Solid-state Buffers Research Articles

Sulfur dissolution capacity of highly hydrated and fluid-saturated dacitic magmas at the lower crust and implications for porphyry deposit formation

Arc basalts with high H2O content are expected to evolve into highly hydrated felsic melts saturated with aqueous fluids at lower crustal pressures, which are potentially important in providing sulfur (S) and metals for porphyry deposit formation. However, the role of such highly hydrated and fluid-saturated magmas in the transfer of S (and metals) remains rarely studied. To investigate the S dissolution capacity of highly hydrated felsic magmas saturated with fluids at the lower crustal conditions, this study performed experiments with dacite + 10 wt% FeS at H2O contents of 10, 15, and 20 wt%. All the experiments were performed in piston-cylinder apparatus at 1.0 GPa and 950 °C with fO2 conditions imposed by the following solid-state buffers: cobalt-cobalt oxide (CCO), nickel-nickel oxide (NNO), rhenium-rhenium oxide (RRO), MnO-Mn3O4 (MMO), and hematite-magnetite (HM), resulting in a fO2 range of FMQ−1.5 to FMQ+3.3 log units (FMQ = fayalite + magnetite + quartz oxgen buffer). The experimental results show that the H2O content in the felsic melt, in addition to fO2 and melt composition, has a strong effect on the concentration of S in the silicate melt at sulfide ± anhydrite saturation. At a given fO2, the S concentration in the melt increases with increasing H2O content (hydration), resulting in a S concentration of up to ∼8000 ppm in the highly hydrated dacitic melts at fO2 ∼ FMQ+2. We found that high melt hydration at an intermediate fO2 not only dramatically increases sulfide and anhydrite solubilities in the dacitic melts but also promotes the saturation of S-rich fluids. The S concentration in the fluid that corresponds to the highest measured S concentrations in the melt (∼8000 ppm) are ∼5 wt%. These results show that highly hydrated felsic magmas saturated with fluids have a high capacity for S dissolution. Therefore, injection of such fluid-saturated magmas from lower crustal to shallow magma reservoirs is a potential mechanism for the effective transport of S (and perhaps metals) to facilitate subsequent porphyry ore deposit formation.

Glass-ionomer dental cements as novel solid-state buffers

This study examined the possible buffering effect of acid-base glass-ionomer cements. Commercial capsulated materials were used, namely: Riva Self Cure, Ketac Molar, Equia Forte and Chemfil Rock. Cements were prepared by vibratory mixing then cylinders (4 mm diameter, 6 mm height) prepared from them using metal moulds. They were cured in the moulds at 37 °C for one hour, then placed in 8 ml deionised water at 22 °C and stored for 24 h or 4 weeks. Then they were crushed to powders with a pestle and mortar. Ten samples per powder (0.05 g) were placed in individual 10 ml volumes of deionised water. For two sets of ten (24 h or 4 weeks old), 1.0 ml of 0.01 M HCl was added, equilibrated and the pH measured by calibrated electrode. For another two sets of ten (24 h or 4 weeks old), 1.0 ml of 0.01 M NaOH was added, equilibrated, and the pH measured. The pH of a control set of solutions (no cement powder) was also measured. Data were tested using 1-way ANOVA and Tukey HSD test. The original acid solution pH was 3.2, which rose to 4.4–5.5 depending on brand and age of cement. The original alkali solution pH was 11.3, which fell to 6.7–8.3 depending on the details of the cement. Differences from initial pH had Tukey HSD p values of 0.001 in all cases. It was concluded that acid-base glass-ionomers can act as solid-state buffers, a finding attributed to the carboxylic acid/carboxylate conjugate pairs within them.

