HR: 0830h
AN: V51H-0384    [PDF]
TI: Simultaneous Thermal Analysis Techniques for Determining the Energetics of Water in Mineral Hydrates
AU: * Neuhoff, P S
EM: neuhoff@ufl.edu
AF: University of Florida, Dept. of Geological Sciences, Gainesville, FL 32611-2120 United States
AB: Hydrate minerals (those containing molecular water; e.g., expandable clays, zeolites, hydrated salts) are important phases in surficial and near-surface Earth systems. These phases can undergo reversible dehydration and dehydration as temperature, pressure, and water fugacity vary, which can have a profound effect on the rheological, hydrological, thermal, and geochemical behavior of the crust. In addition, prediction of the water content of hydrate minerals is essential for assessing their stability. While thermodynamic models necessary for predicting hydration state are well established, experimental data based on equilibrium observations and calorimetry are often insufficient and too inconsistent for accurate depiction of hydration state. This uncertainty largely arises from inadequate knowledge of the heat capacities of these minerals as a function of temperature and hydration state and imprecise knowledge of the actual hydration states of samples during experiments. Simultaneous thermal analysis (STA) combining differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) can overcome many of these difficulties by allowing direct observation of hydration state during calorimetric experiments, thereby eliminating ambiguities in sample composition. Samples can be prepared in virtually any relevant hydration state within an STA apparatus, and subsequently used in calorimetric experiments. Although the precision of heat capacity measurements by DSC is lower than other common methods, direct monitoring of hydration state during experiments permits evaluation and elimination of excess heat of dehydration effects. Isothermal gas immersion measurements of the heat of hydration by DSC can also be directly monitored to assess the initial and final states of the sample, and permit partial molar heats of hydration to be determined over a wide range of hydration state from one experiment. Coupled application of heat capacity and immersion heat measurements by this method are demonstrated on the rock-forming zeolite natrolite. Comparison of the results of the present study with previous calorimetric and equilibrium observations of natrolite dehydration illustrate the critical importance of reliable heat capacity measurements and the experimental efficiency of present technique.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1045 Low-temperature geochemistry
DE: 1094 Instruments and techniques
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3694 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting