Michael Burchard

Thermodynamic modeling of solubility and speciation of silica in H2O-SiO2 fluid up to 1300°C and 20 kbar based on the chain reaction formalism

Recent systematic studies of mineral solubilities in water to high pressures up to 50 kbar call for a suitable thermodynamic formalism to allow realistic fitting of the experimental data and the establishment of an internally consistent data base. The very extensive low-pressure ( 2 in H 2 O has in the last few years been extended to 20 kbar and 1300°C, providing an excellent experimental basis for testing new approaches. In addition, solubility experiments with different SiO 2 -buffering phase assemblages and in situ determinations of Raman spectra for H 2 O-SiO 2 fluids have provided both qualitative and quantitative constraints on the stoichiometry and quantities of dissolved silica species. We propose a thermodynamic formalism for modeling both absolute silica solubility and speciation of dissolved silica using a combination of the chain reaction approach and a new Gibbs free energy equation of water based on a homogeneous reaction formalism. For a given SiO 2 -buffer ( e.g. , quartz) and the coexisting H 2 O-SiO 2 fluid both solubility and speciation of silica can be described by the following two reactions: - monomer-forming standard reaction: \[\mathrm{SiO_{2(s)}\ {+}\ 2(H_{2}O)L\ {=}\ (SiO_{2}){\bullet}(H_{2}O)_{2}}\] - polymer-forming chain reaction: \[\mathrm{(SiO_{2})_{n{-}1}{\bullet}(H_{2}O)_{n}\ {+}\ (SiO_{2}){\bullet}(H_{2}O)_{2}\ {=}\ (SiO_{2})_{n}{\bullet}(H_{2}O)_{n{+}1}\ {+}\ (H_{2}O)_{L},}\] where 2 ≤ n ≤ ∞, and (H 2 O) L stands for “liquid-like” (associated, clustered) water molecules in the aqueous fluid. We show that reactions (A) and (B) lead to the simplified relationships Δ G ° (mono)r, P,T = Δ H ° (mono),r − T Δ S ° (mono),r + Δ Cp ° (mono),r [ T − 298.15 - T ln( T /298.15)] + Δ V ° (mono),r ( P − 1), and Δ G ° (poly),r, P,T = Δ H ° (poly),r − T Δ S (poly),r + Δ V ° (poly),r ( P − 1) (where the Δ G ° r, P,T , are the standard molar Gibbs free energy changes in reactions (A) and (B) as a function of pressure P and temperature T ; the Δ H ° r , Δ S ° r , Δ Cp ° r , and Δ V ° r are standard molar enthalpy, entropy, isobaric heat capacity and volume changes, respectively, in reactions (A) and (B) at reference temperature T o = 298.15 K and pressure T o = 1 bar) that provide excellent descriptions of the available H 2 O-SiO 2 data set in terms of both SiO 2 solubility and silica speciation. Discrepancies between directly determined solubility data and data obtained from in situ Raman spectra are ascribed to (i) possible experimental problems of equilibration and (ii) inherent difficulties of interpreting Raman spectra of dilute H 2 O-SiO 2 solutions. In agreement with recent findings, our model indicates that dissolved silica in quartz-buffered aqueous solutions is considerably polymerized, exceeding 20–25 % at all temperatures above 400°C.

Diamond Pressure and Temperature Sensors for High‐Pressure High‐Temperature Applications

A high-pressure high-temperature diamond based sensor has been developed for the use in diamond anvil cells. The electronic structure of the sensor is that of p-i-p unipolar diode with boron doped p-type regions separated by an i-region containing compensated boron acceptors. The sensor has been fabricated by high-temperature high-energy boron ion implantation followed by high-energy carbon ion irradiation on the working surface of an anvil made of type IIa natural diamond. The performance of the sensor has been tested at pressures up to 70 kbar and temperatures up to 800 C. The sensor has a resolution of 5 bar for pressure and of 0.01 °C for temperature.

