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Quasi-ice-like C_P behavior of molecular H₂O in hemimorphite Zn₄Si₂O₇(OH)₂·H₂O: C_P and entropy of confined H₂O in microporous silicates

¹ Institut für Geowissenschaften, Abteilung Mineralogie, Christian-Albrechts-Universität zu Kiel, D-24098 Kiel, Germany
² Fachbereich Materialforschung und Physik, Abteilung Mineralogie, Universität Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria

Correspondence: ^* E-mail: chg{at}min.uni-kiel.de

Hemimorphite, Zn₄Si₂O₇(OH)₂·H₂O, and its dehydrated analog Zn₄Si₂O₇(OH)₂ were studied by low-temperature relaxation microcalorimetry and their heat capacity determined to analyze the behavior of the confined H₂O between 5 and 300 K. An analysis of the data, which are corrected for the presence of a phase transition, shows that the C_P of H₂O in hemimorphite behaves more similar to the C_P of ice than to liquid water or steam. The H₂O molecule, with its four planar hydrogen bonds in hemimorphite, as well as its tetrahedral coordination in ice, is more rigidly hydrogen bonded in both than in liquid water. This is reflected in their respective C_P behavior. The heat capacity and entropy for the dehydration reaction at 298 K are C_Prxn = – 2.1 ± 3.6 J/(mol·K) and S^rxn = 134.7 ± 4.0 J/(mol·K). C_P behavior at 0 < T < 300 K and entropy values at 298 K for confined H₂O in hemimorphite and hydrous Mg cordierite are compared to those in several zeolites. The entropy for confined H₂O in hemimorphite, analcime, and mordenite is around 54 J/(mol·K) at 298 K. The strength of the interactions (e.g., H bonding) between an H₂O molecule and its surroundings increases approximately from steam > cordierite > analcime > hemimorphite mordenite > heulandite > natrolite scolecite > liquid H₂O > ice and, in the case of microporous silicates, is inversely proportional to the S of the confined H₂O.

Key Words: Hemimorphite • heat capacity • entropy • microporous minerals • confined H₂O

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