High temperature infrared spectra of hydrous microcrystalline quartz

A series of in-situ high temperature infrared (IR) measurements of water in an agate sample and in a milky quartz has been conducted in order to understand the nature of water in silica at high temperatures (50–700?°C) and the dehydration behavior. IR absorption bands of water molecules trapped in the milky quartz showed a systematic decrease in intensities and a shift from 3425?cm^?1 at 50?°C toward 3590?cm^?1 at 700?°C without any loss of water. This indicates a change in IR absorption coefficients corresponding to different polymeric states of water at different temperatures. The broad 3430?cm^?1 band in the agate sample also showed a systematic decrease in IR intensity and a band shift toward higher frequency with increasing temperature (～700?°C). This indicates that the agate sample also contains fluid inclusion-like water. For this agate sample, a dehydration of loosely hydrogen-bonded molecular water occurred at lower temperatures (<200?°C). At higher temperatures (>400?°C), sharp bands around 3660 and 3725?cm^?1 (3740?cm^?1 at 50?°C) due to surface silanols, appeared. This indicates dehydration of H₂O molecules that are hydrogen bonded to surface silanols. SiOH species in the agate are divided into three groups, namely SiOH group located at structural defects, surface silanols hydrogen bonded to each other and free surface silanols. Former two dehydrate below 700?°C and the dehydration rate of the SiOH at structural defects is faster than the other. IR spectra show that SiOH species decrease continuously even after the dehydration of most of H₂O molecules. All these results provide realistic bases for the change in physicochemical states of different OH species in silica at high temperatures.     

Dissolution mechanisms of water in depolymerized silicate melts: Constraints from H and Si NMR spectroscopy and ab initio calculations

Dissolution of water in magmas significantly affects phase relations and physical properties. To shed new light on the this issue, we have applied ¹H and ²⁹Si nuclear magnetic resonance (NMR) spectroscopic techniques to hydrous silicate glasses (quenched melts) in the CaO-MgO-SiO₂ (CMS), Na₂O-SiO₂, Na₂O-CaO-SiO₂ and Li₂O-SiO₂ systems. We have also carried out ab initio molecular orbital calculations on representative clusters to gain insight into the experimental results.The most prominent result is the identification of a major peak at ∼1.1 to 1.7 ppm in the ¹H MAS NMR spectra for all the hydrous CMS glasses. On the basis of experimental NMR data for crystalline phases and ab initio calculation results, this peak can be unambiguously attributed to (Ca,Mg)OH groups. Such OH groups, like free oxygens, are only linked to metal cations, but not part of the silicate network, and are thus referred to as free hydroxyls in the paper. This represents the first direct evidence for a substantial proportion (∼13∼29%) of the dissolved water as free hydroxyl groups in quenched hydrous silicate melts. We have found that free hydroxyls are favored by (1) more depolymerized melts and (2) network-modifying cations of higher field strength (Z/R²: Z: charge, R: cation-oxygen bond length) in the order Mg > Ca > Na. Their formation is expected to cause an increase in the melt polymerization, contrary to the effect of SiOH formation. The ²⁹Si MAS NMR results are consistent with such an interpretation. This water dissolution mechanism could be particularly important for ultramafic and mafic magmas.The ¹H MAS NMR spectra for glasses of all the studied compositions contain peaks in the 4 to 17 ppm region, attributable to SiOH of a range of strength of hydrogen bonding and molecular H₂O. The relative population of SiOH with strong hydrogen bonding grows with decreasing field strength of the network-modifying cations. Ab initio calculations confirmed that this trend largely reflects hydrogen bonding with nonbridging oxygens.     

