Armenite, ideal formula BaCa2Al6Si9O30 center dot 2H(2)O, and its dehydrated analog BaCa2Al6Si9O30 and epididymite, ideal formula Na2Be2Si6O15 center dot H2O, and its dehydrated analog Na2Be2Si6O15 were studied by low-temperature relaxation calorimetry between 5 and 300 K to determine the heat capacity, C-p, behavior of their confined H2O. Differential thermal analysis and thermogravimetry measurements, FTIR spectroscopy, electron microprobe analysis and powder Rietveld refinements were under-taken to characterize the phases and the local environment around the H2O molecule. The determined structural formula for armenite is Ba0.88(0.01)Ca1.99(0.02)Na0.04(0.01)Al5.89(0.03)Si9.12(0.02)O30 center dot 2H(2)O and for epididymite Na1.88(0.03)K0.05(0.004)Na0.01(0.004)Be2.02(0.008)Si6.00(0.01)O15 center dot H2O. The infrared (IR) spectra give information on the nature of the H2O molecules in the natural phases via their H2O stretching and bending vibrations, which in the case of epididymite only could be assigned. The powder X-ray diffraction data show that armenite and its dehydrated analog have similar structures, whereas in the case of epididymite there are structural differences between the natural and dehydrated phases. This is also reflected in the lattice IR mode behavior, as observed for the natural phases and the H2O-free phases. The standard entropy at 298 K for armenite is S degrees = 795.7 +/- 6.2 J/mol K and its dehydrated analog is S degrees = 737.0 +/- 6.2 J/mol K. For epididymite S degrees = 425.7 +/- 4.1 J/mol K was obtained and its dehydrated analog has S degrees = 372.5 +/- 5.0 J/mol K. The heat capacity and entropy of dehydration at 298 K are Delta C-p(rxn) = 3.4 J/mol K and Delta S-rxn = 319.1 J/mol K and Delta C-p(rxn) = -14.3 J/mol K and Delta S-rxn = 135.7 J/mol K for armenite and epididymite, respectively. The H2O molecules in both phases appear to be ordered. They are held in place via an ion dipole interaction between the H2O molecule and a Ca cation in the case of armenite and a Na cation in epididymite and through hydrogen-bonding between the H2O molecule and oxygen atoms of the respective silicate frameworks. Of the three different H2O phases ice, liquid water and steam, the Cp behavior of confined H2O in both armenite and epididymite is most similar to that of ice, but there are differences between the two silicates and from the Cp behavior of ice. Hydrogen-bonding behavior and its relation to the entropy of confined H2O at 298 K is analyzed for various microporous silicates. The entropy of confined H2O at 298 K in various silicates increases approximately linearly with increasing average wave-number of the OH-stretching vibrations. The interpretation is that decreased hydrogen-bonding strength between a H2O molecule and the silicate framework, as well as weak ion dipole interactions, results in increased entropy of H2O. This results in increased amplitudes of external H2O vibrations, especially translations of the molecule, and they contribute strongly to the entropy of confined H2O at T < 298 K. (C) 2010 Elsevier Ltd. All rights reserved.