U3Si2 behavior in H2O environments: Part II, pressurized water with controlled redox chemistry

Recent interest in U3Si2 as an advanced light water reactor fuel has driven assessment of numerous properties, but characterization of its response to H2O environments is sparse in available literature. The behavior of U3Si2 in H2O containing atmospheres is investigated and presented in a two-part series of articles. This work examines the behavior of U3Si2 following exposure to pressurized H2O at temperatures from 300 to 350 °C. Testing was performed using two autoclave configurations and multiple redox conditions. Use of solid state buffers to attain a controlled water chemistry is also presented as a means to test actinide-bearing systems. Buffers were used to vary the hydrogen concentration between 1 and 30 parts per million H2. Testing included UN, U3Si5, and UO2. Both UN and U3Si5 were found to rapidly pulverize in less than 50 h at 300 °C. Uranium dioxide was included as a control for the autoclave system, and was found to be minimally impacted by exposure to pressurized water at the conditions tested for extended time periods. Testing of U3Si2 at 300 °C found reasonable stability through 30 days in 1–5 ppm H2. However, pulverization was observed following 35 days. The redox condition of testing strongly affected pulverization. Characterization of the resulting microstructures suggests that the mechanism responsible for pulverization under more strongly reducing conditions differs from that previously identified. Hydride formation is hypothesized to drive this transition. Testing performed at 350 °C resulted in rapid pulverization of U3Si2 in under 50 h.

The effect of pressure on sulphur speciation in mid- to deep-crustal arc magmas and implications for the formation of porphyry copper deposits

Piston cylinder experiments are used to investigate the effect of oxygen fugacity (ƒO2) on sulphur speciation and phase relations in arc magmas at 0.5–1.5 GPa and 840–950 °C. The experimental starting composition is a synthetic trachyandesite containing 6.0 wt% H2O, 2880 ppm S, 1500 ppm Cl and 3800 ppm C. Redox conditions ranging from 1.7 log units below the Ni–NiO buffer (NNO − 1.7) to NNO + 4.7 were imposed by solid-state buffers: Co–CoO, Ni–NiO, Re–ReO2 and haematite–magnetite. All experiments are saturated with a COH fluid. Experiments produced crystal-bearing trachydacitic melts (SiO2 from 60 to 69 wt%) for which major and volatile element concentrations were measured. Experimental results demonstrate a powerful effect of oxidation state on phase relations. For example, plagioclase was stable above NNO, but absent at more reduced conditions. Suppression of plagioclase stability produces higher Al2O3 and CaO melts. The solid sulphur-bearing phases and sulphur speciation in the melt are strong functions of ƒO2, as expected, but also of pressure. At 0.5 GPa, the anhydrite stability field is intersected at NNO ≥ +2, but at 1.0 and 1.5 GPa, experiments at the same ƒO2 produce sulphides and the stability field of sulphate moves towards higher ƒO2 by ~1 log unit at 1.0 GPa and ~1.5 log units at 1.5 GPa. As a result, models that appeal to high oxidation state as an important control on the mobility of Cu (and other chalcophiles) during crustal differentiation must also consider the enhanced stability of sulphide in deep- to mid-crustal cumulates even for relatively oxidized (NNO + 2) magmas. Experimental glasses reproduce the commonly observed minimum in sulphur solubility between the S2− and S6+ stability fields. The solubility minimum is not related to the Fe content (Fe2+/Fe3+ or total) of the melt. Instead, we propose this minimum results from an unidentified, but relatively insoluble, S-species of intermediate oxidation state.