Determination of hydrogen content in geological samples using elastic recoil detection analysis (ERDA)

The present study illustrates the interest of using the elastic recoil detection analysis (ERDA) method to characterize any geological sample matrix with respect to hydrogen. ERDA is combined with Rutherford back scattering (RBS) and particle induced X-ray emission (PIXE), allowing the simultaneous characterization of the matrix with respect to major and trace elements (Z > 15). Analyses are performed by mapping of a 4 × 16 μm² incident beam of ⁴He⁺ on large areas (50 × 200 μm²). The method is almost not destructive and requires no calibration with respect to well known hydrous samples. Hydrous and nominally anhydrous phases in contact with each other in the same sample may both be characterized. The depth of the analyses is limited to several μm beneath the surface, allowing tiny samples to be investigated, provided their sizes are larger than the incident beam. Our setup has been improved in order to allow H determination on a micrometric scale with a 5-15% relative uncertainty and a detection limit of 94 wt ppm H₂O. We present multi-elemental mappings on a large panel of samples: (1) natural and analogue synthetic glasses from Stromboli volcano (0.44-4.59 wt% H₂O), natural rhyolitic glasses (1466-1616 wt ppm H₂O); (2) magmatic rhyolitic melt inclusions from Guadeloupe Island (4.37-5.47 wt% H₂O) and their quartz host crystal (2020 ± 230 wt ppm H₂O); (3) nominally anhydrous natural (82-260 wt ppm H₂O) and experimentally hydrated (240-790 wt ppm H₂O) olivines; natural clinopyroxenes (159-716 wt ppm H₂O); natural orthopyroxenes (201-452 wt ppm H₂O); a natural garnet (90 wt ppm H₂O). Results show that ERDA is a strong and accurate reference method that can be used to characterize geological sample from various matrix compositions from high to low water contents. It can be used to calibrate other methods of microanalysis such as Fourier Transform Infrared Spectroscopy (FTIR) or secondary ion mass spectrometry (SIMS).     

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.     

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     

Submicron-sized fluid inclusions and distribution of hydrous components in jadeite,quartz and symplectite-forming minerals from UHP jadeite–quartzite in the Dabie Mountains,China: TEM and FTIR investigation

Fluid inclusions and clusters of water molecules at nanometer-to submicron-scale in size have been investigated using transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR) in jadeite, quartz and symplectite aegirine–augite, albite, taramite and magnetite corona minerals from ultrahigh-pressure (UHP) jadeite–quartzite at Shuanghe, the Dabie Mountains, China. Fluid inclusions from 0.003 μm to 0.78 μm in size occur in jadeite and quartz crystals, and a small number of fluid inclusions from 0.001 μm to 0.25 μm have also been detected in symplectite-forming minerals. Most of the fluid inclusions have round or negative crystal morphology and contain aqueous fluids, but some contain CO₂-rich fluids. They are usually connected to dislocations undetectable at an optical scale. The dislocations represent favorable paths for fluid leakage, accounting for non-decrepitation of most fluid inclusions when external pressure decreased at later stages, although there was partial decrepitation of some fluid inclusions unconnected to defect microstructures resulting from internal overpressure. Non-decrepitation and partial decrepitation of fluid inclusions resulted in changes of original composition and/or density. It is clear that identification of hidden re-equilibration features has significant implications for the petrological interpretation of post-peak metamorphic processes. Micro-FTIR results show that all jadeite and quartz samples contain structural water occurring as hydroxyl ions (OH⁻) and free water (H₂O) in the form of clusters of water molecules. The H₂O transformed from OH⁻ during exhumation and could have triggered and enhanced early retrograde metamorphism of the host rocks and facilitated plastic deformation of jadeite and quartz grains by dislocation movement, and thus the H₂O released during decompression might represent early-stage retrograde metamorphic fluid. The nominally anhydrous mineral (NAM) jadeite is able to transport aqueous fluids in concentrations of at least several hundred ppm water along a subduction zone to mantle depths in the form of clusters of water molecules and hydroxyl ions within crystals.     

The hydrogen bond system in natrochalcite-type compounds — an FTIR spectroscopic study of the H3O2 unit

Summary FTIR spectroscopy was used to study the vibrational behaviour of the hydrogen bond system of the H₃O₂ unit, present with widely varying O-H...O distances in natrochalcite-type compounds, MeCu₂(H₃O₂)(ZO₄)₂ (Me = Na, K; Z=S, Se). Absorption bands of powder and polarized single-crystal spectra of non-deuterated and partially deuterated natrochalcite phases are assigned to stretching and bending modes of clearly divided OH^– and H₂O groups. These results are indicative for a not centro-symmetric configuration of the H₃O₂ unit and confirm the presence of a very strong hydrogen bond with a split hydrogen atom position.

