierites synthesized reveal that there is invariably an Si-deficiency against the ideal number of 3.0 p.f.u. In the calculation procedure used here, this deficiency is balanced by assuming tetrahedral Al. The remaining Al, taken to occupy the octahedral sites, is always below the ideal number of 7.0 p.f.u. Charge-balancing the structure with the hydrogen found analytically leads to two different mechanisms of H incorporation: (1) 3H⁺ + octahedral vacancy for Al^[6]; (2) H⁺ + tetrahedral Al for Si^[4]. Dumortierite synthesized at high fluid pressure contains little Al^[4] and, thus, little H⁺ of type 2; its hydrogen is predominantly present as type 1. Conversely, dumortierite formed at low fluid pressures is high in Al^[4] and hydrogen type 2. The amounts of hydrogen type 1 in low-pressure dumortierites decrease with rising temperatures of synthesis. Typical structural formulae are: (Al_6.670.33)[Al_0.49Si_2.51–O_13.53(OH)_1.47](BO₃) for a low-pressure product, and (Al_6.680.32)[Al_0.09Si_2.91O_13.94(OH)_1.06](BO₃) for a high-pressure product. Independently of the synthesis conditions, dumortierite was found always to be orthorhombic, with b₀/a₀ deviating slightly, but significantly from the valid for hexagonal lattice geometry. As a function of increasing Al/Si in the synthetic crystals, their a₀, c₀, and V₀ rise, whereas b₀ decreases. Thus b₀/a₀ decreases most sensitively with rising Al/Si and also with growing Al^[4]. More experimentation is required before the compositional variations of dumortierite found here can be applied successfully to geothermobarometry of natural rocks.     

Aluminum substitution mechanisms in perovskite-type MgSiO<Subscript>3</Subscript>: an investigation by Rietveld analysis

Al-containing MgSiO₃ perovskites of four different compositions were synthesized at 27 GPa and 1,873 K using a Kawai-type high-pressure apparatus: stoichiometric compositions of Mg_0.975Si_0.975Al_0.05O₃ and Mg_0.95Si_0.95Al_0.10O₃ considering only coupled substitution Mg²⁺ + Si⁴⁺ = 2Al³⁺, and nonstoichiometric compositions of Mg_0.99Si_0.96Al_0.05O_2.985 and Mg_0.97Si_0.93Al_0.10O_2.98 taking account of not only the coupled substitution but also oxygen vacancy substitution 2Si⁴⁺ = 2Al³⁺ + V_O¨. Using the X-ray diffraction profiles, Rietveld analyses were performed, and the results were compared between the stoichiometric and nonstoichiometric perovskites. Lattice parameter–composition relations, in space group Pbnm, were obtained as follows. The a parameters of both of the stoichiometric and nonstoichiometric perovskites are almost constant in the X _Al range of 0–0.05, where X _Al is Al number on the basis of total cation of two (X _Al = 2Al/(Mg + Si + Al)), and decrease with further increasing X _Al. The b and c parameters of the stoichiometric perovskites increase linearly with increasing Al content. The change in the b parameter of the nonstoichiometric perovskites with Al content is the same as that of the stoichiometric perovskites within the uncertainties. The c parameter of the nonstoichiometric perovskites is slightly smaller than that of the stoichiometric perovskites at X _Al of 0.10, though they are the same as each other at X _Al of 0.05. The Si(Al)–O1 distance, Si(Al)–O1–Si(Al) angle and minimum Mg(Al)–O distance of the nonstoichiometric perovskites keep almost constant up to X _Al of 0.05, and then the Si(Al)–O1 increases and both of the Si(Al)–O1–Si(Al) angle and minimum Mg(Al)–O decrease with further Al substitution. These results suggest that the oxygen vacancy substitution may be superior to the coupled substitution up to X _Al of about 0.05 and that more Al could be substituted only by the coupled substitution at 27 GPa. The Si(Al)–O1 distance and one of two independent Si(Al)–O2 distances in Si(Al)O₆ octahedra in the nonstoichiometric perovskites are always shorter than those in the stoichiometric perovskite at the same Al content. These results imply that oxygen defects may exist in the nonstoichiometric perovskites and distribute randomly.     

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.     

