Synthesis, structure, thermodynamic properties, and stability relations of K-cymrite, K[AlSi 3 O 8 ]·H 2 O

Abstract

Single-phase K-cymrite, K[AlSi3O8]·H2O, has been synthesized in the P-T range 3≤P(GPa)≤4 and 350≤T(°C)≤650, and characterized by a variety of techniques like SEM, FTIR, and 29Si MAS-NMR. Its thermal expansivity and compressibility have been measured up to 375 °C and 6.0 GPa, respectively. Within the uncertainty of the microchemical determination of H2O by Karl-Fischer titration, it invariably contains 1 mol of H2O per mol of KAlSi3O8. Under the SEM, it appears a small idiomorphic prisms. It is optically negative, with n o=1.553(1) and n e=1.521(1). FTIR spectrum identifies the water in its structure as molecular H2O. Its lattice constants are a=5.3348(1) A, c=7.7057(1) A, V= 189.924 A3, the space group being P6/mmm. The 29Si MAS-NMR suggests a weak short-range order of Al and Si in the symmetrically equivalent tetrahedral sites. A Rietveld structure refinement demonstrates that it is isostructural with cymrite (BaAl2Si2O8·H2O), the structure comprising double tetrahedral sheets with H2O molecules residing in their cavities, K serving as an interlayer cation. Whereas cymrite, with its ordered tetrahedral Al/Si distribution, shows a Pm symmetry, the weak short-range Al/Si order in K-cymrite (abbreviated below as KCym) makes it crystallize in the space group P6/mmm. Three reversal experiments on the reaction K[AlSi3O8]·H2O (KCym)=K[AlSi3O8] (Kfs)+H2O, executed in this study, confirm the earlier results of Thompson (1994) and supplement her data. A simultaneous treatment of those reversals, together with the thermodynamic data for Kfs and H2O available in the literature, helps derive the standard enthalpy of formation (−4233±9.4 kJ/mol) and standard entropy (276.3±10.2 J/K·mol) for K-cymrite. The computed phase relations of KCym in the KAlSi3O8-H2O binary are shown in Figure 4 for three different values of aH 2O. Given a 5 °C/km isotherm in a subducting slab of metasediments in a ultra-high-pressure metamorphic environment, KCym will be expected to grow by hydration of Kfs, unless the aH 2O had been substantially less than 0.5. Nevertheless, how far it can survive exhumation of the subducted terrain will depend critically on the rate of uplifting and on the aH 2O prevailing during that process.