Diffusion of phosphorus in olivine and molten basalt

The diffusivity of phosphorus in San Carlos olivine (SCO) was measured at near-atmospheric pressure and 650–850 °C by in-diffusion of P from a surface powder source consisting of pre-reacted SCO and AlPO 4 . The experiments were conducted in evacuated silica-glass ampoules at oxygen fugacities fixed by solid-state buffers, generally Ni-NiO but also including two experiments buffered at wustite-magnetite. Phosphorus uptake profiles were characterized by Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA). The temperature dependence of P diffusion in SCO conforms to the expected Arrhenius relation D = D 0 exp(− E a /R T ), where the constants are as follows: log( D 0 , m 2 /s) = −10.06 ± 0.80 and E a = 229 ± 16 kJ/mol. These values characterize P as a relatively slow diffuser in olivine—slower by about an order of magnitude than Cr and Ca at basalt near-liquidus temperatures—but substantially faster than Si. With a view toward modeling P uptake during rapid growth of natural olivines, P diffusion was also characterized in dry MORB basalt melt over the temperature range 1250–1500 °C at 1 GPa, using traditional diffusion couples contained in graphite. Phosphorus diffusion profiles in the quenched and depressurized samples were quantified by laser-ablation ICP/MS. Phosphorus diffusion in basaltic melt is similar to that of Si, with log( D 0 , m 2 /s) = −6.30 ± 0.7 and E a = 147 ± 22 kJ/mol. The new data for P diffusion in olivine and basalt melt can be used to explore the acquisition of fine-scale zoning in natural olivine phenocrysts through kinetic models, as well as the survival of P zoning in olivine with time spent at elevated temperature. Models of growth entrapment of a P-enriched near-surface layer in the olivine lattice indicate that crystal growth at plausible sustained rates is indeed likely to result in regions of anomalously high P content in the resulting crystal. Phosphorus concentrations above the equilibrium partitioning value can also result from development of a diffusive boundary layer in the melt against a rapidly growing crystal, but this mechanism is ineffective at typical sustained olivine growth rates, requiring dendrite-forming growth speeds. Preservation of P zoning on the scale of a few micrometers apparently requires cooling within a few months of formation of the zoning.

Dependence of dislocation creep of dunite on oxygen fugacity: Implications for viscosity variations in Earth's mantle

[1] Significant variations in flow behavior are known to exist both vertically and laterally in Earth's upper mantle. The sources of such variation may be thermal, compositional, or reflect differences in the chemical activity of components, such as oxygen, silica, and water. We report on the effects of oxygen fugacity on dislocation creep of dunite, with a view to understanding potential strength heterogeneity in the mantle. Although room pressure experiments on single crystals of olivine have shown a clear dependence of creep rate on oxygen fugacity, no prior deformation study of polycrystalline olivine-rich rocks has demonstrated such a dependency under high-pressure conditions. In this study we performed a series of dry creep experiments on a natural dunite under carefully controlled thermochemical conditions, including oxygen fugacity. The samples, cored from coarse-grained Aheim dunite with a grain size of ∼0.9 mm, were deformed under triaxial compression at oxygen fugacities fixed by either the iron/wustite or the nickel/nickel oxide solid state buffers, temperatures between 1150° and 1277°C, and differential stresses up to 300 MPa. The results of a global fit to all experimental data indicate a power law dependence of creep rate on oxygen fugacity, with an oxygen fugacity exponent of m = 0.20 ± 0.01, n = 3.6 ± 0.1, A = 102.6±0.3 s−1 MPa−3.6 Pa−0.2, and an activation energy for creep of 449 ± 7 kJ/mol. This activation energy is significantly less than the commonly used value of 535 kJ/mol because the earlier experiments made no corrections for the effects of oxygen fugacity. When applied to planetary interiors, an increase in oxygen fugacity by a factor of ∼103.5, from the iron/wustite to the fayalite-magnetite-quartz buffers, will result in a factor of ∼5 decrease in viscosity.

PI-based fractionation of serum proteomes versus anion exchange after enhancement of low-abundance proteins by means of peptide libraries

The pre-treatment of biological extracts with the aim of detecting very low-abundance proteins generates complexity requiring a proper fractionation. Therefore the success of identifying all newly detectable species depends on the selected fractionation methods. In this context and starting from a human serum, where the dynamic concentration range was reduced by means of a preliminary treatment with a combinatorial hexapeptide ligand library, we fractionated the sample using a novel method based on the differences in isoelectric points of proteins by means of Solid-State Buffers (SSB) associated with cation exchangers. The number of fractions was limited to four and was compared to a classical anion exchange method generating the same number of fractions. What was observed is that when using SSB technology the protein redundancy between fractions was significantly reduced compared to ion exchange fractionation allowing thus a better detection of novel species. The analysis of trypsinized protein fractions by nanoLC-MS/MS confirmed that the SSB technology used is more discriminant than anion exchange chromatography fractionation. A sample fractionation by SSB after the reduction of dynamic concentration range can be accomplished without either adjustment of pH and ionic strength or protein concentration and cleanup. Both advantages over either classical chromatography or isoelectric fractionations allow approaching the discovery of markers of interest under easier conditions applicable in a variety of fields of investigation.

