Nephrite in Bargradient Conditions

Larissa Ivanova (med@gpg.crust.irk.ru) & Vladimir Medvedev

Institute of the Earth's Crust SB RAS, Lermontov str. 128, 664033 Irkutsk, Russia

The kinetics of fluid passing in the nephrite in the area of T=300-500 C and P=1000 atm. in the H₂-H₂O-O₂ system was studied. The investigations were done both using the isobar-isothermal endurance and a direct study of barodiffusive flows with gradients of fluid pressure up to 1000 atm/cm. The migration of the fluid into the sample is found in both cases. The reaction oxidizing-recovery of iron, limited by the H₂ or O₂ inflows, takes place on the migration front. The FeO/Fe₂O₃ ratio changes with time. The experiments were carried out for conic samples (cone angle is 12-25^°), the sample diameter ranges from 10 to 25 cm. The velocity of fluid passing varies from 10^-8 to 10^-5 g/sec depending in the sample permeability and compression pressure. The differentiation fluid-flow takes places when moving through the sample. It results in the formation of zones different in terms of FeO/Fe₂O₃ ratio depending on time of the experiment and the sample size. The study of the barodiffusive flows for the system nephrite-H₂O-H₂ indicated that if the primary hydrogen concentration in the fluid-flow is 20 mol% its concentration changes with time, decreasing in the studied time interval up to 0.05 mol%. The separation is found for the system nephrite-H₂O-O₂. However, it is less effective. Thus, the differentiation fluid-flow in the barodiffusive process creates a principal possibility of forming the zones with different redox regime in one and the same sample. In the case with the nephrite these are the zones of different color. Varying in terms of the temperature, pressure, experiment duration and character of primary samples it is possible to obtain a wide range of nephrite color. Supported by the Russian Foundation for Basic Research (grant 98-05-65107).

Glass Transition Temperatures of Silicates: New Measurements and a Model of Calculation

Philippe Jarry (jarry@ipgp.jussieu.fr)¹, Fabrice Lafon (fabrice.lafon@sgr.saint-gobain.com)² & Pascal Richet (richet@ipgp.jussieu.fr)¹

¹ Laboratoire de Physique des Géomatériaux, Institut de Physique du Globe de Paris, Tour 14-15, 4 Place Jussieu, 75252 Paris Cedex 05, France

² Saint-Gobain Recherche, BP 135, 39 Quai Lucien Lefranc, 93303 Aubervilliers Cedex, France

As it marks the onset of configurational changes the glass transition is an important parameter characterizing atomic dynamics under definite kinetic conditions. In this contribution, it has been measured for more than 40 silicate glasses of industrial interest by differential scanning calorimetry and beam-bending viscometry at a heating rate of 5 K/min. These compositions include from 39 mol% to 65 mol% SiO₂ and up to 18 mol% Al₂O₃. A model has been developed to predict the variation with composition of the glass transition temperature. On the average, it reproduces the new data to within 15 K. Although empirical, this model has structural and rheological implications that will also be discussed.

Viscosity of CaAl₂Si₂O₈ - CaSiO₃ Melts and Calculation of the Anorthite - Pseudowollastonite Phase Diagram

Philippe Jarry (jarry@ipgp.jussieu.fr) & Pascal Richet (richet@ipgp.jussieu.fr)

Laboratoire de Physique des Géomatériaux, Institut de Physique du Globe de Paris, 4 Place Jussieu, 75252 Paris Cedex 05, France

Magma compositions are too diverse and the pressure and temperature conditions relevant to magmatic processes are too varied so that models are needed in petrology to interpolate or extrapolate the experimental determinations of phase equilibria or of physical properties. Modelling of solid-liquid equilibria is important in this respect, but is fraught with difficulties stemming from lack of enthalpy of mixing and especially of entropy of mixing data. However, determinations of configurational entropies from viscosity measurements through the "configurational entropy" theory of relaxation processes could make it possible to calculate phase diagrams from a purely thermochemical approach, i.e., without any adjustable parameters. To check this possibility, we have considered the binary anorthite-pseudowollastonite which represents a simplified model of some igneous rocks. In this contribution we will thus compare the experimental phase diagram of Osborn (1942) to the diagram calculated thermochemically. For this calculation, we used the enthalpies of mixing of Tarina et al. (1994) and entropies of mixing determined for a series of melts from available high-temperature viscosities and data newly measured between 10¹⁰ and 10 ¹³ Poises. The potential and practical limitations of the method will also be discussed.

Osborn EF, Amer. J. Sci, 240, 751-788, (1942).

Tarina I, Navrotsky A & Gan H, Geochim. Cosmochim. Acta, 58, 3665-3673, (1994).