Structure of hydrous aluminosilicate glasses along the diopside-anorthite join: A comprehensive one- and two-dimensional H and Al NMR study

We have taken a systematic approach utilizing advanced solid-state NMR techniques to gain new insights into the controversial issue concerning the dissolution mechanisms of water in aluminosilicate melts (glasses). A series of quenched anhydrous and hydrous (∼2 wt% H₂O) glass samples along the diopside (Di, CaMgSi₂O₆)—anorthite (An, CaAl₂Si₂O₈) join with varying An components (0, 20, 38, 60, 80, and 100 mol %) have been studied. A variety of NMR techniques, including one-dimensional (1D) ¹H and ²⁷Al MAS NMR, and ²⁷Al → ¹H cross-polarization (CP) MAS NMR, as well as two-dimensional (2D) ¹H double-quantum (DQ) MAS NMR, ²⁷Al triple-quantum (3Q) MAS NMR, and ²⁷Al → ¹H heteronuclear correlation NMR (HETCOR) and 3QMAS/HETCOR NMR, have been applied. These data revealed the presence of SiOH, free OH ((Ca,Mg)OH) and AlOH species in the hydrous glasses, with the last mostly interconnected with Si and residing in the more polymerized parts of the structure. Thus, there are no fundamental differences in water dissolution mechanisms for Al-free and Al-bearing silicate melts (glasses), both involving two competing processes: the formation of SiOH/AlOH that is accompanied by the depolymerization of the network structure, and the formation of free OH that has an opposite effect. The latter is more important for depolymerized compositions corresponding to mafic and ultramafic magmas.Aluminum is dominantly present in four coordination (Al^IV), but a small amount of five-coordinate Al (Al^V) is also observed in all the anhydrous and hydrous glasses. Furthermore, six-coordinate Al (Al^VI) is also present in most of the hydrous glasses. As Al of higher coordinations are favored by high pressure, Al^VIOH and Al^VOH may become major water species at higher pressures corresponding to those of the Earth’s mantle.     

Substitution mechanisms and solubility of titanium in phlogopites from rocks of probable mantle origin

Previously proposed substitution mechanisms for Ti in phlogopites, based on experimental studies and crystal chemistry, have been examined using data for 81 phlogopites from mantle-derived rocks (primarily as nodules in kimberlites and also from alkali basalts, lamprophyres and carbonatites), 49 phlogopites from high-K rocks with basaltic affinities, and from 32 phlogopites crystallized in high pressure experiments mainly on high-K rock compositions. For the majority of phlogopites from the kimberlite group and for all those crystallized in the experimental studies, the substitution of Ti can be represented by a combination of the mechanisms represented by 2Mg^[VI]⇌ Ti^[VI]□^[VI] and Mg^[VI]2Si^[IV]⇌Ti^[VI]2Al^[IV]. Some phlogopites in ultrapotassic rocks have only the former substitution mechanism. The Ti contents of phlogopites generally increase with decreasing octahedral site occupancy and decreasing Si+Al^[VI]. For the phlogopites crystallized in the experiments on high-K rocks, the solubility of Ti increases with increasing fO₂ and temperature, and possibly with decreasing pressure at constant fO₂. The effect of the composition of the liquids used in the experimental studies from which these phlogopites have crystallized has only minor effect on either the substitution mechanism or the solubility of Ti in phlogopites. This suggests that phlogopite in high-K rocks may be a potential geothermometer and possibly a geobarometer.     

The effect of Fe on Si adsorption by Bacillus subtilis cell walls: insights into non-metabolic bacterial precipitation of silicate minerals

Si adsorption onto Bacillus subtilis and Fe and Al oxide coated cells of B. subtilis was measured both as a function of pH and of bacterial concentration in suspension in order to gain insight into the mechanism of association between silica and silicate precipitates and bacterial cell walls. All experiments were conducted in undersaturated solutions with respect to silicate mineral phases in order to isolate the important adsorption reactions from precipitation kinetics effects of bacterial surfaces. The experimental results indicate that there is little association between aqueous Si and the bacterial surface, even under low pH conditions where most of the organic acid functional groups that are present on the bacterial surface are fully protonated and neutrally charged. Conversely, Fe and Al oxide coated bacteria, and Fe oxide precipitates only, all bind significant concentrations of aqueous Si over a wide range of pH conditions. Our results are consistent with those of Konhauser et al. [Geology 21 (1993) 1103; Environ. Microbiol. 60 (1994) 49] and Konhauser and Urrutia [Chem. Geol. 161 (1999) 399] in that they suggest that the association between silicate minerals and bacterial surfaces is not caused by direct Si–bacteria interactions. Rather, the association is most likely caused by the adsorption of Si onto Fe and Al oxides which are electrostatically bound to the bacterial surface. Therefore, the role of bacteria in silica and silicate mineralization is to concentrate Fe and Al through adsorption and/or precipitation reactions. Bacteria serve as bases, or perhaps templates, for Fe and Al oxide precipitation, and it is these oxide mineral surfaces (and perhaps other metal oxide surfaces as well) that are reactive with aqueous Si, forming surface complexes that are the precursors to the formation of silica and silicate minerals.     