A pI-based protein fractionation method using solid-state buffers

The analysis of very complex proteomes is dependent on efficient fractionation methods with low level of carry over from fraction to fraction. Among various possibilities the separation by ranges of isoelectric points for further analysis appears as attractive, but current methods involving an electrically driven migration in the presence of ampholyte carriers are not exempt of technical complications. In the present work a new separation concept is described involving the use of so-called solid-state buffers, in association with ion exchangers, to separate protein categories of different pI ranges with a low level of protein overlapping. Resin blends packed in separated columns are used under a cascade configuration of increasing or decreasing pH and, once proteins of different pI are adsorbed by individual resin blends, the columns are dissociated. From each column protein mixtures corresponding to a given pI range are collected by competitive desorption with salts so as to be ready for proteomic analysis. The process is rapid and does not involve electrical fields nor addition of carrier ampholyte material. The presence of potassium chloride during the separation prevents protein precipitation at the vicinity of their isoelectric points. The fractions thus obtained can be used for two dimensional electrophoresis and mass spectrometry analysis after the removal of salts.

Cutinase activity in supercritical and organic media: water activity, solvation and acid–base effects

We performed a comparative study on the activity of Fusarium solani pisi cutinase immobilized on zeolites NaA and NaY, in n-hexane, acetonitrile, supercritical ethane (sc-ethane) and sc-CO 2, at two different water activity ( a W) values set by salt hydrate pairs in situ and at acid–base conditions fixed with solid-state buffers of aqueous p K a between 4.3 and 10.6. The reaction studied was the transesterification of vinyl butyrate by ( R,S)-2-phenyl-1-propanol. The transesterification activity of cutinase was highest and similar in sc-ethane and in n-hexane, about one order of magnitude lower in acetonitrile and even lower in sc-CO 2. Activity coefficients ( γ) generated for the two substrates indicated that they were better solvated in acetonitrile and thus less available for binding at the active site than in the other three solvents. γ data also suggested higher reaction rates in sc-ethane than in n-hexane, as observed, and provided evidence for a direct negative effect of sc-CO 2 on enzyme activity. Manipulation of the acid–base conditions of the media did not afford any improvement of the initial rates of transesterification relative to the blanks (no added acid–base buffer, only salt hydrate pair), except in the case of cutinase immobilized on zeolite NaA in sc-ethane at a W = 0.7. The poor performance of the blank in this case and the great improvement observed in the presence of a basic buffer suggest a deleterious acidic effect in the medium which, an experiment without additives confirmed, was not due to the known acidic character of the salt hydrate pair used to set a W = 0.7. In acetonitrile, increasing a W was accompanied by a decrease in initial rates of transesterification, unlike in the other solvents. There was considerable hydrolysis in acetonitrile, where initial rates of hydrolysis increased about 20-fold from a W = 0.2 to 0.7. Hydrolysis was less pronounced in sc-ethane and in n-hexane, and only at a W = 0.7, and in sc-CO 2 butyric acid was detected only at very long reaction times, in agreement with a generally low catalytic activity. Cutinase enantio-selectivity towards the alcohol substrate was low and unaffected by any manipulation of medium conditions.

High-activity biocatalysts in organic media: solid-state buffers as the immobilisation matrix for protein-coated microcrystals.