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Das Wasserstoffbrückensystem in Verbindungen des Natrochalcit-Typs - eine FTIR spektroskopische Untersuchung der H₃O ₂ ^– Gruppe

Zusammenfassung FTIR Spektroskopie wurde zur Untersuchung des Schwingungsverhaltens des Wasserstoff-Brückenbindungssystems der H302 Gruppe herangezogen, die mit stark variierenden O-H... 0 Abständen in Verbindungen des Natrochalcit-Typs, MeCu₂(H₃O₂)(ZO₄)₂ (Me = Na, K; Z = S, Se) auftritt. Absorptionsbanden von Pulverund polarisierten Einkristallspektren nicht deuterierter und teilweise deuterierter Natrochalcit-Phasen, werden den Streck- und Knickschwingungen klar getrennter OH^– und H2O Gruppen zugeordnet. Diese Ergebnisse weisen auf einen nichtzentrosymmetrischen Aufbau der H₃O ₂ ^– -Gruppe hin und bestätigen das Auftreten einer sehr starken Wasserstoffbrücke mit einer aufgespaltenen Position des Wasserstoffatoms.

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With 4 Figures     

OH incorporation in nearly pure MgAl2O4 natural and synthetic spinels

Four nearly pure MgAl₂O₄ spinels, of both natural and synthetic occurrence, have been studied by means of X-ray single crystal diffraction and FTIR spectroscopy in order to detect their potential OH content. Absorption bands that can be assigned to OH incorporated in the spinel structure were only observed in spectra of a non-stoichiometric synthetic sample. The absorption intensity of two bands occurring at 3350 and 3548 cm⁻¹ indicate an OH content of 90 ppm H₂O. Based on correlations of OH vibrational frequencies and O-H?O distances, the observed absorption bands correspond to O-H?O distances of 2.77 and 2.99 Å, respectively, which is close to the values obtained by the structure refinements for ^VIO-O_unsh (2.825 Å) and ^IVO-O (3.001 Å). This indicates that one probable local position for hydrogen incorporation is the oxygens coordinating a vacant tetrahedral site. The present spectra demonstrate that the detection limit for OH in Fe-free spinels is in the range 10-20 ppm H₂O. However, at appreciable Fe²⁺ levels, the detection of OH bands becomes hampered due to overlap with strong absorption bands caused by electronic d-d transitions in Fe²⁺ in the tetrahedral position.     

The crystallization temperature of vein quartz estimated from the concentration of the titanium paramagnetic center in quartz: A case study of the Peschanka porphyry copper–molybdenum–gold deposit,Western Chukchi Peninsula,Russia

The concentration of the Al and Ti paramagnetic impurity centers in pre-ore and ore-stage quartz at the Peschanka porphyry copper–molybdenum–gold deposit in the Western Chukchi Peninsula, Russia were determined using electron paramagnetic resonance spectroscopy (EPR). The [AlO ₄ ^- /h⁺]⁰ concentration in pre-ore and ore-stage quartz varies from 29 to 124 and from 13 to 101 at. ppm, respectively. The contents of the [TiO ₄ ^- /Li⁺]⁰ and [TiO ₄ ^- /H⁺]⁰ centers reach 20 and 6.3 at. ppm, respectively. Pre-ore quartz associated with the formation of biotite–potassium feldspar–quartz alteration and ore-stage quartz associated with the formation of quartz–sericite rocks followed by the ore deposition differ considerably in the Ti center content, especially the [TiO ₄ ^- /H⁺]⁰ center. The [TiO ₄ ^- /H⁺]⁰ concentration is much higher in the pre-ore quartz (>2 at. ppm) than that in the ore-stage quartz related to copper mineralization (<2 at. ppm). The [TiO ₄ ^- /Li⁺]⁰ concentration also decreases from pre-ore to ore-stage quartz. Taking the data we obtained into account, the formation temperature of pre-ore and ore-stage quartz estimated from a titaniumin-quartz geothermometer is 590–470°C (weighted average 520°C) and 510–310°C (weighted average 430°C), respectively. The obtained temperature range of 590 to 310°C is similar to that determined from homogenization of fluid inclusions in quartz.     

