Calorimetry of silicate melts at 1773 K: measurement of enthalpies of fusion and of mixing in the systems diopside-anorthite-albite and anorthite-forsterite     

Alexandra Navrotsky  David Ziegler  Richard Oestrike  Papu Maniar 《Contributions to Mineralogy and Petrology》1989,101(1):122-130

Transposed-temperature-drop calorimetry, using a Setaram HT 1500 calorimeter, was used to study directly the melting at 1773 K of mixtures of crystalline albite, anorthite, and diopside and of anorthite and forsterite. The enthalpy of albite at 1000–1773 K, starting with both crystalline and glassy samples, was also measured. The results confirm previously measured enthalpies of fusion of albite, diopside and anorthite (Stebbins et al. 1982, 1983; Richet and Bottinga 1984,1986). The new results use thermochemical cycles which completely avoid the glassy state by transforming crystals directly to melts. The enthalpy of fusion of forsterite is estimated to be 89±12 kJ/mol at 1773 K and 114±20 kJ/mol at its melting point of 2163 K. The data allow semiquantitative evaluation of heats of mixing in the molten silicates. Along the Ab-An join, enthalpies of mixing in the liquid at 773 K are the same or somewhat more negative than those in the glass at 986 K, whereas along Ab-Di and An-Di, enthalpies of mixing in the liquid are distinctly more positive than in the glass. These differences correlate with excess heat capacities in the liquids suggested by Stebbins et al. (1984).  相似文献   

Phase transition in CaGeO3 perovskite: Evidence from X-ray powder diffraction,thermal expansion and heat capacity     

X. Liu  Y. Wang  R. C. Liebermann  P. D. Maniar  A. Navrotsky 《Physics and Chemistry of Minerals》1991,18(4):224-230

High-temperature x-ray powder diffraction study by the full pattern Rietveld method of orthorhombic CaGeO₃ (Pbnm at ambient condition) perovskite confirms the previously observed phase transition at Tc=520 K. The measured volumetric thermal expansion coefficients are 3.1 x 10^-5 (K^-1) below Tc and 3.5x 10^-5 (K^-1) above Tc. The space group at T>Tc has been tentatively identified as Cmcm. Such a transition involves the disappearance of one of the two octahedral rotations in the (001) plane, and the doubling of the unit cell volume, with c axis unchanged. Although this transition should be of first order from symmetry considerations, the distortion of the Pbnm phase decreases continuously as the temperate approaches Tc and there is no observable volume discontinuity at Tc. The measured heat capacity places an upper limit on the enthalpy of transition of 50 J/mol, which is quite reasonable in terms of the crystallographic nature of this phase transition.A National Science Foundation Science and Technology Center  相似文献   

Energetics of glasses in the system diopside-anorthite-forsterite     

Alexandra Navrotsky  Papu Maniar  Richard Oestrike 《Contributions to Mineralogy and Petrology》1990,105(1):81-86

Enthalpies of mixing in glasses in the experimentally accessible region of the system Di-An-Fo are generally less than 4 kJ/mol in magnitude. Enthalpies of mixing of liquids in this and in several other petrologically relevant melts are also small; thus, if magmas of different compositions mix isothermally, the heat released or absorbed can be neglected in any consideration of the thermal evolution of the magmas, unless melting or crystallization takes place. The enthalpy of vitrification of Mg₂SiO₄ (to form a hypothetical glass at 700°C) is estimated to be 61±4 kJ/mol, in contrast to the enthalpy of fusion at 1500°C, 89±12 kJ/mol (Navrotsky et al. 1989). This suggests an average difference in heat capacity ( C _p) between liquid and crystal of 35 J/K. mol, half that estimated by Ghiorso and Carmichael (1980, 1987).  相似文献