Recently, we reported a new high-activity biocatalyst for use in organic media termed protein-coated microcrystals (PCMC) (Kreiner et al. [2001] Chem Commun 12:1096-1097). These novel particles consist of water-soluble micron-sized crystalline particles coated with the given biocatalyst(s) and are prepared in a one-step rapid dehydration process. In this study we extended the choice of immobilisation matrix from a simple inorganic salt, K(2)SO(4), to other compounds, both inorganic and zwitterionic, that act as solid-state buffers for biocatalysis in organic media. The catalytic activity of serine proteases subtilisin Carlsberg (SC) and alpha-chymotrypsin (CT) were significantly increased when coated onto the surface of solid-state buffers, as measured in acetonitrile/1wt% H(2)O. SC-PCMC with both organic and inorganic buffer carriers (Na-AMPSO, Na(2)CO(3), and NaHCO(3)) showed a 3-fold greater activity than that observed when using the unbuffered system (PCMC-SC/K(2)SO(4)). In comparison with freeze-dried preparations, this represents an approximately 3,000-fold increase in catalytic activity. Importantly, there is no improvement in catalytic activity upon external addition of any of the solid-state buffers to the reaction mixture. When acting in a solid-state buffer capacity, good buffering capacity was observed with SC-PCMC (3 wt% protein loading) prepared from a 1:1 mixture of AMPSO and AMPSO-Na. Alternatively, increasing the amount of solid-state buffer in the system allows improvement of the buffering. This can be achieved either by decreasing the protein loading of the SC/Na-AMPSO-PCMC or by addition of further external solid-state buffer to the reaction mixture. The catalytic activity of lipase-PCMC prepared from solid-state buffers was found less responsive to immobilisation.

Crystal chemistry of Fe-containing sphalerites

Cell dimensions and solvus properties of Fe-containing sphalerites, depending on temperature and sulfur fugacity, were investigated using equilibrated powdered materials synthesized from elements and binary sulfides under vacuum. The Fe solvus in sphalerite, determined by optical microscopy and microprobe analysis, are directly correlated with increasing temperature and decreasing sulfur fugacity controlled by solid-state buffers. The increase of lattice parameters with Fe correlates with an increase of FeS independent of sulfur fugacity up to 10 mol% FeS within ZnS. Above about 10 mol% the lattice parameters are strongly depending on the sulfur fugacity controlled Fe3+/Fe2+ ratios. The Fe3+/Fe2+ ratios determined by Moessbauer spectroscopy and involving metal vacancies depend on the sulfur fugacity. The critical Fe2+ content determined by experimental simulations as well as the minimal Fe3+/ Fe2+ ratios agree with the required minimal Fe content for CuFeS2-DIS in sphalerite. The critical Fe2+ content also agrees with the change of Moessbauer signal from a singlet to a doublet for Fe2+ containing sphalerite. Pyrrhotite exsolutions in sphalerite caused by higher sulfur fugacity show orientationally intergrown with the sphalerite matrix. Density data calculated from lattice parameters and composition are compared with experimental density measurements.

Effect of solid-state buffers on the catalytic activity of papain in low water media

The catalytic activity of papain in the synthesis of Z-Gly-Phe-NH 2 in tert-butanol has been studied in the presence of solid-state acid–base buffers (acids and their sodium salts). All buffer pairs tested reduced the reaction rate compared with the control, particularly the most acidic and basic (assessed by either aqueous p K a or the response of an organic phase indicator). The highest rates, close to the control, were found with glutamic acid/glutamate-Na, PIPES/PIPES-Na and NaH 2PO 4/Na 2HPO 4. However, these pairs were unable to erase the pH memory phenomenon, or to overcome the effect of spiking with acetic acid. Hence, at least these buffers do not seem to be able to affect the protonation state and catalytic activity of papain. In the last aqueous solution before drying, the presence of activating agents (cysteine plus EDTA) was more important than buffer ions.

Tuning lipase enantioselectivity in organic media using solid-state buffers.

