The heat capacities, CP, and heats of solution, ΔHsoln, of six synthetic merlinoite samples having various Si/Al and Na/(Na + K) ratios were measured by low-temperature, adiabatic calorimetry and hydrofluoric acid solution calorimetry, respectively. Merlinoite heat capacities were measured from 10 to 320 K, and enthalpies of formation from the elements, ΔH°f, were determined from measurements of the heats of solution of Si02 (α-quartz), Al(OH)3 (gibbsite), NaF, KF, and the merlinoite samples in 24.4 wt% HF(aq) at 35 °C and 50 °C. The heat capacities of these samples show one large and two smaller glass-type transitions attributed to changes in the entropic state of the zeolitic water molecules.

Tables of the thermodynamic functions CP0,ST0S00,(HT0H00)/T, and (GT0H00)/T are presented for these six samples from 5 to 310 K. Configurational entropy contributions to the third-law entropies resulting from mixing on exchangeable cation, tetrahedral, and water sites were estimated. The configurational entropy contribution was estimated to be about 17% of the calorimetrically determined value of S2980S00. The following values for ST0 and ΔHf,2980 at 298.15 K (25 °C) were obtained:

T,298ΔH°f,298
Merlinoite composition[J/(mol·K)](kJ/mol)
(Na0.81K0.19AlSi1.94O5.88·2.13H2O) 328.8 ± 15.3 -3591.2 ± 2.9 
(K0.80Na0.20AlSi1.94O5.88·1.81 H2O) 324.4 ± 15.8 -3519.0 ± 2.9 
(KAlSi1.94O5.88· 1.69H2O) 320.7 ± 15.3 -3481.8 ± 3.0 
(Na0.81K0.19AlSi1.81O5.62· 2.18H2O) 319.2 ± 14.7 -3488.3 ± 2.8 
(K0.91Na0.09AlSi1.81O5.62·l.79H2O) 305.9 ± 15.0 -3387.3 ± 2.8 
(KAlSi, 1.81O5.62 ·1.69H2O) 304.3 ± 14.7 -3360.0 ± 2.8 
T,298ΔH°f,298
Merlinoite composition[J/(mol·K)](kJ/mol)
(Na0.81K0.19AlSi1.94O5.88·2.13H2O) 328.8 ± 15.3 -3591.2 ± 2.9 
(K0.80Na0.20AlSi1.94O5.88·1.81 H2O) 324.4 ± 15.8 -3519.0 ± 2.9 
(KAlSi1.94O5.88· 1.69H2O) 320.7 ± 15.3 -3481.8 ± 3.0 
(Na0.81K0.19AlSi1.81O5.62· 2.18H2O) 319.2 ± 14.7 -3488.3 ± 2.8 
(K0.91Na0.09AlSi1.81O5.62·l.79H2O) 305.9 ± 15.0 -3387.3 ± 2.8 
(KAlSi, 1.81O5.62 ·1.69H2O) 304.3 ± 14.7 -3360.0 ± 2.8 

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