Heterogeneity of water in garnets from UHP eclogites, eastern Dabieshan, China

Garnets in UHP eclogites from Bixiling in Dabieshan were investigated by Fourier transform infrared spectroscopy (FTIR). The results indicate that all garnets contain structural water that occurs as hydroxyl (OH⁻) and non-structural molecular water (H₂O) possibly in the form of sub-microscopic fluid inclusions. The structural hydroxyl contents range from 92 to 1735 ppm (H₂O wt.) and most are between 200 and 1000 ppm. Therefore, garnet in eclogite can recycle surface water into the mantle. Various water contents were observed among different samples of the same outcrop (∼150 m) and in different domains of the same sample (∼1 cm). This variability in structural H₂O contents suggests that the mobility of fluids during UHP metamorphism was very limited, and that both subduction and exhumation processes of UHP rocks occurred in a short time interval.     

Composition and origin of the volatile components released from the Pesyanoe aubrite by stepwise crushing and heating

Aubrites are achondritic meteorites (enstatite pyroxenites) that were formed in highly reduced magmatic environments on a differentiated parent body sharing a common oxygen isotope reservoir with enstatite chondrites (EC), Earth and Moon, and could be considered as a geochemical model of the early proto-Earth. Some pyroxenes of the Pesyanoe aubrite have high abundance of gaseous inclusions, captured during the crystallization of the rocks. Investigation of the inclusions by IR spectroscopy reveals presence of OH⁻ groups and C–H bonds. The former are assigned to protonated point defects in enstatite lattice and the latter to compounds occupying void walls. Molecular water and CO₂ were not observed. Volatile components released from the samples of the Pesyanoe enstatite by stepwise crushing and heating are composed of CO₂, H₂O and a non-condensable phase. Hydrogen isotopic composition of volatiles extracted in form of molecular water in Px-separates varies in the range δD = −61 – −84‰ with mean value of δD = −73 ± 16‰ VSMOW and is within the ranges of ECs and Earth’s mantle. The total abundance of H₂ in the pyroxene of Pesyanoe were estimated as at least 0.047 ppm that is too low in comparison with that of enstatite chondrites (≥30 ppm H₂) and could indicate nearly complete degassing of the Pesyanoe primitive precursor material during the Pesyanoe parent body accretion or a mantle degassing in igneous differentiation process. In a last case a primitive precursor could have D/H ratio different from that of enstatite chondrites.     

Correlations between 29Si, 17O and 1H NMR properties and local structures in silicates: an ab initio calculation

