IR spectroscopic characterization of NH4-analcime

 The IR spectrum of ammonium-exchanged natural analcime (basalt, Nidym River, Siberian platform) exhibits several features that suggest a lowered symmetry for the NH₄ ⁺ ion and that this is influenced by hydrogen bonding within the framework. These features are: the pronounced splitting into three components and high-frequency shift of the ν₄-bending mode; appearance of the ν₁-stretching mode which is predicted to become IR-active when the ideal T _d symmetry of NH₄ ⁺ ion is violated, and the low-frequency shift of the ν₁- and ν₃-stretching modes. The absence of absorption lines in the 1800–2400-cm⁻¹ region indicates that hydrogen bonding between the framework and the NH⁺ ₄ ion is very weak. The three-component splitting of the ν₄-bending mode indicates that the symmetry of NH⁺ ₄ ion is lower than C_3v. This implies that at least two N–H bonds of the NH⁺ ₄ ion are disturbed by hydrogen bonding. Computer analysis of the normal vibrations of the NH⁺ ₄ molecule for different symmetry types (using harmonic approximation) indicates that the best fit to the observed ν₄ triplet frequencies for C₁ symmetry implies a deviation of the valent angle ∠H–N–H from ideal T _d symmetry of around ±2.5°. The factors governing the behaviour of the NH⁺ ₄ ion in the analcime structure are discussed. The geometry of the nearest environment of the NH⁺ ₄ ion in the analcime structure is analyzed with respect to the present IR data. Received: 2 January 2002 / Accepted: 26 June 2002 Acknowledgements We thank Dr. D. Harlov and an anonymous reviewer for their helpful comments, as well as Dr. I.A. Belitsky and Dr. S.V. Goryainov for discussion of the material. This work is supported by RFBR grants 01-05-65414, 00-05-65305 and 02-05-65313.     

Raman spectra of gypsum under pressure

Raman sprectra of a gypsum crystal were made at pressures between 0.001 and 7 kbar using He gas as the pressure medium. \(\frac{{{\text{d}}v}}{{dP}}\) values for bands in the range 3,600–100 cm^?1 were obtained. Comparison of results with \(\frac{{{\text{d}}v}}{{{\text{d}}T}}\) from the literature for temperatures of 77 and 300° K. shows that the internal modes of the SO₄ units are more sensitive to pressure than to temperature. The effect is small. Coupled H₂O-SO₄ translational modes are greatly affected by both pressure and temperature while coupled Ca-SO₄ mode are less so. It was found that stretching vibrations of water molecules were affected differently under pressure. The band at 3,500 cm^?1 is more greatly displaced by pressure \(\left( {\frac{{{\text{d}}v}}{{{\text{d}}P}} = {\text{2}}{\text{.11cm}}^{{\text{ - 1}}} /{\text{kbar}}} \right)\) than the band at 3,400 cm^?1 \(\left( {\frac{{{\text{d}}v}}{{{\text{d}}P}} \simeq {\text{2}}{\text{.11cm}}^{{\text{ - 1}}} /{\text{kbar}}} \right)\) . Assuming two different hydrogen bond intensities for the water molecules, one can attribute this difference in behavior of stretching modes to and increase in hydrogen bonding of one of the hydrogens which is exterior to the double H₂O planes in the gypsum structure. The great variety of pressure derivatives for the different types of vibrational modes observed indicates that each molecular unit readjusts internally to pressure induced volume changes and the some of the chemical bonds between the units are significantly affected.     