Pressure‐volume‐temperature behavior of diaspore and corundum: An in situ X‐ray diffraction study comparing different pressure media

The lattice parameters of synthetic diaspore (AlO(OH)) and synthetic and natural corundum (Al2O3) have been measured at simultaneously elevated pressure and temperature up to 7 GPa and 800°C (1000°C for corundum) using a MAX 80 cubic anvil high-pressure apparatus. Several experimental runs were carried out for both minerals, in which the samples were first compressed at room temperature and then heated; NaCl served as an internal standard for pressure calibration. In most runs the samples were mixed with Vaseline or NaCl to ensure hydrostatic pressure-transmitting conditions. Some runs, mainly at pressures <4 GPa, were performed without using a pressure medium. A Birch-Murnaghan equation of state (EOS) was fitted to the experimental data obtained at room temperature (assuming K′ = 4), yielding bulk moduli K (diaspore) = 134.4±1.4 GPa in contrast to 230 GPa obtained from diamond anvil cell measurements without using a pressure medium recently, and K (corundum) = 219.1±3.5 GPa, consistent with previous results based on X-ray diffraction measurements in conjunction with multi-anvil high-pressure techniques. The high-temperature Birch-Murnaghan EOS was fitted to the high-temperature high-pressure diaspore data, P = 1.5KT[VT,r−7 - VT,r−5][1 - 0.75 (4 - K′T) VT,r−2 - 1], VT,r ≡ (V/VT,0)1/3, KT = K + (∂KT/∂T)P(T - 298.15), VT,0 = 117.84 A3 exp (∫a+bTdT), resulting in K = 133.6±4.0 Gpa,K′T = 4.1±1.5, (∂KT/∂T)P = −0.017±0.007 Gpa K−1, a = 2.22±0.40 10−5 K−1, and b = 2.23±0.79 10−8 K−2.

Common high-pressure metamorphic history of eclogite lenses and surrounding metasediments : a case study of calc-silicate reaction zones (Erzgebirge, Germany)

The Erzgebirge dome in the Central European Variscides is a stack of crustal slices including some high- and ultrahigh-pressure rocks related to continent–continent collision. One such slice is the Mica-Schist/Eclogite Unit, in which the predominantly metasedimentary country rocks appear to record maximum metamorphic pressures of at most 11–13 kbar, whereas volumetrically minor eclogite lenses within the unit indicate pressures up to 27 kbar. In the present study, the P – T evolution of rocks found in calc–silicate reaction zones between eclogite and country rock marble near the locality Stumpelfelsen, a crag composed of eclogite near the village of Hammerunterwiesenthal, has been established via conventional geothermobarometry. A metasomatic marble–eclogite interchange can be documented for the earliest stages of prograde metamorphism. Significant amounts of fluorine infiltrated the metabasic rocks from the marbles, leading to fluorine-bearing amphibole as well as phengite, and even fluorine-rich growth zones in garnet with 0.62 wt.% F and 1.2 wt.% OH. This appears to be the first description of a F-bearing member of an almandine–grossular solid solution poor in andradite component. The P – T path of the eclogite–marble reaction zone reaches a maximum pressure of 26 kbar at 520–640 °C, just below the quartz–coesite transition. The exhumation path can be traced to 10 kbar and 450–600 °C, where it is then coincident with the published P – T paths of the mica schists and orthogneisses of the Mica-Schist/Eclogite Unit. This study indicates that the Stumpelfelsen eclogite lens and the surrounding metasedimentary country rock of the area must share a common high-pressure metamorphic history. The critical question arising for future studies is how much of the Mica-Schist/Eclogite Unit has actually “travelled the high-pressure eclogite route”, and how much of it was never subducted to pressures greater than 11–13 kbar.

