Dachs, E. and Geiger, C. A. (2009) Heat-capacity behaviour of hemimorphite, Zn(4)Si(2)O(7)(OH)(2)center dot H(2)O, and its dehydrated analogue Zn(4)Si(2)O(7)(OH)(2): a calorimetric and thermodynamic investigation of their phase transitions. European Journal of Mineralogy, 21 (5). pp. 971-983. DOI 10.1127/0935-1221/2009/0021-1975.

Abstract

The heat capacity, C(p), of hemimorphite, Zn(4)Si(2)O(7)(OH)(2)center dot H(2)O, was measured using relaxation calorimetry and DSC methods in the temperature range 5 to 464 K and that of dehydrated hemimorphite, Zn(4)Si(2)O(7)(OH)(2), from 5 to 764 K. The experimental C(p) data for hemimorphite show a prominent lambda-anomaly Lit 101.8 K that is related to a structural phase transition. An additional weak C(p) anomaly occurs around 40 K, suggesting a possible second phase transition. The C(p) data of Zn(4)Si(2)O(7)(OH)(2) exhibit a lambda-anomaly at 86.3 K. At T > 280 K the C(p) behaviour of this phase is given by: C(p)(Zn4Si2O7(OH)2) = 537.7 - 3693.2 . T(-0 5) - 5.7766.10(6).T(-2) + 7.80821.10(8). T(-3). Two different model approaches were undertaken to describe low-temperature C(p) behaviour and to derive phase-transition thermodynamic properties for both phases. In the first approach, heat capacities outside the region of the respective phase transitions (i.e., below 50 K and above 120 K for hemimorphite and above 110 K for Zn(4)Si(2)O(7)(OH)(2)) were modelled using a combination of Debye, Einstein and Schottky functions. The excess heat capacity for the transition, Delta C(p) was then calculated by Subtracting interpolated model C(p) values from the experimental heat capacities in the temperature region of the lambda-anomaly. In the second model approach, the heat capacity of the high-temperature phase was extrapolated into the stability field of the low-temperature phase by use of the Komada-Westrum model. The model C(p) values give "base-line" C(p) behaviour in the temperature region of the lambda-anomaly. A Landau analysis shows that the transitions in both phases are principally first order in character, but are close to a tricritical point with T(c) = 101.8 K for hemimorphite and T(c) = 86.3 K for Zn(4)Si(2)O(7)(OH)(2). Theexcess heat capacity, Delta C(p), was fitted to a tricritical Landau expression Delta C(p) = aT/(4 root T(c)(T(c)- T)) and the determined thermodynamic phase-transition properties are Delta H(tr)=494 +/- 13 J/mol and Delta S(tr)=7.3 +/- 0.3 J/mol.K for hemimorphite and Delta H(tr)=360 +/- 11 J/mol and Delta S(tr)=6.3 +/- 0.2 J/mol.K for Zn(4)Si(2)O(7)(OH)(2) (a = 14.6 +/- 1.4 Jmol.K for hemimorphite and a = 12.5 +/- 0.8 J/mol.K for Zn(4)Si(2)O(7)(OH)(2)). A possible crystal-chemical explanation for the transition in both phases is that dynamic proton disorder, associated with the OH groups of the framework in the high-temperature phase, is quenched below the transition temperature. Around 40 K Delta C(p) behaviour for hemimorphite is not described well by a Landau model, thus indicating a second phase transition. Its excess C(p) in addition to the Landau Delta C(p), gives Delta H(tr) = 86 +/- 3 j/mol and Delta S(tr) = 1.8 +/- 0.1 J/mol.K. This transition at similar to 40 K could be related to changes in the weak H-bonding arrangement in the micropores of hemimorphite involving the H(2)O molecules. This proposal is strengthened by the fact that a similar transition is not observed in Zn(4)Si(2)O(7)(OH)(2). Standard entropies, obtained using the first model approach are S(degrees) = 369.5 +/- 3.0 J/mol.K for hemimorphite and S degrees = 315.8 +/- 2.5 J/mol.K for Zn(4)Si(2)O(7)(OH)(2). These values agree within error with those derived from the second model approach.

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