An infrared and Raman spectroscopic study of gypsum at high pressures

 We present Raman and infrared spectra of gypsum to 21 GPa at 300 K. Our measurements encompass the internal modes of the (SO₄)⁻⁴ group that lie between 400 and 1150 cm⁻¹, hydroxyl-stretching vibrations between 3200 and 3600 cm⁻¹, and a libration and bending vibrations of the molecular H₂O group. All vibrations of the sulfate group have positive pressure shifts, while the hydroxyl-stretching and -bending vibrations have a mixture of positive and negative pressure shifts: the effect of pressure on the hydrogen bonding of the water molecule thus appears to be complex. Near 5 GPa, the two infrared-active bending vibrations of the water molecule coalesce, and the morphology of the hydroxyl-stretching region of the spectrum shifts dramatically. This behavior is consistent with a pressure-induced phase transition in gypsum in the vicinity of 5–6 GPa, which is observed to be reversible on decompression to zero pressure. The spectral observations are consistent with the onset of increased disorder in the position of the water molecule in gypsum: the sulfate vibrations are largely unaffected by this transition. The Raman-active symmetric stretch of the sulfate group undergoes an apparent splitting near 4 GPa, which is interpreted to be produced by Fermi resonance with an overtone of the symmetric bending vibration. The average mode Grüneisen parameter of the 20 vibrational modes we sample is less than 0.05, in contrast to the bulk thermal Grüneisen parameter of 1.20. Accordingly, the vibrations of both water and sulfate units within gypsum are highly insensitive to volumetric compaction. Therefore, in spite of the changes in the bonding of the water unit near 5 GPa, metastably compressed gypsum maintains strongly bound molecular-like units to over 20 GPa at 300 K. Received: 31 July 2000 / Accepted: 5 April 2001     

An infrared spectroscopic study of lawsonite to 20 GPa

The infrared spectrum of CaAl₂Si₂O₇ · H₂O-lawsonite, has been characterized to pressures of 20 GPa at 300 K. Our results constrain the response to compression of the silicate tetrahedra, hydroxyl units, and water molecules in this material. The asymmetric and symmetric stretching and bending vibrations of the Si₂O₇ groups (at zero pressure frequencies between 600 and 1000 cm⁻¹) increase in frequency with pressure at rates between 3.6 and 5.9 cm⁻¹/GPa. All silicate modes appear to shift continuously with pressure to 20 GPa, although the lowest frequency stretching vibration becomes unresolvable above 18 GPa, and a splitting of the main bending vibration is observed near this pressure. The O-H stretches of the hydroxyl units exhibit a discontinuity in their mode shifts at ∼8–9 GPa, which we interpret to be produced by a pressure-induced change in hydrogen bonding. The stretching and bending vibrations of the water molecule are relatively unaffected by compression to 20 GPa, thus demonstrating that the structural cavities in which water molecules reside are relatively rigid. Significant changes in the amplitude of the O-H stretches of the hydroxyl and water units are observed at this pressure as well; nevertheless, our results demonstrate that the dominant structural units in lawsonite persist metastably at 300 K with only modest structural modifications well beyond the known stability field of this phase. Received: 10 July 1998 / Revised, accepted: 23 October 1998     

Water–rock interaction during the process of steam stimulation exploitation of viscous crude oil in Liaohe Shuguang Oil Field, Liaoning, China

In the process of steam stimulation exploitation of viscous crude oil, the injected water, at high temperature and under high pressure, reacts intensively with the host rock. This kind of water–rock interaction in Liaohe Shuguang Oil Field was studied on the basis of analysis of water composition changes, laboratory experiments, mineral saturation indices analysis, and mass balance calculation. Compared with the injected water, the changes of the composition of discharged water are mainly the distinct decrease of pH, Na⁺, SiO₂ and Cl⁻, as well as the increase of K⁺, Ca²⁺, Mg²⁺, SO ₄ ²⁻ and HCO ₃ ⁻ . Laboratory experiments under field conditions showed: the dissolution sequence of minerals quantitatively is quartz>potassium feldspar>albite, and the main change of clay minerals is the conversion of kaolinite to analcime. Mass balance calculation indicated during the process of steam stimulation, large quantities of analcime are precipitated with the dissolution of large amounts of quartz, kaolinite, potassium feldspar, and CO₂. These results correlated very well with the experimental results. The calculated results of Liaohe Shuguang Oil Field showed that during the steam stimulation for viscous crude oil, the amounts of minerals dissolved (precipitated) are huge. To control the clogging of pore spaces of oil reservoirs, increased study of water–rock interaction is needed.     