The enantioselectivity exhibited by Candida antarctica lipase B (CALB) in predominantly organic media has been studied for different enzyme protonation states. Alcoholysis of (+/-)-2-phenyl-4-benzyloxazol-5(4H)-one (1) using butan-1-ol as the nucleophile in low-water organic solvents was used as a model reaction. Using either organo-soluble bases or the newly introduced solid-state buffers of known pK(a), the protonation state of the lipase was altered. By choice of the appropriate solid-state buffer or organic base, the enantioselectivity could be selectively tuned. Both Et(3)N and the solid-state buffer pair CAPSO/CAPSO.Na were found to increase the enantioselectivity of the reaction catalyzed by CALB and that of another lipase (Mucor miehei). Significant differences to both the enantioselectivity and catalytic rate were observed, especially under hydrated conditions where byproduct acid was formed.

Enzymatic dehalogenation of gas phase substrates with haloalkane dehalogenase

Haloalkane dehalogenase is an enzyme capable of catalyzing the conversion of short-chained (C2–C8) aliphatic halogenated hydrocarbons to a corresponding primary alcohol. Because of its broad substrate specificity for mono-, di-, and trisubstituted halogenated hydrocarbons and cofactor independence, haloalkane dehalogenases are attractive biocatalysts for gas-phase bioremediation of pollutant halogenated vapor emissions. A solid preparation of haloalkane dehalogenase from Rhodococcus rhodochrous was used to catalyze the dehalogenation reaction of 1-chlorobutane or 1,3-dichloropropane delivered in the gas phase. For optimal gas-phase dehalogenase activity, a relative humidity of 100%, aw = 1, was desired. With a 50% reduction in the vapor-phase hydration level, an 80% decrease in enzymatic activity was observed. The enzyme kinetics for the gas-phase substrates obeyed an Arrhenius-“like” behavior and the solid haloalkane dehalogenase preparation was more thermally stable than its water-soluble equivalent. Triethylamine was added to the gaseous reaction environment in efforts to increase the rate of reaction. A tenfold increase in the dehalogenase activity for the vapor-phase substrates was observed with the addition of triethylamine. Triethylamine altered the electrostatic environment of haloalkane dehalogenase via a basic shift in local pH, thereby minimizing the effect of the pH-reducing reaction product on enzyme activity. Both organic phase and solid-state buffers were used to confirm the activating role of the altered ionization state. © 2000 John Wiley & Sons, Inc., Biotechnol Bioeng 69: 235–241, 2000.

Acid-base control for biocatalysis in organic media: new solid-state proton/cation buffers and an indicator

Although great care is generally taken to buffer aqueous enzyme reactions, active control of acid-base conditions for biocatalysis in low-water media is rarely considered. Here we describe a new class of solid-state acid-base buffers suitable for use in organic media. The buffers, composed of a zwitterion and its sodium salt, are able to set and maintain the ionisation state of an enzyme by the exchange of H+ and Na+ ions. Surprisingly, equilibrium is established between the different solid components quickly enough to provide a practical means of controlling acid-base conditions during biocatalysed reactions. We developed an organosoluble chromoionophore indicator to screen the behaviour of possible buffer pairs and quantify their relative H+/Na- exchange potential. The transesterification activity of an immobilised protease, subtilisin Carlsberg, was measured in toluene in the presence of a range of buffers. The large observed difference in rates showed good correlation with that expected from the measured exchange potentials. The maximum water activities accessible without formation of hydrates or solutions of the buffers are reported here. The indicator was also used to monitor, for the first time in situ, changes in the acid-base conditions of an enzyme-catalysed transesterification reaction in toluene. We found that even very minor amounts of an acidic by-product of hydrolysis were leading to protonation of the enzyme, resulting in rapid loss of activity. Addition of solid-state buffer was able to prevent this process, shortening reaction times and improving yields. Solid-state buffers offer a general and inexpensive way of precisely controlling acid-base conditions in organic solvents and thus also have potential applications outside of biocatalysis.

