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Heat-capacity behaviour of hemimorphite, Zn (sub 4) Si (sub 2) O (sub 7) (OH) (sub 2) .H (sub 2) O, and its dehydrated analogue Zn (sub 4) Si (sub 2) O (sub 7) (OH) (sub 2) ; a calorimetric and thermodynamic investigation of their phase transitions

Edgar Dachs and Charles A. Geiger
Heat-capacity behaviour of hemimorphite, Zn (sub 4) Si (sub 2) O (sub 7) (OH) (sub 2) .H (sub 2) O, and its dehydrated analogue Zn (sub 4) Si (sub 2) O (sub 7) (OH) (sub 2) ; a calorimetric and thermodynamic investigation of their phase transitions
European Journal of Mineralogy (October 2009) 21 (5): 971-983

Abstract

The heat capacity, C (sub p) , of hemimorphite, Zn (sub 4) Si (sub 2) O (sub 7) (OH) (sub 2) .H (sub 2) O, was measured using relaxation calorimetry and DSC methods in the temperature range 5 to 464 K and that of dehydrated hemimorphite, Zn (sub 4) Si (sub 2) O (sub 7) (OH) (sub 2) , from 5 to 764 K. The experimental C (sub p) data for hemimorphite show a prominent lambda -anomaly at 101.8 K that is related to a structural phase transition. An additional weak C (sub p) anomaly occurs around 40 K, suggesting a possible second phase transition. The C (sub p) data of Zn (sub 4) Si (sub 2) O (sub 7) (OH) (sub 2) exhibit a lambda -anomaly at 86.3 K. At T>280 K the C (sub p) behaviour of this phase is given by: C (sub p) (super ZN4Si2O7(OH)2) =537.7-3693.2.T (super -0.5) -5.7766.10 (super 6) . T (super -2) + 7.80821.10 (super 8) .T (super -3) . Two different model approaches were undertaken to describe low-temperature C (sub 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 (sub 4) Si (sub 2) O (sub 7) (OH) (sub 2) , were modelled using a combination of Debye, Einstein and Schottky functions. The excess heat capacity for the transition, Delta C (sub p) , was then calculated by subtracting interpolated model C (sub 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 (sub p) values give "base-line" C (sub 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 (sub c) =101.8 K for hemimorphite and T (sub c) =86.3 K for Zn (sub 4) Si (sub 2) O (sub 7) (OH) (sub 2) . The excess heat capacity, Delta C (sub p) , was fitted to a tricritical Landau expression Delta C (sub p) =aT/(4T (sub c) (T (sub c) -T)) and the determined thermodynamic phase-transition properties are Delta H (sub tr) =494+ or -13 J/mol and Delta S (sub tr) =7.3+ or -0.3 J/mol.K for hemimorphite and Delta H (sub tr) =360+ or -11 J/mol and Delta S (sub tr) =6.3+ or -0.2 J/mol.K for Zn (sub 4) Si (sub 2) O (sub 7) (OH) (sub 2) (a=14.6+ or -1.4 J/mol.K for hemimorphite and a=12.5+ or -0.8 J/mol.K for Zn (sub 4) Si (sub 2) O (sub 7) (OH) (sub 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 (sub p) behaviour for hemimorphite is not described well by a Landau model, thus indicating a second phase transition. Its excess C (sub p) , in addition to the Landau Delta C (sub p) , gives Delta H (sub tr) =86+ or -3 J/mol and Delta S (sub tr) =1.8+ or -0.1 J/mol.K. This transition at approximately 40 K could be related to changes in the weak H-bonding arrangement in the micropores of hemimorphite involving the H (sub 2) O molecules. This proposal is strengthened by the fact that a similar transition is not observed in Zn (sub 4) Si (sub 2) O (sub 7) (OH) (sub 2) . Standard entropies, obtained using the first model approach are S degrees =369.5+ or -3.0 J/mol.K for hemimorphite and S degrees =315.8+ or -2.5 J/mol.K for Zn (sub 4) Si (sub 2) O (sub 7) (OH) (sub 2) . These values agree within error with those derived from the second model approach.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 21
Serial Issue: 5
Title: Heat-capacity behaviour of hemimorphite, Zn (sub 4) Si (sub 2) O (sub 7) (OH) (sub 2) .H (sub 2) O, and its dehydrated analogue Zn (sub 4) Si (sub 2) O (sub 7) (OH) (sub 2) ; a calorimetric and thermodynamic investigation of their phase transitions
Affiliation: Universitaet Salzburg, Abteilung Mineralogie, Salzburg, Austria
Pages: 971-983
Published: 200910
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 27
Accession Number: 2009-098192
Categories: General geophysics
Document Type: Serial
Bibliographic Level: Analytic
Annotation: Includes appendices
Illustration Description: illus. incl. 6 tables
Secondary Affiliation: Christian-Albrechts-Universitaet Kiel, DEU, Federal Republic of Germany
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200952
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