The system NaAlSi3O8−H2O−H2 (1200° C, 2 kbar): the solubility and interaction mechanism of fluid species with melt

The H₂O and H₂ solubilities in an albite melt at 1200° C and 2 kbar over the entire range of gas phase composition, from pure hydrogen to pure water were studied in gas-media pressure vessels. The water solubility initially increases with increasing hydrogen content until a maximum of 9.19 wt% H₂O atXH ₂ ^v =0.1 is reached, withXH ₂ ^v >0.1 the water solubility decreases. The hydrogen solubility curve has a maximum atXH ₂ ^v =0.42 where the concentration reaches 0.206 wt% H₂O. Over the entire compositional range¹H NMR (nuclear magnetic resonance) spectra show distinct absorption lines due to protons bound to OH groups and to isolated firmly bound water molecules. In NMR and Raman spectra there were no bands attributable to the H–H vibrations of molecular hydrogen. The X-ray photo-electronic spectra of hydrogen-bearing glasses show the Si2p (99 eV) band which corresponds to the zero-valency silicon. The formation of OH groups and molecular water during interaction between hydrogen-bearing fluids and melts under reducing conditions has a qualitative effect, the same as for water dissolution. Another point of interest is that hydrogen-bearing melts undergo more depolymerization than do hydrous melts.     

An experimental investigation of hydroxyl solubility in jadeite and Na-rich clinopyroxenes

The solubility and incorporation mechanisms of water in synthetic, water-saturated jadeite and Na-rich clinopyroxenes have been experimentally investigated. Infrared spectra for water-saturated jadeite synthesised from 2.0 to 10 GPa show two prominent sharp peaks at 3,373 and 3,613 cm^–1 together with several weaker features in the OH-stretching region, indicating that there are at least 5 distinct modes of hydrogen incorporation in the structure. Water solubility in pure jadeite reaches a maximum of about 450 ppm by weight at 2 GPa and slowly decreases with increasing pressure to about 100 ppm at 10 GPa. Solubility can be described by the function c_OH=A f_H2O^0.5 exp (–PV_Solid/RT), where c_OH is water solubility in ppm H₂O by weight, A is 7.144 ppm/bar^0.5, f_H2O is water fugacity, and V_Solid=8.019 cm³/mol is the volume change of the clinopyroxene upon incorporation of OH. Jadeite provides a good model for understanding hydrogen incorporation mechanisms in more complex omphacite compositions. Assignment of absorption bands in IR spectra verifies the importance of cation vacancies on the M2 site in providing mechanisms for hydrogen incorporation. However, results also suggest that substitution of lower valency cations onto the M1 site may also be important. Solid solution of jadeite with diopside and in particular, with Ca-Eskola component leads to a drastic increase of water solubility, and the bulk composition has a more important effect on the capacity of omphacite to store water than pressure and temperature. Omphacite is expected to be the major carrier of water in a subducted eclogite after the breakdown of hydrous minerals.Editorial responsibility: W. Schreyer     

Analcime: A characteristic authigenic phase of andean alluvium,northern chile

Analcime (NaAlSi₂O₆.H₂O) forms a characteristic authigenic phase in Andean alluvium of northern Chile. Three stratigraphic units ranging from the lower Cretaceous to the Miocene were studied in order to investigate the presence of analcime in diagenetic assemblages of different ages. The units were deposited in forearc, foreland/back-arc, and intra-arc basinal settings. Characteristics of other sedimentary analcime occurrences were examined in order to define conditions favourable for analcime precipitation. Zeolite zonation and evidence for the original presence of volcanic glass, common to many sedimentary analcime occurrences, are absent in the studied units. Analcime precipitation requires a high Na⁺/H⁺ ratio, relatively low Si⁴⁺/Al³⁺ activity and low H₂O activity. Diagenetic examination of the three stratigraphic units revealed early quartz and feldspar overgrowths, mechanical clay infiltration, and dissolution of labile rock and mineral fragments, particularly intermediate plagioclase (An_30–80). Conditions for these processes are favoured by oxidizing groundwater at near neutral pH. Later diagenesis involved the precipitation of analcime, non-ferroan calcite, quartz, and authigenic clays as the main cementing phases. These formed due to the concentration of oxidizing groundwater and resulted from its continued reaction with detrital feldspars and rock fragments of andesitic and dacitic composition. Analcime precipitation in Andean alluvium resulted from the evolution of saline, alkaline brines under an arid, evaporative climate in closed hydrographic basins with circulating groundwater systems.     