Enzymatic dehalogenation of gas phase substrates with haloalkane dehalogenase

Haloalkane dehalogenase is an enzyme capable of catalyzing the conversion of short-chained (C(2)-C(8)) aliphatic halogenated hydrocarbons to a corresponding primary alcohol. Because of its broad substrate specificity for mono-, di-, and trisubstituted halogenated hydrocarbons and cofactor independence, haloalkane dehalogenases are attractive biocatalysts for gas-phase bioremediation of pollutant halogenated vapor emissions. A solid preparation of haloalkane dehalogenase from Rhodococcus rhodochrous was used to catalyze the dehalogenation reaction of 1-chlorobutane or 1,3-dichloropropane delivered in the gas phase. For optimal gas-phase dehalogenase activity, a relative humidity of 100%, a(w) = 1, was desired. With a 50% reduction in the vapor-phase hydration level, an 80% decrease in enzymatic activity was observed. The enzyme kinetics for the gas-phase substrates obeyed an Arrhenius-"like" behavior and the solid haloalkane dehalogenase preparation was more thermally stable than its water-soluble equivalent. Triethylamine was added to the gaseous reaction environment in efforts to increase the rate of reaction. A tenfold increase in the dehalogenase activity for the vapor-phase substrates was observed with the addition of triethylamine. Triethylamine altered the electrostatic environment of haloalkane dehalogenase via a basic shift in local pH, thereby minimizing the effect of the pH-reducing reaction product on enzyme activity. Both organic phase and solid-state buffers were used to confirm the activating role of the altered ionization state.

Solid-state proton/sodium buffers: “chemical pH stats” for biocatalysts in organic solvents

The useful application of enzymes in organic synthesis requires reliable and straightforward methods for maximising efficiency and selectivity. Here we describe how to control the protonation state of enzymes using a new class of solid-state buffers, applied for the first time in polar organic solvents. Remarkably these insoluble buffers are able to rapidly exchange H+ and Na+ ions with both the protein and reaction mixture as demonstrated here using propanol rinsed enzyme preparations (PREPs) of subtilisin Carlsberg and chymotrypsin. The buffers tested were generally mixtures of a zwitterionic biological buffer and its Na+ salt with each buffer pair setting a characteristic fixed ratio of H+ activity to Na+ activity (aH+/aNa+) within the system. Dependent upon the solid buffer pair selected a wide range of different enzymatic activities could be observed. The variation in rate showed a fairly good but not exact correlation with the aqueous pKa of the buffers, indicating that crystal lattice energies have less affect on acid–base strength than might be expected. The solid-state buffers were able to prevent detrimental changes to enzyme activity caused by the presence or build up of acids or bases in the organic reaction mixture (often undetected). They could also be used advantageously to tune the enzyme protonation state in solvent if a previous aqueous preparation step needs to be carried out at a pH not optimal for catalysis. Such buffering systems are expected to find wide-spread use as ‘chemical pH stats’ for reactions in non-aqueous media.

A M�ssbauer and X-ray diffraction study of annites synthesized at different oxygen fugacities and crystal chemical implications

A refined set of Mossbauer parameters (isomer shifts, quadrupole splittings, Fe2+/Fe3+ ratios) and lattice parameters were obtained from annites synthesized hydrothermally at pressures between 3 and 5 kbars, temperatures ranging from 250 to 780° C and oxygen fugacities controlled by solid state buffers (NNO, QMF, IM, IQF). Mossbauer spectra showed Fe2+ and Fe3+ on both the M1 and the M2 site. A linear relationship between Fe3+ content and oxygen fugacity was observed. Towards low Fe3+ values this linear relationship ends at ≈10% of total iron showing that the Fe3+ content cannot be reduced further even if more reducing conditions are used. This indicates that in annite at least 10% Fe2+ are substituted by Fe3+ in order to match the larger octahedral layer to the smaller tetrahedral layer. IR spectra indicate that formation of octahedral vacancies plays an important role for charge balance through the substitution 3 Fe2+ → 2 Fe3+ + ▪(oct).