In order to gain insight into the correlations between ²⁹Si, ¹⁷O and ¹H NMR properties (chemical shift and quadrupolar coupling parameters) and local structures in silicates, ab initio self-consistent field Hartree-Fock molecular orbital calculations have been carried out on silicate clusters of various polymerizations and intertetrahedral (Si-O-Si) angles. These include Si(OH)₄ monomers (isolated as well as interacting), Si₂O(OH)₆ dimers (C₂ symmetry) with the Si-O-Si angle fixed at 5° intervals from 120° to 180°, Si₃O₂(OH)₈ linear trimers (C₂ symmetry) with varying Si-O-Si angles, Si₃O₃(OH)₆ three-membered rings (D₃ and C₁ symmetries), Si₄O₄(OH)₈ four-membered ring (C₄ symmetry) and Si₈O₁₂(OH)₈ octamer (D₄ symmetry). The calculated ²⁹Si, ¹⁷O and ¹H isotropic chemical shifts (δ_i ^Si, δ_i ^O and δ_i ^H) for these clusters are all close to experimental NMR data for similar local structures in crystalline silicates. The calculated ¹⁷O quadrupolar coupling constants (QCC) of the bridging oxygens (Si-O-Si) are also in good agreement with experimental data. The calculated ¹⁷O QCC of silanols (Si-O-H) are much larger than those of the bridging oxygens, but unfortunately there are no experimental data for similar groups in well-characterized crystalline phases for comparison. There is a good correlation between δ_i ^Si and the mean Si-O-Si angle for both Q ¹ and Q ², where Q ⁿ denotes Si with n other tetrahedral Si next-nearest neighbors. Both the δ _i ^O and the ¹⁷O electric field gradient asymmetry parameter, η of the bridging oxygens have been found to depend strongly on the O site symmetry, in addition to the Si-O-Si angle. On the other hand, the ¹⁷O QCC seems to be influenced little by structural parameters other than the Si-O-Si angle, and is thus expected to be the most reliable ¹⁷O NMR parameter that can be used to decipher Si-O-Si angle distribution information. Both the ¹⁷O QCC and the ²H QCC of silanols decrease with decreasing length of hydrogen bond to a second O atom (Si-O-H···O), and the δ _i ^H increase with the same parameter. Received: 18 July 1997 / Revised, accepted: 23 February 1998     

A deuterium-hydrogen fractionation mechanism in magnesium oxide

Water dissolved in a nominally anhydrous oxide like MgO forms not only OH^? ions, as is usually assumed, but also molecular H₂ and peroxy anions O₂^2?. The specific lattice site where this reaction occurs has been identified for the model-type MgO structure: it is the fully OH^? compensated cation vacancy. In partially deuterated synthetic MgO, molecular D₂ forms preferentially to H₂, by a factor of the order of 4. Three methods have been used to study this very large D/H fractionation experimentally: mass spectrometry, electron spin resonance and infrared spectroscopy. All give consistent results and confirm that internal D/H fractionation occurs and is a function of the temperature. Theoretically the D/H fractionation is explained by the lower zeropoint energy of D₂ with respect to H₂, amplified by secondary processes through which the molecular hydrogen species become mobilized and eventually lost via diffusion. The possible consequences of such internal D/H fractionation processes are indicated for the interpretation of water and hydrogen release patterns, with varying D/H ratios, from terrestrial and extraterrestrial samples.     

Structural state and differusion of impurities in natural quartz of different genesis

Impurity inhomogeneities and other structural defects have been studied by means of transmission electron microscopy (TEM), X-ray microanalysis and electron paramagnetic resonance (EPR) in untreated and heat-treated quartz samples of three genetic types: hydrothermal, pegmatitic and magmatic. The impurities present are Al, Na and H₂O, which occupy tetrahedral (Al³⁺) or interstitial (Na⁺, H₂O) positions in the quartz lattice. Impurities form imperfections of various degrees of segregation: from point defects to micropores with a gas-liquid content. Their size, form, density and distribution in the lattice depend on the formation conditions of the quartz, the presence of dislocations and plane defects serving as sinks for the impurity atoms, and the heat treatment regime. Experimental data indicate that gas-liquid inclusions of dimensions up to some microns are the result of impurity segregation during postcrystallizational cooling. Crystalline quartz amorphizes upon electron irradiation. A model of structural water explaining experimentally observed features of this phenomenon is proposed whereby the water molecule, represented as a dipole, enters microregions of the silica lattice with a high impurity content and there forms a bond between ‘defective’ [SiO₃]^2? and [AlO₄]^5? tetrahedra. On irradiation, the Si---O donor-acceptor bonds trap nonelastically scattered electrons and are ruptured as a result. The water released by this lattice discontinuity forms microbubbles that diffuse along sinks into the larger micropores thus further increasing their volume.     