Zeolitic diagenesis of late Quaternary fluviolacustrine sediments and associated calcrete formation in the Lake Bogoria Basin, Kenya Rift Valley

Late Quaternary fluviolacustrine siltstones, mudstones and claystones (Loboi Silts) on the northern margins of the saline, alkaline Lake Bogoria in the Kenya Rift Valley contain up to c. 40% authigenic analcime and minor natrolite. The zeolitic sediments are reddish brown and up to 1 m thick. The amount of analcime increases upward in the profile, but decreases with distance from the lake. The altered sediments show many pedogenic features including zeolitic root mats, rootmarks, concretions and carbonate rhizoliths. Residual patches of calcrete locally cap the zeolitic rocks. The profile is interpreted as an exhumed palaeosol and land surface on the former margins of the lake. The analcime occurs as submicroscopic (0–5–2–5 μ.m) subhedral and euhedral crystals, which have an average Si/A 1 ratio of 2–33 (as determined by X-ray microanalysis) or 2–18 (d-value of 639 analcime peak). The analcime formed in lake marginal sediments (soils) by reaction of silicate detritus with Na₂CO₃ rich pore waters concentrated close to the land surface by evaporative pumping and evapotranspiration. Poorly ordered clay minerals were probably the main reactants. Authigenic illite may have been a by-product of the reactions. Chemical analyses suggest that pore waters supplied some of Na⁺, and possibly K⁺ and SiO₂. The associated calcrete and rhizoliths were formed during or shortly after the main period of zeolitic alteration. The Ca²⁺ may have originated from infiltrating dilute runoff and groundwater. Authigenic smectite was precipitated in open porosity following analcime formation. The zeolitic alteration at Lake Bogoria provides a relatively recent analogue for lake marginal zeolites found in many ancient saline, alkaline lake sediments.     

Equation of state and pressure-induced structural changes in mirabilite (Na2SO4·10H2O) determined from ab initio density functional theory calculations

We have carried out ab initio calculations using density functional theory to determine the bulk elastic properties of mirabilite, Na₂SO₄·10H₂O, and to obtain information on structural trends caused by the application of high pressure up to ~60 GPa. We have found that there are substantial isosymmetric discontinuous structural re-organisations at ~7.7 and ~20 GPa caused by changes in the manner in which the sodium cations are coordinated by water molecules. The low-pressure and intermediate-pressure phases both have sodium in sixfold coordination but in the high-pressure phase the coordination changes from sixfold to sevenfold. These coordination changes force a re-arrangement of the hydrogen-bond network in the crystal. The trend is towards a reduction in the number of hydrogen bonds donated to the sulphate group (from twelve down to six over the range 0–60 GPa) and an increase in hydrogen bonding amongst the Na-coordinated water molecules and the two interstitial water molecules. Ultimately, we observe proton transfers from the interstitial waters (forming OH^? ions) to two of the Na-coordinated waters (forming a pair of H₃O⁺ ions). The equation of state in the athermal limit of the low-pressure phase of mirabilite, parameterised by fitting an integrated form of the third-order Birch-Murnaghan expression to the calculated energy as a function of unit-cell volume, yields the zero-pressure unit-cell volume, V ₀ = 1468.6(9) ų, the incompressibility, K ₀ = 22.21(9) GPa, and the first pressure derivative K ₀′ = (?K/?P)₀ = 5.6(1).     

