Abstract

Enthalpies of drop-solution (Delta H (sub drop-soln) ) of a suite of stuffed quartz-derivative phases with the composition Li (sub 1-x) Al (sub 1-x) Si (sub 1+x) O 4 (0< or =x< or =1) have been measured in molten 2PbO.B 2 O 3 at 974 K. Substitution of Si (super 4+) for Li (super +) +Al (super 3+) results in more exothermic enthalpies of drop-solution, which is consistent with behavior seen in other crystalline and glassy aluminosilicates. Al/Si ordering serves to stabilize these phases, and long-range ordering for compositions with x approximately <0.3 can be discerned in both calorimetric data and in structural data obtained by electron and synchrotron X-ray diffraction (XRD). In contrast, a structural but not an energetic discontinuity is apparent at x congruent to 0.65, which corresponds to a compositionally induced alpha -beta quartz transition with a small enthalpy of transformation. An enthalpy for the Al/Si order-disorder reaction in beta -eucryptite was measured as 25.9+ or -2.6 kJ/mol. Standard molar enthalpies of formation of the stuffed quartz-derivative phases from constituent oxides (Delta H 0 (sub f,ox) ) and elements (Delta H 0 (sub f,el) ) at 298 K also are presented. Delta H 0 (sub f,ox) = -69.78+ or -1.38 kJ/mol and Delta H 0 (sub f,el) = -2117.84+ or -2.50 kJ/mol for beta -eucryptite, which are in good agreement with results previously determined by HF solution calorimetry at 346.7 K (Barany and Adami 1966). The enthalpies of formation of other compositions are reported for the first time.

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