Extending the pressure and temperature limits of hydrothermal diamond anvil cells

A modified hydrothermal diamond anvil cell (HDAC) has been developed. The cell is designed primarily for use with anvils incorporating internal heaters, but also solves several technical problems limiting the pressures and temperatures that can be reached with conventional HDACs. The essential features of the cell are (i) the pressurizing mechanics ensure strictly parallel alignment and preclude accidental collision of the anvils, (ii) the mechanical anvil holders provide both robust fixation of the anvils without any glue and reliable electrical contacts with the anvil facets, (iii) the diamond anvils can be heated while the anvil backing plates and the holders remain comparatively cool, (iv) stable hydrothermal working conditions to at least 4.5 GPa and 1000 °C can be achieved during the course of sluggish reactions typical of geoscience research thrusts, and (v) large sample volumes are possible, such as diameters of 0.5 mm and heights of 0.1 mm for a 1 mm culet.

The heavy ion micro-projection setup at Bochum

Abstract A new type of microbeam setup is presented allowing MeV ion projection imaging of large area structures using the final lens of a high energy microprobe. Key issue of the new technology is the use of high aspect ratio high resolution stencil masks capable of withstanding high beam power which are now available. While the theoretical lateral resolution limit for this setup should allow the production of features well below 40 nm in size, the practical imaging resolution demonstrated so far is at 300 nm with heavy ions and typical implantation fluence. Microstructure formation is possible on substrates which do not allow the application of surface contact masks or in situations where such masks would result in sample contamination due to sputtering. A compact sample heater provides high substrate temperatures up to 1300°C during implantation for minimum damage implantation of crystalline samples. This setup opens a fascinating new field of applications in materials science since many structures can be exposed simultaneously. Fast structured implantation of stoichiometric doses of e.g. rare earth element ions gives access to the production of new materials of high purity at different substrate temperatures within reasonable time.

The solubility of natural grossular-rich garnet in pure water at high pressures and temperatures

The solubility of a natural grossular-rich garnet of composition Ca 2.86 Fe 2+ 0.07 Mg 0.07 Fe 3+ 0.10 Al 1.90 Si 3.00 O 12 has been experimentally determined in pure water at pressures from 1 to 5 GPa and temperatures ranging from 400 to 800 °C with the weight-loss technique in piston–cylinder apparatus. Grossular dissolves congruently in this pressure–temperature region. The amount of dissolved grossular increases with both increasing pressure and temperature and ranges from 0.1 to 7.0 wt.%. In comparison to available data on other phases in the CaO–Al 2 O 3 –SiO 2 system, the solubility of grossular is considerably lower than that of quartz (SiO 2 ) or wollastonite (CaSiO 3 ), but higher than that of corundum (Al 2 O 3 ) at comparable pressure–temperature conditions. Using a model based on the familiar correlation between the equilibrium constant of a dissolution reaction and the density of water, ρ H 2 O, we suggest that the following expression provides an acceptable description of the solubility of pure grossular, Ca 3 Al 2 Si 3 O 12 , in water for the above pressure–temperature range: \[log\ (\mathit{m}_{grs})\ =\ 0.8639\ {-}\ 3519.71{\ast}\mathit{T}^{{-}1}\ +\ 676921.07{\ast}\mathit{T}^{{-}2}\ +\ 4502.85{\ast}\mathit{T}^{{-}3}\ +\ log{\rho}_{H_{2}O}{\ast}\ (7.2557\ +\ 773.65{\ast}\mathit{T}^{{-}1}\ {-}\ 106080.98{\ast}\mathit{T}^{{-}2}),\] where m grs is the molality of dissolved grossular and T is in °C. Uncertainties estimated from the outer bounds of experimental data scatter indicate a maximum of ±0.045 mol grs /kg H 2 O. Comparison with available experimental solubility data on quartz, wollastonite, corundum and kyanite provides indirect evidence for the presence of aqueous Al–Si-species.