Hydrogen bonding interactions in phase A [Mg7Si2O8(OH)6] at ambient and high pressure

Neutron powder diffraction data of phase A (Mg₇Si₂O₈(OH)₆) were collected at ambient pressure and 3.2?GPa (calculated from the compressibility of phase A) from the deuterated compound, and the structure was refined using the Rietveld method. The derived crystal structure implies that hydrogen atoms occupy two distinct sites in phase A, both forming hydrogen bonds of different lengths with the same oxygen atom. This picture is supported by IR spectra, which exhibit two absorption bands at 3400 and 3513?cm^?1 corresponding to OH stretching vibrations, and proton NMR spectra, which display two peaks with equal intensities and isotropic chemical shifts of 3.7 and 5?ppm. The D-D distance [D(1)-D(2) distance] at ambient pressure was found to be 2.09?±?0.02?Å from the neutron diffraction data and 2.09?±?0.05?Å from the NMR spectra. At 3.2?GPa, there is no statistically significant increase in the O-D interatomic distance while the hydrogen bonding interaction D···O appears to increase for one of the hydrogen sites, D(1), which has the stronger hydrogen bonding interaction compared with the other hydrogen, D(2), at ambient pressure. The O-D bond valences, determined indirectly from the D···O distances were 0.86 and 0.91 at ambient pressure, and 0.83 and 0.90?at 3.2?GPa, for D(1) and D(2), respectively.     

Experimente zur Zeolithbildung durch hydrothermale Umwandlung

The formation of zeolites by hydrothermal alteration has been investigated by taking trass from the Laach volcanic area as a sample. Zeolites to be found are chabazite, phillipsite and analcime, all of which originated from the same phonolitic glass. This paper aims at explaining the formation of zeolites by means of experimental alteration of the pumice with various solutions. NaOH and KOH solutions were used in the experiments, these limited the formation conditions of chabazite, phillipsite, analcime in alkaline environments. Moreover, experiments were carried out with H₂O dist and with solutions that formed during the alteration of pumice by reacting with H₂O. These experiments were conducted to supply clues concerning the formation of zeolites in the Laach volcanic area. With NaOH solutions zeolites were formed from pumice within a temperature range of 70 to 250° C and a concentration range of 0.001 to 1.0 n; with KOH solutions they were formed in the same concentration range within a temperature range of 100 to 250° C. The formation of zeolites proved to depend much on temperature and concentration: At low temperatures high concentrations are necessary, higher temperatures need lower concentrations. With NaOH solutions the zeolites are formed in the succession, chabazite, phillipsite, analcime at increasing temperatures and increasing concentrations. With KOH solutions the same succession is to be found with rising temperature. With rising concentration, however, this succession is changed at higher temperatures: Phillipsite appears in place of analcime at high temperatures and concentrations. By the alteration of pumice with H₂O, zeolites are formed from 200° C onward. With solutions that had been formed during the alteration of pumice by reacting with H₂O, the minimum temperature for the formation of zeolites was 180° C. Concerning the genesis of zeolites in the Laach volcanic area, the experiments showed that the temperature for the formation of analcime was about 250° C; for the formation of chabazite and phillipsite it was between 150 and 190° C. The pH of the reacting solutions may have been between 7 and 8. Thus the experiments proved that from the same material different zeolites, chabazite, phillipsite, analcime may be formed by temperature change of the reacting solutions (H₂O to slightly alkaline solutions).     

Water in the structure of minerals from mantle peridotites as controlled bythermal and redox conditions in the upper mantle

