Thermochemistry of stuffed quartz-derivative phases along the join LiAlSiO₄-SiO₂

Enthalpies of drop-solution (ΔH(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·B₂O₃ at 974 K. Substitution of Si⁴⁺ for Li⁺ + Al³⁺ 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 κ 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 (ΔH(f,ox)⁰) and elements (ΔH(f,el)⁰) at 298 K also are presented. ΔH(f,ox)⁰ = -69.78 ± 1.38 kJ/mol and ΔH(f,el)⁰ = -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.