Geochemical features of ore fluid for gold deposits related to alkaline rocks in China

Fluid inclusion studies of 5 gold deposits connected with alkaline rocks show that quartz separated from auriferous quartz veins contains abundant three-phase CO2-NaCl-H2O inclusions and two-phase CO2-dominated ones,measuring 5-20um in diameter,Homogenization temperatures of the fluid inclusions are mostly within the range of 150-300℃,and the salinities,mainly 0.2wt%-12 wt%(NaCl),Gold mineralizations occurred at depths of 1.4-2.8km,The most striking character of fluid composition is that among the cations,Na^ in dominant,followed by K^ ,Ca^2 ,among the anions,Cl^- is slightly higher than SO4^2-,In the evaporate,H2O is dominant,followed by CO2,The pH values are mainly within the range of 6.5-8.5,indicating that the ore-forming solutions are alkaline in nature.The hydrogen and oxygen isotopic ratios indicate that the ore fluid is composed mainly of magmatic water.With the dropping of temperature in the ore fluid,the contents of CO2 decreased while the salinity increased.The relations between Au and other components of the ore fluid are discussed in the paper,and it is concluded that in these deposits,Chlorides,H2S,SiO2,CO2,etc.in the fluid all are involved in the migration and concentration of Au.     

The nature of water in chalcedony and opal-C from brazilian agate geodes

The water species (H₂O_(SiOH) and H₂O_(mol)) of length-fast chalcedony and opal-C in Brazilian agates were studied with thermoanalytical, chemical and infrared absorption methods. Specific surfaces were measured with the BET nitrogen adsorption method and the specific densities were determined. Chalcedony and opal-C have fully hydrated crystal surfaces at the open porosity. They contain additional water at inner surfaces, which are closed micropores in the case of opal-C and regions of accumulated defects (e.g. twinlamellae boundaries) in chalcedony. All surfaces are covered with silanole groups, hydrogen-bonded to molecular water. Additional hydroxyl groups, weakly hydrogen bonded to the structural framework within the crystallites, are located at structural point defects. Wall-lining chalcedony ranges from translucent gray to milky white bands corresponding with decreasing total water content, the H₂O_(SiOH)/H₂O_(mol)-ratio, BET-surfaces and increasing density. The H₂O_(SiOH)/H₂O_(mol)-ratio is sensitive to subsequent hydrothermal treatment and indicates a low temperature formation of chalcedony.     

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.     

Hydrogen bonding in basic copper salts: a spectroscopic study of malachite,Cu2(OH)2CO3, and brochantite,Cu4(OH)6SO4

Infrared and Raman spectra of the basic copper salts malachite, Cu₂(OH)₂CO₃, and brochantite, Cu₄(OH)₆SO₄, as well as of deuterated and ¹³C substituted samples are presented and discussed in terms of group theory and the hydrogen bonds present. The main results are that (i) the hydrogen donor strengths of the OH^? ions are strongly increased due to the very great synergetic effect of the copper ions, (ii) the acceptor strengths of the H-bond acceptor groups (SO₄ ^2-, CO₃ ^2-, and OH^? ions) are significantly modified by the linkage and coordination of the acceptor atoms — this complicates true assignment of the OH bands observed to the two and six different OH^? ions present in malachite and brochantite, respectively -, and (iii) the Cu — O stretching modes at 430–590 cm^?1 and 420–520 cm^?1 for malachite and brochantite, respectively, exhibit strong, partially covalent Cu — O bonding.     

Trace element incorporation into quartz: A combined study by ICP-MS, electron spin resonance, cathodoluminescence, capillary ion analysis, and gas chromatography