Structural properties of reduced Upton montmorillonite

Reduction of octahedral Fe in the crystalline structure of smectites influences, possibly controls, surface-sensitive physical and chemical properties. The purpose of this study was to investigate if reduction of structural Fe by Na-dithionite or bacteria affects the chemical environment of constituent cations in montmorillonite, employing solid state multinuclear (²⁹Si and ²⁷Al) magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Reduction of structural Fe resulted in a positive (down field) chemical shift of the main Si Q³ (Q³(0Al)) site which was strongly correlated with Fe(II) content and inferred that distortions in Si-OT (T=Si, Al) bond angles and Si-O bond lengths occur with increasing layer charge. The line width (W) of the ²⁹Si Q³ signal also increased with increasing levels of reduction. No change occurred in the position of the peak maximum for the octahedral Al (²⁷Al_VI) signal; however, an increased W was observed for this peak with increasing Fe(II) content. These results are attributed to decreases in Si-O-T bond angles and Si-O bond distances, corresponding to a better fit between the tetrahedral and octahedral sheets brought about by the presence of Fe(II) in the clay structure. The increased ²⁷Al_VI signal width (W) may also be due to a lessening of the paramagnetic influence of Fe(III) nuclei and enhancement of ²⁷Al_VI signals with different quadrupole coupling constants (QCC). Multinuclear MAS NMR analyses of dithioniteand microbially-reduced montmorillonite indicate that reduction of structural Fe caused reversible changes in the smectite structure, at least as far as this method could discern.     

Transformation of meta-stable calcium silicate hydrates to tobermorite: reaction kinetics and molecular structure from XRD and NMR spectroscopy

Understanding the integrity of well-bore systems that are lined with Portland-based cements is critical to the successful storage of sequestered CO₂ in gas and oil reservoirs. As a first step, we investigate reaction rates and mechanistic pathways for cement mineral growth in the absence of CO₂ by coupling water chemistry with XRD and NMR spectroscopic data. We find that semi-crystalline calcium (alumino-)silicate hydrate (Al-CSH) forms as a precursor solid to the cement mineral tobermorite. Rate constants for tobermorite growth were found to be k = 0.6 (± 0.1) × 10^-5 s^-1 for a solution:solid of 10:1 and 1.6 (± 0.8) × 10^-4 s^-1 for a solution:solid of 5:1 (batch mode; T = 150°C). This data indicates that reaction rates for tobermorite growth are faster when the solution volume is reduced by half, suggesting that rates are dependent on solution saturation and that the Gibbs free energy is the reaction driver. However, calculated solution saturation indexes for Al-CSH and tobermorite differ by less than one log unit, which is within the measured uncertainty. Based on this data, we consider both heterogeneous nucleation as the thermodynamic driver and internal restructuring as possible mechanistic pathways for growth. We also use NMR spectroscopy to characterize the site symmetry and bonding environment of Al and Si in a reacted tobermorite sample. We find two ^[4]Al coordination structures at δ _iso = 59.9 ppm and 66.3 ppm with quadrupolar product parameters (P_Q) of 0.21 MHz and 0.10 MHz (± 0.08) from ²⁷Al 3Q-MAS NMR and speculate on the Al occupancy of framework sites by probing the protonation environment of Al metal centers using ²⁷Al{¹H}CP-MAS NMR.     

