Joseph V. Chernosky

The stability of tremolite; new experimental data and a thermodynamic assessment

Abstract The equilibria: tremolite + forsterite = 2diopside + 5enstatite + H2O (1) tremolite + 3calcite + 2quartz = 5diopside + 3CO2 + H2O (4) have been reversed experimentally at Pfluid = PH₂O = 0.5 kbar, 1.0 kbar, and 5.0 kbar and at Pfluid = PH₂O + PCO₂ = 5 kbar, respectively. Starting materials consisted of natural tremolite (St. Gotthard, Switzerland) and quartz (Brazil), and synthetic calcite, forsterite, diopside, and enstatite mixed in stoichiometric proportions. Reaction direction was determined by comparing XRD patterns of reactant and product assemblages and by examining surface features of experimental products with an SEM. Our new experimental data for Equilibrium 1 are consistent with the natural-tremolite results of Skippen and McKinstry (1985), who used St. Gotthard tremolite, whereas the new bracket for Equilibrium 4 is ≈25 ℃ lower than that of Slaughter et al. (1975) who also used St. Gotthard tremolite. Comparison of our results with other studies indicates that use of the St. Gotthard tremolite in place of synthetic tremolite in the starting material displaces these equilibria toward higher temperatures by about 25 and 5 ℃. respectively. Tremolite stability differences reflected in these data, as well as m phase equilibrium data for nine additional equilibria involving synthetic and natural tremolite can be accounted for with a simple ideal on-site mixing model to describe tremolite compositional differences. Our analysis leads us to conclude, however, that tremolite growth in some experiments near the equilibrium boundary occurs with respect to metastable end-member pyroxenes used in starting materials, whereas pyroxene-stable half-brackets involve growth of stable pyroxene compositions. Thennodynamic properties for end-member tremolite, retrieved by mathematical programming analysis of the experimental phase equilibrium data with these assumptions, provide the most sound basis for prediction of calcic aniphi- bole stability relationships in natural assemblages, as well as improved calibration of quantitative amphibole geothermobarometers. Our success hi extracting consistent thermodynamic properties for end-member tremolite from experimental data obtained with both synthetic and natural tremolite. assuming the former to contain 10 mol% magnesiocum- mingtonite component (Jenkins 1987), can be taken either as support for the validity of this assumption or as an indication that chain multiplicity faults (Maresch et al. 1994) produce a similar degree of stabilization as this solid solution.

Thermal decomposition of aluminium-bearing compounds

The thermal decomposition of aluminium sulfate, potassium aluminium sulfate, ammonium aluminium sulfate and alunite have been studied by dynamic thermogravimetry and X-ray diffraction. Specifically, the factors that affect the sulfate decomposition reaction have been investigated. The sulfate decomposition is found to be significantly influenced by the presence of an inert material such as potassium sulfate in the solid matrix.ZusammenfassungMittels dynamischer Thermogravimetrie und Röntgendiffraktionsverfahren wurde der thermische Zerfall von Aluminiumsulfat, Kaliumaluminiumsulfat, Ammoniumaluminiumsulfat und Alaunstein untersucht. Es wurden insbesondere diejenigen Faktoren untersucht, die die Zersetzungsreaktion der Sulfate beeinflussen. Es konnte nachgewiesen werden, daß die Sulfatezersetzung durch die Gegenwart von inerten Materialien, z. B. von Kaliumsulfat in der Festkörpermatrix eindeutig beeinflusst wird.РезюмеС помощью динамическ ой термогравиметрии и рентгено-диффракцио нного анализа изучено термическое разложение сульфата алюминия, калийалюминий сульф ата, аммонийалюминий сульфата и алунита. Де тально исследованы ф акторы, затрагивающие реакц ию разложения сульфата. Найдено, что разложен ие сульфата в значительной степе ни затрагивается присутствием в тверд ой матрице какого-либ о инертного вещества, как наприме р, сульфата калия.

KINETIC MODELING OF THERMAL DECOMPOSITION OF ALUMINUM SULFATE

The kinetics of thermal decomposition of aluminum sulfate was studied by dynamic and isothermal thermogravimetry. The nature of the solid phases was further examined by powder x-ray diffraction. Various kinetic models such as nucleation, phase boundary movement, diffusion and the homogeneous reaction were tested. Effectiveness of integral and differential methods of data reduction was evaluated. The activation energies obtained for the sulfate decomposition, calculated from different models and methods of analysis have been compared. The results indicate that aluminum sulfate decomposition can be adequately described by phase boundary movement models with an activation energy of 235 kJ/mole. It is further observed that the alumina formed at 900°C is amorphous in nature.