The enthalpies of solution, in lead-cadmium-borate at 700°C, of samples of Amelia albite heated for 0-35 days at 1045 ± 5°C have been measured. For the transformation low albite→high albite at 700°C we obtained ΔH= 3.4±0.25 kcal/mole. The heat content of high albite at 700°C was measured by “transposed temperature” drop calorimetry. Ihis permits a comparison of the enthalpy of transformation at 700°C writh corresponding data determined near room temperature.

The enthalpy of solution data indicate that the process low albite→high albite takes place in two steps. The first step, which in our samples was essentially complete after three days at 1045°C, is associated with an enthalpy change of about 2.4 kcal/mole. I he second step of about 1 kcal/mole was nearly complete after three weeks at 1045°C. We attribute the first step in the transformation to partial disordering of the Al+Si atoms to form an “intermediate” albite, Na [Al0.5Si0.5]2Si2O5.

Based on various assumptions regarding the configurational entropies in low, intermediate and high albite, we have estimated approximate temperatures of transformation for the process low albite→high albite and for low albite→intermediate albite. The calculated transformation temperatures are consistent with the work of McConnell and McKie, who suggested the existence of a “smeared” polymorphic transformation around 600°.

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