The reaction
NaAlsi2O6+SiO2=NaAlSi3O8JadeiteQuartzhigh albite
has been determined by direct experiment in the temperature range 600–1200°C, and the P–T line can be described by the equation
P=0.35+0.0265T(°C)±0.50kbar
The brackets, when compared with the gas-apparatus determinations of this reaction by Hays and Bell (1973) and by Birch and LeComte (1960), show clearly that pressure corrections are vanishingly small when the NaCl pressure cell is used with the piston–cylinder device if piston-out methods are used. The slope (dP/dT = 26.5 bar °C−1), in conjunction with tabulated entropies for quartz and jadeite, leads to a value for the entropy of synthetic high albite. The disordering entropy of albite is (14.2±2.4 J K−1 mol−1), and the enthalpy is (12950±3320 J mol−1). The disordering enthalpy is in excellent agreement with recent calorimetric measurements on Amelia albite and its heat-treated modification. The best values from phase equilibria for the 298 K, 1 bar, thermochemical data for synthetic high albite are
ΔfH298,1bar=3922170±4300J mol1S298,1bar=221.6±2.5J K1mol1
The lack of curvature of the reaction in the P–T plane implies that high albite does not undergo ordering down to 600°C, at least within the duration time of the experiments. This is also confirmed by the X-ray patterns of albites from runs at 600°C.
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