High-temperature creep of olivine single crystals, 2. dislocation structures

Dislocation structures and densities in experimentally deformed single crystals of San Carlos olivine were examined using the oxidation-decoration technique. The high-temperature deformation experiments were conducted at various temperatures ( T), differential stresses (σ), and oxygen fugacities ( ƒ o 2 ); the samples were buffered against either orthopyroxene (opx) or magnesiowustite (mw) solid-state powders and compressed along one of the three 45° orientations. In samples that were deformed and subsequently quenched under load, seven distinct dislocation structures were observed. (a) For samples compressed parallel to [110] c at 16 < σ < 50 MPa and ƒ o 2 = 10 −6 atm , two different dislocation arrangements were identified. For opx-buffered samples at 1400 < T < 1475° C, the dislocation structure was composed of curved screw segments that had usually cross-slipped and straight edge segments of comparable length. At 1300 < T < 1350° C for opx-buffered samples and at 1350 < T < 1500° C for mw-buffered samples, the dislocation structure is dominated by arrays of very long edge or mixed dislocations with only a few screw dislocations, (b) For both opx-buffered and mw-buffered samples compressed parallel to [101] c at 1340 < T < 1400° C and 30 < σ <60 MPa, the dislocation configurations are characterized by zigzag near-edge dislocations at ƒ o 2 = 10 −5 atm and by straight, cross-slipped screw dislocations plus pinned edge dislocations at ƒ o 2 = 10 −9 atm . (c) For samples compressed parallel to [011] c at T = 1400° C and 70 < σ < 150MPa, the primary dislocations change from long, straight edge dislocations (for ƒ o 2 = 10 −4 atm and opx buffer) to a combination of straight edge dislocations and polygon-shaped half loops (for ƒ o 2 = 10 −4 atm and mw buffer), to gently curved near-edge dislocations (for ƒ o 2 = 10 −9 atm for both solid-state buffers). Abundant long, straight screw dislocations were present in each case for the [011] c samples. All of the above variations in dislocation structure are paralleled by changes in the measured power-law equation that describes the behavior of creep mechanisms of olivine. Thus, the different dislocation structures are associated with different rate-controlling creep mechanisms. For all of the dislocation structures, the dislocation density (ρ) increases with differential stress according to the relation ρ ∞ σ 1.4. Dislocation structures in samples that were deformed and then statically annealed have also been studied. For both [110] c and [011] c samples deformed over a wide range of experimental conditions, the dislocation structures on the slip planes generally consist of short, curved edge or mixed dislocation segments. In [101] 1 samples deformed at low ƒ o 2 , 50 MPa and 1400°C, the dislocation structure was much less altered by annealing than the dislocation structures in [110] c and [011] c samples. The density of dislocations decreased much less for these [101] c samples than for the [110] c and [011] c samples. Due to climb of the edge segments, the straight screw segments found in quenched samples became curved and many low-angle (100) tilt boundaries, connected by straight [100] screw dislocations, developed during the annealing process.

Crystal chemistry of a natural schorlomite and Ti-andradites synthesized at different oxygen fugacities

Ti-andradites were synthesized at a pressure of P(H2O)=3 kbar and temperatures of 700–800° C. Oxygen fugacities were controlled by solid state buffers (Ni/NiO; SiO2 + Fe/Fe2SiO4). The Fe2+-and Fe3+-distribution was determined by low temperature Mossbauer spectroscopy. The water content was measured by a solid's moisture analyzer. The chemical composition of the synthetic and the natural sample has been determined by electron microprobe. Ti-andradites from runs at high oxygen fugacities have Fe3+ on octahedral and tetrahedral sites; Ti-andradites from runs at low oxygen fugacities have tetrahedrally and octahedrally coordinated Fe2+ as well. These “reduced” garnets must also contain Ti3+ on octahedral sites. Charge balance is maintained due to substitution of O2− by (OH)− by two mechanisms: (SiO4)4− ⇌ (O4H4)4− and (Fe3+O6)9− ⇌ (Fe2+O5OH)9−. FTIR spectra of the synthetic samples do show the presence of structurally bound (OH)−. In a natural sample tetrahedrally and octahedrally coordinated Fe3+ are observed together with Fe2+ on all three cation sites of the garnet structure.