SYNTHESIS, STRUCTURE, THERMODYNAMIC PROPERTIES, AND STABILITY RELATIONS OF K-CYMRITE, K[ALSI3O8]CENTER-DOT-H2O

Single-phase K-cymrite, K[AlSi3O8]. H2O, has been synthesized in the P -T range 3 less than or equal to P(GPa) less than or equal to 4 and 35 0 less than or equal to T(degrees C)less than or equal to 650, and cha racterized by a variety of techniques like SEM, FTIR, and Si-29 MAS-NM R. Its thermal expansivity and compressibility have been measured up t o 375 degrees C and 6.0 GPa, respectively. Within the uncertainty of t he microchemical determination of H2O by Karl-Fischer titration, it in variably 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 i n its structure as molecular H2O. Its lattice constants are a = 5.3348 (1) Angstrom, c = 7.7057(1) Angstrom, V = 189.924 Angstrom(3), the spa ce group being P6/mmm. The Si-29 MAS-NMR suggests a weak short-range o rder of Al and Si in the symmetrically equivalent tetrahedral sites. A Rietveld structure refinement demonstrates that it is isostructural w ith cymrite (BaAl2Si2O8 . H2O), the structure comprising double tetrah edral 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 o rder in K-cymrite (abbreviated below as KCym) makes it crystallize in the space group P6/mmm. Three reversal experiments on the reaction K[A lSi3O8]. 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 therm odynamic data for Kfs and H2O available in the literature, helps deriv e the standard enthalpy of formation (-4233 +/- 9.4 kJ/mol) and standa rd 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 a(H2O). Given a 5 degrees C/km isotherm in a subducting slab of metasediments in a ultra-high-pressure metamorphi c environment, KCym will be expected to grow by hydration of Kfs, unle ss the a(H2O) had been substantially less than 0.5. Nevertheless, how far it can survive exhumation of the subducted terrain will depend cri tically on the rate of uplifting and on the a(H2O) prevailing during t hat process.