Potassium-bearing clinopyroxene: a review of experimental, crystal chemical and thermodynamic data with petrological applications

Adsorption of chlorinated organic compounds (COC) on activated charcoal and determination of ionic chlorine by gas chromatography were used for assessment of water pollution by COC. Content of adsorbing COC was the following (micrograms CI-/l): in river water 10-470, in drinking water 10-1180, in underground water 90-150; content of volatile COC was (micrograms CI-/l): in drinking water 2-117, in underground water 14-16

Thermodynamics of pyrope-majorite, Mg3Al2Si3O12-Mg4Si4O12, solid solution from atomistic model calculations

Static lattice energy calculations, based on empirical pair potentials have been performed for a large set of different structures with compositions between pyrope and majorite, and with different states of order of octahedral cations. The energies have been cluster expanded using pair and quaternary terms. The derived ordering constants have been used to constrain Monte Carlo simulations of temperature-dependent properties in the ranges of 1073 3673K and 0 20 GPa. The free energies of mixing have been calculated using the method of thermodynamic integration. At zero pressure the cubic/tetragonal transition is predicted for pure majorite at 3300 K. The transition temperature decreases with the increase of the pyrope mole fraction. A miscibility gap associated with the transition starts to develop at about 2000K and xmaj 0.8, and widens with the decrease in temperature and the increase in pressure. Activity composition relations in the range of 0 20 GPa and 1073 2673K are described with the help of a high-order Redlich Kister polynomial

Thermodynamic properties of MgSi(3) majorite and phase transitions near 660 km depth in MgSiO(3) and Mg(2)SiO(4): A first principles study

Thermodynamic properties of MgSiO3 tetragonal majorite have been calculated at high pressures and temperatures within the quasi-harmonic approximation based on density functional theory using the local density approximation (LDA) and the generalized gradient approximation (GGA). The LDA results compare exceptionally well with measured thermodynamic properties. A classical Monte Carlo simulation based on results from a cluster expansion method demonstrates that disorder between magnesium and silicon in the octahedral sites in MgSiO3 majorite does not occur below 3600 K at transition zone pressures. The ensuing calculations on phase boundaries of MgSiO3 between majorite, perovskite, and ilmenite show that a much better agreement with experiment can be obtained by using GGA rather than LDA, for LDA underestimates the transition pressures by as much as 11 GPa. The Clapeyron slopes predicted by GGA and LDA are close to each other: 0.9-1.7 MPa/K for majorite-perovskite transition, 6.9-7.9 MPa/K for majorite-ilmenite transition, and -7--3 MPa/K for ilmenite-perovskite transition. The triple point predicted by GGA is located at 21.8 +/- 1 GPa and 1840 +/- 200 K which is similar to 400 K lower in temperature than most experimental estimates. This result suggests that ilmenite is restricted to lower temperatures and that the majorite to ilmenite transition may occur in cold subducting slabs in the transition zone. Our calculation also reveals that wadsleyite decomposes to an assemblage of majorite plus periclase above 2280 K with a large negative Clapeyron slope (-22--12 MPa/K) and that ringwoodite decomposes to ilmenite plus periclase below 1400 K (1.2 MPa/K). These two decomposition transitions may influence hot plumes and cold slabs near 660 km depth, respectively. Further calculations show that discontinuities in density, bulk modulus, and bulk sound velocity associated with the majorite to perovskite transition in MgSiO3 are much larger than those from the postspinel transition in Mg2SiO4 at conditions close to 660 km depth. This suggests that the large density discontinuity at 660 km depth as proposed by PREM (9.3%) might be accounted by a piclogite compositional model or marginally accounted by a pyrolite compositional model with, for example, 50 vol % ringwoodite, 45 vol % majorite, and 5 vol % other phases (such as calcium perovskite) at the bottom of the transition zone, provided that the density contrast between majorite and perovskite will not be greatly altered by the presence of other elements such as Fe, Al, Ca, and H. On the other hand, the smaller density discontinuity at 660 km depth as derived from impedance studies (4-6%) disfavors sharp contributions to seismic discontinuities from the majorite to perovskite transition

High temperature elastic properties of Mg-cordierite: experimental studies and atomistic simulations

The temperature dependence of the elastic stiffness coefficients of natural orthorhombic Mg-cordierite was studied between 295 K and 1573 K using resonant ultrasound spectroscopy. The measurements revealed a continuous decrease of all the elastic constants with increasing temperature. The bulk modulus softens from about 129(2) GPa at 295 K to 110(2) GPa at 1473 K. Irreversible anomalies in the temperature evolution of the resonance frequencies of certain eigenmodes were observed above 920 K due to the escape of volatiles and the occurrence of microcracks. However, the dehydrated samples still showed integrity on the macroscopic scale. Therefore, despite the occurrence of the micro-cracks, a reasonable quantitative analysis of the high-temperature RUS data of cordierite samples was still feasible. The thermal expansion was studied between 100 K and 1570 K using dilatometry. The new data are consistent with earlier experimental results and confirm the expansion of the a and b unit cell parameters and the contraction of the c parameter with increasing temperature. Possible contributions of the Al/Si disorder to the elastic properties of Mg-cordierite were estimated on the basis of force-field and quantum mechanical calculations. The behaviour of individual elastic stiffness coefficients was followed across the order/disorder transition by Monte Carlo simulations. The simulations predicted a decrease in the bulk modulus with increasing Al/Si disorder. However, this effect is much smaller than that observed experimentally. The measured decrease in the elastic stiffness coefficients is mainly due to phonon softening effects