Pegmatite quartz from different occurrences in Norway and Namibia was investigated by a combination of ICP-MS, Electron Spin Resonance (ESR), Capillary Ion Analysis (CIA) and Gas Chromatography (GC) to quantify trace elements in very low concentrations and to determine their position in the quartz structure.The studied quartz samples show similar geochemical characteristics with low contents of most trace elements. Remarkable are the elevated concentrations of Al (36-636 ppm), Ti (1.6-25.2 ppm), Ge (1.0-7.1 ppm), Na (5.2 to >50 ppm), K (1.6 to >100 ppm) and Li (2.1-165.6 ppm). These elements are preferentially incorporated into the quartz lattice on substitutional (Al, Ti, Ge) and interstitial (Li, Na, K) positions. Li⁺ was found to be the main charge compensating ion for Al, Ge and Ti, whereas some ppm of Na and K may also be hosted by fluid inclusions. Ti may be incorporated as substitutional ion for Si or bound on mineral microinclusions (rutile). The results of the ESR measurements show that there may be a redistribution of alkali ions during irradiation. The diamagnetic [AlO₄/M⁺]⁰ center transforms into the paramagnetic [AlO₄]⁰ center, whilst the compensating ions diffuse away and may be captured by the diamagnetic precursor centers of [GeO₄]⁰ and [TiO₄]⁰ to form paramagnetic centers ([TiO₄/Li⁺]⁰, [GeO₄/Li⁺]⁰).In general, fluid inclusions in pegmatite quartz can be classified as H₂O-CO₂-NaCl type inclusions with water as the predominant volatile. Among the main elements hosted by fluid inclusions in quartz are Na, K, NH₄, Ca, Mg and the anionic complexes Cl⁻, NO₃⁻, HCO₃⁻ and SO₄²⁻. Gas analysis of trapped fluids shows volatile components in the following order of abundance: H₂O > CO₂ > N₂(+) ≥ CH₄ > COS > C₂ and C₃ hydrocarbons. Additionally, traces of Co, Ni, Zn, Pb, and Cu were detected by CIA in fluid inclusions of some samples. There are indications that the REE and Rb are also bound in fluid inclusions, however, the concentrations of these elements are too low to be measured by CIA. Assuming that the REE preferentially occur in fluid inclusions, the typical chondrite normalized REE distribution patterns with tetrad effects and a distinct negative Eu anomaly would reflect the composition of the mineralizing fluid.For a number of elements, especially those with extremely low concentrations, the “type” of incorporation in quartz could not directly be determined. We conclude that these ions either are too large to substitute for the small Si⁴⁺ ion or they are not soluble in the mineralizing fluids to be hosted by fluid inclusions. Some of these elements, which are concentrated in the specific mineralization of certain pegmatites, are not present in elevated concentrations in the paragenetic pegmatite quartz itself. This was observed, for instance, for Be, Cs and Rb in the Li (Be-Cs-Rb) pegmatites of Rubicon or for Nb and Ta for Nb-Ta bearing pegmatites from Norway. It may be concluded that the concentrations of these trace elements in quartz do not reflect the mineralization and that these elements thus, cannot be used as petrogenetic indicator.     

Quartz from Allchar as monitor for cosmogenic 26Al: Geochemical and petrogenetic constraints

Summary Results of a multidisciplinary study on quartz concentrates (mineralogically separated) and etched concentrates (stoichiometric quartz) from three locations at Allchar (Macedonia) are presented. The investigation of quality and composition of these quartz samples is of great interest because the same material has been previously used as monitor for ²⁶Al Acceleration Mass-Spectrometry (AMS) erosion rate estimates. Two genetically different types of quartz are distinguished in the studied samples which petrologically can be described as hydrothermally altered dacites or quartz latites; i.e. volcanic (Q_V) and hydrothermal (Q_H) quartz with relative proportions of Q_H:Q_V around 3:2. Q_H is genetically related to the Allchar Sb–As–Tl–S mineralization having very high Sb (85–785 ppm), As (7.6–78 ppm) and (Tl 3.3–4.0 ppm) contents. This type of quartz is also characterized by very high Li (129–138 ppm), Al (2424–2520 ppm) and Ti (153–219 ppm) concentrations. Q_V appears to be much less enriched in trace elements having Al and K contents ranging from 0 to 280 ppm and from 50 to 85 ppm, respectively. ²⁶Al AMS measurements were done on the samples containing two genetically different types of quartz but this had no effects on the interpretation and erosion rate determinations. However, the extremely high Al concentrations in the analyzed quartz have generally negative effects, mainly by decreasing ²⁶Al/²⁷Al ratios and thus causing an increase of the detection limit. The disagreement between the results of ²⁶Al AMS analyses and quantitative geomorphologic data for one location is probably caused by different geographical position with respect to the direction of cosmic rays.