60 The Open Mineralogy Journal, 2008, 2, 60-65 1874-4567/08 2008 Bentham Open Open Access Anomalous Temperature Dependence of the Heat of Hydration of Natrolite Jie Wang and Philip S. Neuhoff * Department of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, FL 32611-2120, USA Abstract: The temperature dependence of the heat of hydration of natrolite was studied by isothermal adsorption calo-rimetry from 412 to 472 K. The heat capacity of hydration implied by these results is about 5 times greater than the calo-rimetric heat capacity of reaction, suggesting atypical behavior across this solid solution. Key Words: Natrolite, hydration, differential scanning calorimetry, enthalpy. 1. INTRODUCTION Natrolite is a natural zeolite with an essentially stoichio- metric composition (Na 2 Al 2 Si 3 O 10 ·2H 2 O; [1]). Its crystal structure is well-known, including positions of H 2 O mole-cules (in fact, it was the first zeolite structure refined; [1-5]). The framework of natrolite is composed of Si- and Al-centered tetrahedra. The arrangement of Si and Al in the tetrahedral sites is variable but tends to be largely ordered [6, 7]. Channels within the structure contain two Na + ions and two H 2 O molecules per ten framework oxygens oriented in zigzag chains with each Na + coordinated to four framework oxygens and two H 2 O molecules [3, 8, 9]. The importance of natrolite as a rock-forming mineral [e.g., 1, 7] and its regular compositional and structural prop-erties have led many workers to use this mineral as a refer-ence for studying zeolite dehydration reactions [9-13]. How-ever, unlike most zeolite dehydration reactions that proceed in a continuous fashion with increasing temperature, imply-ing complete solution between the hydrated forms [e.g. 1, 13-16], the dehydration of natrolite occurs abruptly as a function of temperature, as indicated by both isothermal, equilibrium measurements [10] and scanning heating ther-mogravimentric analysis (TGA; Fig. 1) [see also 1, 13] indi-cate that dehydration of natrolite occurs abruptly as a func-tion of temperature. This can be seen in the TGA curve of Fig. (1), where it can be seen that the mass of natrolite ini-tially decreases gradually with temperature between 400 and 550 K and then decreases dramatically at ~ 575 K. In con-trast to the sharp “right angle” topology of the TGA curve of Fig. (1) at 600 K, most zeolites exhibit a more gradual, curved topology in TGA signals as complete dehydration is reached. In addition, a pronounced hysteresis is observed in TGA studies of dehydration/rehydration of natrolite under constant water vapor pressure (P H 2 O ) with rehydration occur-ring at significantly lower temperatures [13] in contrast to the behavior of many zeolites. The cause(s) of these phe-nomena are not known, in part because little data are avail- *Address correspondence to this author at the Department of Geological Sciences, 241 Williamson Hall, P.O. Box 112120, Gainesville, FL 32611-2120, USA; Tel: 1-352-846-2413; Fax: 1-352-392-9294; E-mail: neuhoff@ufl.edu able for evaluating the thermodynamic properties of natrolite dehydration. The present study investigates the thermodynamic behav-ior of the natrolite-H 2 O system through isothermal adsorp-tion heat measurements as a function of temperature. Reac-tion behavior exhibited in these experiments indicates that a solvus exists between natrolite and dehydrated natrolite. For the first time, the temperature dependence of the heat of hy-dration has been directly determined and compared to that calculated from the heat capacities of hydrated and dehy-drated natrolite and water vapor. These results demonstrate excess heat capacity across the natrolite-dehydrated natrolite solid solution. Combined, the observations of this study pro-vide evidence for a solvus in the natrolite-dehydrated natro-lite solid solution that explains many of the anomalous ther-mal analysis behaviors exhibited by this mineral. 2. MATERIALS AND METHODOLOGY The sample of natrolite was previously described and characterized by Neuhoff et al. [7; sample NAT001]. It was collected as veins within a metabasaltic tectonic inclusion at the famous Dallas Gem Mine benitoite and neptunite local-ity, San Benito County, California. Phase pure separates were hand picked, ground in an agate mortar, and sieved to a 20-40 μm size fraction. Sample identification and purity were confirmed by X-ray powder diffraction. The composi-tion was determined by electron probe microanalysis at Stan-ford University to be essentially stoichiometric (Na 2 Al 2 Si 3 O 10 ·nH 2 O). Water content of the sample was determined in this study by thermogravimetric heating to 1023 K after the equilibration with a room temperature atmosphere of 50 % relative humidity. The mass loss is about 9.49% of total sample mass, very close to the ideal water content of natro-lite (9.48%), and the water content taken to be 2 moles of water per formula unit. All the experiments in this study were conducted on the Netzsch STA 449C Jupiter simultaneous thermal analysis system at the University of Florida. The core component of the system is a vacuum-tight liquid nitrogen cooled furnace enclosing a sample carrier with an electrode for measure-ment of temperature differences between the sample and a
