ALBERT

Description: The position of hydrogen in the structure of topaz-OH was determined by means of ab-initio quantum-mechanic calculations. Static lattice energy calculations predict the existence of four non-equivalent positions of protons, which are characterized by O4–H1... O1, O4–H2... O2, O4–H3... O3 and O4–H4... O4 hydrogen bonds. The distribution of the protons between positions of local equilibrium is controlled by the proton–proton avoidance rule and the strength of the hydrogen bonds. The most favourable configuration of hydrogen atoms is achieved for adjacent protons, which form O4–H3... O3 and O4–H4... O4 hydrogen bonds, respectively. The thermal excitation of atoms at a temperature of 55 K is large enough for the hydrogen atoms occasionally to change their positions to form O4–H1... O1 and O4–H2... O2 bonds. At ambient pressures and higher temperatures the protons are in a dynamic exchange between the allowed positions of local minima. As a consequence, for nearly room-temperature conditions, the dynamic change between different structural configurations leads to the violation of all possible symmetry elements and with that to space group #E5/E5#1. The flipping of the protons between different sites is achieved by simple rotation of the OH-dipole and does not produce any significant distortion of the framework of topaz, whose symmetry remains that of the space group Pbnm. Therefore, no reduction of symmetry has been observed in former X-ray studies on topaz-OH. Calculated IR absorption spectra of topaz-OH were found to be in good agreement with measured spectra. According to the calculations, the two favourable configurations of protons might correspond to the measured peak splitting within the OH-stretching range. An experimentally observed low-frequency band at 3520 cm–1 was assigned to the OH-stretching of the O4–H3... O3 bond, while the band at 3600 cm–1 was attributed to OH-stretching of the O4–H4... O4 hydrogen bond. The broad peak in FAR-IR frequency range at 100–150 cm–1 is attributed to the stretching of H3... O3 and H4... O4 contacts. The rate of proton exchange at 670 K among different sites was estimated by ab-inito molecular dynamic simulations. The calculations predict that flipping of adjacent protons between O4–H3... O3 and O4–H4... O4 bonds at 670 K occur at a rate of about 1.96 THz.

Description: 〈P〉Phase equilibria in the system CaO-MgO-SiO〈FONT SIZE="-1"〉2〈/FONT〉-CO〈FONT SIZE="-1"〉2〈/FONT〉-H〈FONT SIZE="-1"〉2〈/FONT〉O-NaCl are calculated to illustrate phase relations in metacarbonates over a wide range of P-T-X[H〈FONT SIZE="-1"〉2〈/FONT〉O-CO〈FONT SIZE="-1"〉2〈/FONT〉-NaCl] conditions. Calculations are performed using the equation of state of Duan et al. (1995) for H〈FONT SIZE="-1"〉2〈/FONT〉O-CO〈FONT SIZE="-1"〉2〈/FONT〉-NaCl fluids and the internally consistent data set of Gottschalk (1997) for thermodynamic properties of solids. Results are presented in isothermal-isobarical plots showing stable mineral assemblages as a function of fluid composition. It is shown that in contact-metamorphic P-T regimes the presence of very small concentrations of NaCl in the fluid causes almost all decarbonation reactions to proceed within the two fluid solvus of the H〈FONT SIZE="-1"〉2〈/FONT〉O-CO〈FONT SIZE="-1"〉2〈/FONT〉-NaCl system.〈BR〉 Substantial flow of magma-derived fluids into marbles has been documented for many contact aureoles by shifts in stable isotope geochemistry of the host rocks and by the progress of volatile-producing mineral reactions controlled by fluid compositions. Time-integrated fluid fluxes have been estimated by combining fluid advection/dispersion models with the spatial arrangement of mineral reactions and isotopic resetting. All existing models assume that minerals react in presence of a single phase H〈FONT SIZE="-1"〉2〈/FONT〉O-CO〈FONT SIZE="-1"〉2〈/FONT〉 fluid and do not allow for the effect that fluid immiscibility has on the flow patterns. 〈BR〉 It is shown that fluids emanating from calc-alkaline melts that crystallize at shallow depths are brines. Their salinity may vary depending mainly on pressure and fraction of crystallized melt. Infiltration-driven decarbonation reactions in the host rocks inevitably proceed at the boundaries of the two fluid solvus where the produced CO〈FONT SIZE="-1"〉2〈/FONT〉 is immiscible and may separate from the brine as a low salinity, low density H〈FONT SIZE="-1"〉2〈/FONT〉O-CO〈FONT SIZE="-1"〉2〈/FONT〉 fluid. Most parameters of fluid-rock interaction in contact aureoles that are derived from progress of mineral reactions and stable isotope resetting are probably incorrect because fluid phase separation is disregarded.〈/P〉