Hydrous species and the amount of water (OH^? ions and crystal hydrate H₂O) in structures of nominally anhydrous rock-forming minerals (olivine, ortho- and clinopyroxenes) were studied with Fourier spectroscopy in peridotite nodules (19 samples) from Cenozoic alkali basalts of the Baikal-Mongolia region (Dariganga Plateau, Taryat Depression, and Vitim Plateau). Single-crystal samples oriented relative to the crystallographic axes of minerals were examined with an FTIR spectrometer equipped with an IR microscope at the points of platelets free from fluid inclusions. FTIR spectra were measured in regions of stretching vibrations of OH^? and H₂O (3800–3000 cm^?1) and deformation vibrations of H₂O (1850–1450 cm^?1). The water content in mineral structures was determined from integral intensities. To estimate the conditions of entrapment and loss of structural water in minerals, their chemical composition, including Fe²⁺ and Fe³⁺ contents, was determined with an electron microprobe analysis and Mössbauer spectroscopy. The bulk chemical composition of some nodules was determined with XRF and ICP MS. The total water content (OH^? + H₂O) varies from 150 to 1140 ppm in olivines, from 45 to 870 ppm in clinopyroxenes, and from 40 to 1100 ppm in orthopyroxenes. Both water species in the mineral structures are retained down to a depth of 150–160 km in wide temperature and pressure ranges (1100–1500 °C, 32–47 kbar) at the oxygen fugacity of ?1.4 to ?0.1 log units relative to that of the quartz-fayalite-magnetite buffer.     

安徽马鞍山磷铝石宝石矿物学特征研究

近年来在安徽马鞍山地区所在的绿松石矿体附近,相继发现一种绿色、半透明的磷铝石,部分达到宝石级别。本文采用电子探针、X射线粉晶衍射仪、扫描电镜、傅里叶变换红外光谱仪、紫外可见光谱仪等测试技术,对该地区磷铝石的化学成分、矿物成分、微观结构和光谱特征进行对比验证和综合分析,研究其水的赋存形式,进而对磷铝石的呈色机制作了深入探讨。电子探针分析显示该地区磷铝石的化学成分主要以Al、P元素组成,含微量的Fe、V元素。X射线粉晶衍射与红外吸收光谱分析表明主要矿物为磷铝石,基本不含有其他杂质矿物;磷铝石是一种水合磷酸盐矿物,含有结晶水以及少量结构水的矿物,且结晶水与结构水多与Al3+(Fe3+)相结合的形式存在。偏光显微镜和扫描电镜观察显示磷铝石整体以鳞片状集合体产出,微观上多以短柱状及板片状堆积,单个晶体显示斜方晶系结晶生长习性。紫外可见吸收光谱中639 nm处吸收谱峰由Fe3+与V3+联合所致,300、423、864 nm处吸收峰由Fe3+所致,说明Fe3+与V3+的共同作用是马鞍山地区磷铝石呈现绿色的主要原因。本研究对于认识该类磷铝石的宝石矿物学性质以及颜色成因具有一定意义。     

Combined inelastic neutron scattering and solid-state DFT study of dynamics of hydrogen atoms in trioctahedral 1M phlogopite

Inelastic neutron scattering (INS) was used to study the vibrational dynamics of the hydrogen atoms in natural trioctahedral phlogopite, K_0.93Na_0.03(Mg_2.47Fe_0.22Al_0.16Fe_0.04Tl_0.06)[Si_2.84Al_1.16]O₁₀OH_1.71F_0.28Cl_0.01, within the 50–1,000?cm^?1 energy range. The INS spectra collected using direct geometry spectrometer SEQUOIA (ORNL) were interpreted by means of the solid-state DFT calculations covering both normal mode analysis and molecular dynamics. To optimize the structure and to calculate the vibrational modes under harmonic approximation, both a hybrid PBE0 and the AM05 functional were used, while the molecular dynamics calculations (60?ps/1?fs) were performed only with the computationally less-demanding AM05 functional. The main contributions to the dominant band within ~750–550?cm^?1 are symmetric and antisymmetric Mg–O–H bending modes, overlapping with the skeletal stretching and bending modes causing weaker secondary movements of H atoms of inner hydroxyl groups. Signatures of the Mg–O–H bending modes appear down to ~400?cm^?1, where a region of octahedra deformation modes starts. These deformations cause just shallow movements of the hydrogen atoms and are mirrored by the modes with close vibrational energies. The region from ~330?cm^?1 down to the low-energy end of the spectrum portrays induced vibrations of the H atoms caused by deformation of individual polyhedra, translational vibrations of the parts of the 2:1 layer relative one to another, and librational and translational vibrations of the layer. The main difference between the INS spectrum of dioctahedral Al-muscovite and trioctahedral Mg-phlogopite is that the Mg–O–H modes are all assigned to in-plane vibrations of the respective hydrogen atoms.     