Modelling high-pressure aqueous fluids in the system CaO–SiO2–H2O: A comprehensive semi-empirical thermodynamic formalism

There is a distinct need for predictive equations of state for supercritical aqueous solutions, both for understanding fluids in deeper levels of Earth’s crust or in subduction zones, as well as in experimental work on mineral solubility. Here we develop a semi-empirical approach introduced by Gerya & Perchuk (1997) based on the P-T partition function of statistical thermodynamics, using experimental data on quartz and wollastonite solubility, as well as data on speciation of dissolved solution components derived from first principles molecular dynamics simulations. Two approaches are possible, differing in the degree of explicit information provided on the nature of the solution modelled and also in the amount of basic data needed to implement them. Both have their potential fields of application. A “simple” model using only the semi-empirical formulation for the H 2 O solvent is useful if independent data on speciation are lacking, overall neutrality of the dissolving species can be assumed, and the number of components is relatively low. Computation is fairly straightforward, because the system can be treated as a simple “mixing” problem, and adopted effective dissolved species are characterized by conventional thermodynamic properties that allow interpolation and extrapolation of fitted experimental solubility data. Application to the system CaO–SiO 2 –H 2 O shows that experimental data on fluids coexisting with wollastonite + quartz/coesite can be successfully modelled up to 900 °C and 4 GPa. This simple semi-empirical formulation can lay the groundwork for an “internally consistent data set” allowing descriptions of fluids in relatively simple fluid-rock systems at high pressures. With the addition of independent data on actual speciation, and an optimized model for the dissociation of H 2 O from available literature data, the semi-empirical approach can be extended to a comprehensive description of aqueous solutions in the CaO–SiO 2 –H 2 O system. Both models have one positive feature in common. Because standard thermodynamic properties for dissolved species, oxides or fictive aggregates/clusters can be derived, solutions of arbitrary compositions can be modelled from data obtained from experiments in which fluids are saturated with a given solid phase or phases.

Ca2+ solvation as a function of p, T, and pH from ab initio simulation.

First principles molecular dynamics simulations have been carried out at various temperatures and pressures starting with either Ca(2+) or CaO in a reactive volume of 63 H(2)O molecules. In the case of aqueous Ca(2+), the ion is surrounded by six H(2)O molecules in the first hydration shell at 300 K/0.3 GPa, with rare exchange between first and second hydrations shells. At 900 K/0.9 GPa, the coordination number in the first hydration shell fluctuates between six and eight, the average being 7.0. CaO immediately reacts with the surrounding H(2)O molecules to form Ca(2+) + 2OH(-). The hydroxyl ions form transient Ca(OH)(+) and Ca(OH)(2) complexes and have a mean residence time in the first coordination shell of Ca(2+) of 6 ± 4 ps at 500 K and 3 ± 3 ps at 900 K, respectively. At 500 K/0.5 GPa, the time-averaged relative concentrations of the transient Ca(2+), Ca(OH)(+), and Ca(OH)(2) species are 14%, 55%, and 29%, while at 900 K/0.9 GPa, they are 2%, 34%, and 63%.

Multipurpose high-pressure high-temperature diamond-anvil cell with a novel high-precision guiding system and a dual-mode pressurization device

A novel diamond-anvil cell (DAC) design has been constructed and tested for in situ applications at high-pressure (HP) operations and has proved to be suitable even for HP sample environments at non-ambient temperature conditions. The innovative high-precision guiding mechanism, comparable to a dog clutch, consists of perpendicular planar sliding-plane elements and is integrated directly into the base body of the cylindrically shaped DAC. The combination of two force-generating devices, i.e., mechanical screws and an inflatable gas membrane, allows the user to choose independently between, and to apply individually, two different forcing mechanisms for pressure generation. Both mechanisms are basically independent of each other, but can also be operated simultaneously. The modularity of the DAC design allows for an easy exchange of functional core-element groups optimized not only for various analytical in situ methods but also for HP operation with or without high-temperature (HT) application. For HP-HT experiments a liquid cooling circuit inside the specific inner modular groups has been implemented to obtain a controlled and limited heat distribution within the outer DAC body.