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  • 1
    Publication Date: 2004-10-01
    Description: Phase equilibria in the system CaO–MgO–SiO_2–CO2–H_2O–NaCl are calculated to illustrate phase relations in metacarbonates over a wide-range of P–T–X[H_2O–CO2–NaCl] conditions. Calculations are performed using the equation of state of Duan et al. (Geochim Cosmochim Acta 59:2869–2882, 1995 ) for H_2O–CO2–NaCl fluids and the internally consistent data set of Gottschalk (Eur J Mineral 9:175–223, 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_2O–CO2–NaCl system. 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 the presence of a single phase H_2O–CO2 fluid and do not allow for the effect that fluid immiscibility has on the flow patterns. 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 CO2 is immiscible and may separate from the brine as a low salinity, low density H_2O–CO2 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. ©2004 Springer-Verlag
    Print ISSN: 0010-7999
    Electronic ISSN: 1432-0967
    Topics: Geosciences
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  • 2
    Publication Date: 2004-04-01
    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. ©2004 Springer-Verlag Berlin Heidelberg
    Print ISSN: 0342-1791
    Electronic ISSN: 1432-2021
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 3
    Publication Date: 2003-02-01
    Description:  Recently, the Hy-2 a hydrous olivine (MgH2 SiO4)·3(Mg2SiO4) occurring as nanometre-sized inclusions in mantle olivines has been found by TEM, and has been suggested to be a new DHMS phase (Khisina et al. 2001). A model of the crystal structure of Hy-2 a has been proposed as a 2 a -superstructure of olivine with one Me2+ -vacant octahedral layer in the (1 0 0) plane per Hy-2 a unit cell (Khisina and Wirth 2002). In the present study the crystal structure of Hy-2 a hydrous olivine is optimized by ab initio calculations. The aims of this study are: (1) verification of the suggested models of Hy-2 a hydrous olivine structure; (2) calculation of the most stable configurations for Hy-2 a structure with minimum static lattice energy, by assuming a possible formation of Me2+ vacancies in either M1 or M2 octahedral sites; (3) determination of the position of protons and hydrogen bonds in the Hy-2 a structure. Several different possible configurations of the Hy-2 a structure are optimized. The results support the idea of a stable olivine structure with ordered planar-segregated OH-bearing defects oriented parallel to (1 0 0). The data obtained indicate a preferred stability of the Hy-2 a structure with the protons associated with M1 vacancies and bonded with O1 and O2 oxygen sites. The relative energy values of the optimized Hy-2 a structure configurations correlate as a rule with the average shifts of atoms from their positions in pure forsterite structure. ©2003 Springer-Verlag Berlin Heidelberg
    Print ISSN: 0342-1791
    Electronic ISSN: 1432-2021
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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