Chemical alteration of volcanic glasses and related control by secondary minerals: Experimental studies

A detailed experimental mineralogical and geochemical study on hydrothermal alteration processes of volcanic glasses with a different chemical composition and leucites sampled in the Roman Comagmatic Region was carried out. 2g samples of different grain sizes and 50 ml of deionized water or seawater were sealed in bronze Teflon-lined autoclaves and placed in a rotating sample-holder at 200°C. The internal pressure was 16.2 bars. At arbitrary intervals, the pressure vessel was quenched to room temperature and both solids and solutions were separated by filtration. The solids were used to identify possible alteration products by means of X-ray powder diffraction, SEM and microprobe analyses. In all deionized water systems the contact solution reached pH basic values, but in the various systems the consumption of H⁺ ions occurred in different ways. This is probably linked to different concentrations of released cations. In seawater systems, however, pH values initially fell sharply but subsequently increased slightly. After 2 days, cation concentrations were clearly buffered by reaction products in all solutions. These were mainly zeolites and clay minerals. The following crystallization sequences in all glass/deionized water experiments were observed: the first reaction product was represented by phillipsite, followed by analcime and illite in the alkali-trachytic run; chabasite and analcime in the phonolitic-tephritic run; analcime and finally feldspar in shoshonitic and trachytic runs. Phillipsite and smectite crystallized together only in the basaltic run. In leucite/deionized water systems, however, the first reaction phase was illite, followed by analcime and then phillipsite. The reaction products in glass/seawater systems were smectite and anydrite. The chemical composition of synthetic zeolites was clearly controlled by the chemistry of initial glasses. These synthetic zeolites are like the natural ones in volcaniclastic products from central and southern Italy. This suggests that extensive zeolitization processes of these volcaniclastic rocks may occur through interaction of volcanic glasses with fluids at a very low salinity and a temperature close to 200°C.     

Diurnal Variations of Hydrochemistry in a Travertine-depositing Stream at Baishuitai, Yunnan, SW China

Diurnal variations of hydrochemistry were monitored at a spring and two pools in a travertine-depositing stream at Baishuitai, Yunnan, SW China. Water temperature, pH and specific conductivity were measured in intervals of 5 and 30 min for periods of 1 to 2 days. From these data the concentrations of Ca²⁺, HCO₃⁻, calcite saturation index, and CO₂ partial pressure were derived. The measurements in the spring of the stream did not show any diurnal variations in the chemical composition of the water. Diurnal variations, however, were observed in the water of the two travertine pools downstream. In one of them, a rise in temperature (thus more CO₂ degassing) during day time and consumption of CO₂ due to photosynthesis of submerged aquatic plants accelerated deposition of calcite, whereas in the other pool, where aquatic plants flourished and grew out of the water (so photosynthesis was taking place in the atmosphere), the authors suggest that temperature-dependent root respiration underwater took place, which dominated until noon. Consequently, due to the release of CO₂ by the root respiration into water, which dominated CO₂ production by degassing induced by temperature increase, the increased dissolution of calcite was observed. This is the first time anywhere at least in China that the effect of root respiration on diurnal hydrochemical variations has been observed. The finding has implications for sampling strategy within travertine-depositing streams and other similar environments with stagnant water bodies such as estuaries, lakes, reservoirs, pools and wetlands, where aquatic plants may flourish and grow out of water.     

