Thermochemistry of stuffed quartz-derivative phases along the join LiAlSiO 4 -SiO 2

Enthalpies of drop-solution (∆H drop-soln ) of a suite of stuffed quartz-derivative phases with the composition Li 1-x Al 1-x Si 1+x O4 (0 ≤ x ≤ 1) have been measured in molten 2PbO·B 2 O 3 at 974 K. Substitution of Si 4+ for Li + +Al 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 ≅ 0.65, which corresponds to a compositionally induced α-β quartz transition with a small enthalpy of transformation. An enthalpy for the Al/Si order-disorder reaction in β-eucryptite was measured as 25.9 ± 2.6 kJ/ mol. Standard molar enthalpies of formation of the stuffed quartz-derivative phases from constituent oxides (∆H 0 f,ox ) and elements (∆H 0 f,el ) at 298 K also are presented. ∆H 0 f,ox = -69.78 ± 1.38 kJ/mol and ∆H 0 f,el = -2117.84 ± 2.50 kJ/mol for β-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.