Influence of glass composition and alteration solution on leached silicate glass structure: A solid-state NMR investigation

A multinuclear solid-state NMR investigation of the structure of the amorphous alteration products (so called gels) that form during the aqueous alteration of silicate glasses is reported. The studied glass compositions are of increasing complexity, with addition of aluminum, calcium, and zirconium to a sodium borosilicate glass. Two series of gels were obtained, in acidic and in basic solutions, and were analyzed using ¹H, ²⁹Si, and ²⁷Al MAS NMR spectroscopy. Advanced NMR techniques have been employed such as ¹H-²⁹Si and ¹H-²⁷Al cross-polarization (CP) MAS NMR, ¹H double quantum (DQ) MAS NMR and ²⁷Al multiple quantum (MQ) MAS NMR. Under acidic conditions, ²⁹Si CP MAS NMR data show that the repolymerized silicate networks have similar configuration. Zirconium as a second nearest neighbor increases the ²⁹Si isotropic chemical shift. The gel porosity is influenced by the pristine glass composition, modifying the silicon-proton interactions. From ¹H DQ and ¹H-²⁹Si CP MAS NMR experiments, it was possible to discriminate between silanol groups (isolated or not) and physisorbed molecular water near Si (Q²), Si (Q³), and Si (Q⁴) sites, as well as to gain insight into the hydrogen-bonding interaction and the mobility of the proton species. These experiments were also carried out on heated samples (180 °C) to evidence hydrogen bonds between hydroxyl groups on molecular water. Alteration in basic media resulted in a gel structure that is more dependent on the initial glass composition. ²⁷Al MQMAS NMR data revealed an exchange of charge compensating cations of the [AlO₄]⁻ groups during glass alteration. ¹H-²⁷Al CP MAS NMR data provide information about the proximities of these two nuclei and two aluminum environments have been distinguished. The availability of these new structural data should provide a better understanding of the impact of glass composition on the gel structure depending on the nature of the alteration solution.     

The compositional variation of synthetic sodic amphiboles at high and ultra-high pressures

Sodic amphiboles in high pressure and ultra-high pressure (UHP) metamorphic rocks are complex solid solutions in the system Na₂O–MgO–Al₂O₃–SiO₂–H₂O (NMASH) whose compositions vary with pressure and temperature. We conducted piston-cylinder experiments at 20–30?kbar and 700–800?°C to investigate the stability and compositional variations of sodic amphiboles, based on the reaction glaucophane=2jadeite+talc, by using the starting assemblage of natural glaucophane, talc and quartz, with synthetic jadeite. A close approach to equilibrium was achieved by performing compositional reversals, by evaluating compositional changes with time, and by suppressing the formation of Na-phyllosilicates. STEM observations show that the abundance of wide-chain structures in the synthetic amphiboles is low. An important feature of sodic amphibole in the NMASH system is that the assemblage jadeite–talc?±?quartz does not fix its composition at glaucophane. This is because other amphibole species such as cummingtonite (Cm), nyböite (Nyb), Al–Na-cummingtonite (Al–Na-Cm) and sodium anthophyllite (Na-Anth) are also buffered via the model reactions: 3cummingtonite?+?4quartz?+?4H₂O=7talc, nyböite?+?3quartz=3jadeite?+?talc, 3Al–Na-cummingtonite + 11quartz + 2H₂O=6jadeite + 5talc, and 3 sodium anthophyllite?+?13quartz?+?4H₂O=3 jadeite + 7talc. We observed that at all pressures and temperatures investigated, the compositions of newly grown amphiboles deviate significantly from stoichiometric glaucophane due to varying substitutions of Al^IV for Si, Mg on the M(4) site, and Na on the A-site. The deviation can be described chiefly by two compositional vectors: [Na^AAl^IV]<=>[□^ASi] (edenite) toward nyböite, and [Na^(M4)Al^VI]<=>[Mg^(M4)Mg^VI] toward cummingtonite. The extent of nyböite and cummingtonite substitution increases with temperature and decreases with pressure in the experiments. Similar compositional variations occur in sodic amphiboles from UHP rocks. The experimentally calibrated compositional changes therefore may prove useful for thermobarometric applications.     

29Si and 27Al MAS NMR spectroscopy of mullite

A ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance study is reported for differently synthesized mullites. The ²⁹Si MAS NMR spectra of all samples are essentially identical. They consist of a main resonance at -86.8 ppm, a shoulder around -90 ppm and a second resonance at -94.2 ppm. The main resonance is interpreted as being due to a sillimanite-type geometry around Si and the second one is tentatively assigned to a Si environment typical for mullite. The ²⁷Al MAS NMR spectra of sinter- and fused-mullite measured at different Larmor frequencies revealed clearly the presence of three distinct Al sites in mullite, i.e. of octahedral (M1), tetrahedral (M2) and distorted tetrahedral (Al^*) sites.