Water and the density of silicate glasses

A review of published and newly measured densities for 40 hydrous silicate glasses indicates that the room-temperature partial molar volume of water is 12.0 ± 0.5 cm³/mol. This value holds for simple or mineral compositions as well as for complex natural glasses, from rhyolite to tephrite compositions, prepared up to 10–20 kbar pressures and containing up to 7 wt% H₂O. This volume does not vary either with the molar volume of the water-free silicate phase, with its degree of polymerization or with water speciation. Over a wide range of compositions, this constant value implies that the volume change for the reaction between hydroxyl ions and molecular water is zero and that, at least in glasses, speciation does not depend on pressure. Consistent with data from Ochs and Lange (1997, 1999), systematics in volume expansion for SiO₂–M₂O systems (M=H, Li, Na, K) suggests that the partial molar thermal expansion coefficient of H₂O is about 4 × 10⁻⁵ K⁻¹ in silicate glasses. Received: 30 June 1999 / Accepted: 5 November 1999     

Pressure dependence of hydroxyl solubility in coesite

 The solubility of hydroxyl in coesite was investigated in multianvil experiments performed at 1200 °C over the nominal pressure range 5–10 GPa, at an f _O2 close to the Ni-NiO buffer. The starting material for each experiment was a cylinder of pure silica glass plus talc, which dehydrates at high P and T to provide a source of water and hydrogen (plus enstatite and excess SiO₂). Fourier-transform infrared (FTIR) spectra of the recovered coesite crystals show five sharp bands at 3606, 3573, 3523, 3459, and 3299 cm⁻¹, indicative of structurally bonded hydrogen (hydroxyl). The concentration of hydrogen increases with pressure from 285 H/10⁶ Si (at 5 GPa) to 1415 H/10⁶ Si (at 10 GPa). Assuming a model of incorporation by (4H)_Si defects, the data are fit well by the equation C _OH=Af ² _H2<\INF>Oexp(−PΔV/RT), with A=4.38 H/10⁶ Si/GPa, and ΔV=20.6 × 10⁻⁶ m³ mol⁻¹. An alternative model entailing association of hydrogen with cation substitution can also be used to fit the data. These results show that the solubility of hydroxyl in coesite is approximately an order of magnitude lower than in olivines and pyroxenes, but comparable to that in pyropic garnet. However, FTIR investigations on a variety of ultrahigh pressure metamorphic rocks have failed in all cases to detect the presence of water or hydrogen in coesite, indicating either that it grew in dry environments or lost its hydrogen during partial transformation to quartz. On the other hand, micro-FTIR investigations of quartz crystals replacing coesite show that they contain varying amounts of H₂O. These results support the hypothesis that preservation of coesite is not necessarily linked to fast exhumation rates but is crucially dependent on limited fluid infiltration during exhumation. Received: 23 August 1999 / Accepted: 10 April 2000     

Al/Si interdiffusion in albite: effect of pressure and the role of hydrogen

The effect of pressure on the rate of Al/Si disorder in albite has been determined at temperatures from 800° C to 1050° C and at pressures up to 24 kbar, using dried samples in welded Pt containers, in piston-cylinder devices and internally-heated gas apparatus. In the piston-cylinder device with NaCl medium, the effect of pressure is profound. A pure low albite from Clear Creek, California reaches the equilibrium state of disorder at 850° C and 22 kbar in 10 h, whereas at 6 kbar it has not equilibrated in three weeks, and at one bar it probably cannot be disordered at 850° C in the laboratory. The enhancement of Al/Si interdiffusion takes place under dry conditions: any H₂O penetrating the samples would have produced melting, and none was observed. Hydrogen, however, is produced by dissociation of moisture in the pressure medium and can penetrate the Pt sample capsules. If the samples are deprived of hydrogen by replacing NaCl with glass or by embedding the samples in a hydrogen getter such as Fe₂O₃ or ZnO, the order-disorder reaction is greatly inhibited.A mechanism is suggested in which OH^– groups are formed by hydrogen hopping in albite. The smaller charge on the tetrahedral complex induces transient coordination of Al greater than four at elevated pressures, facilitating Al/Si interchange during the bond-breaking process. This results in an exponential pressure dependence of diffusion. It is also suggested that the presence of hydrogen at high pressures can greatly effect the mechanical properties and reactivity of deep crustal and mantle rocks. In nature, as well as in the laboratory, equilibration at elevated pressures may be as much dependent on the availability of hydrogen as on time or temperature.