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

Enthalpies of drop-solution (Delta Hdrop-soln) of a suite of stuffed quartz -derivative phases with the composition Li1-xAl1-xSi1+xO4 (0 less than or e qual to x less than or equal to 1) have been measured in molten 2PbO . B2O3 at 974 K. Substitution of Si4+ for Li++Al3+ results in more exothermic ent halpies of drop-solution, which is consistent with behavior seen in other c rystalline 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 obt ained by electron and synchrotron X-ray diffraction (XRD). In contrast, a s tructural but not an energetic discontinuity is apparent at x congruent to 0.65, which corresponds to a compositionally induced alpha-beta quartz tran sition with a small enthalpy of transformation. An enthalpy for the Al/Si order-disorder reaction in beta-eucryptite was me asured as 25.9 +/- 2.6 kJ/mol. Standard molar enthalpies of formation of th e stuffed quartz-derivative phases from constituent oxides (Delta H-f,ox(0) ) and elements (Delta H-f,el(0)) at 298 K also are presented. Delta H-f,ox( 0) = -69.78 +/- 1.38 kJ/mol and Delta H-f,el(0) = -2117.83 +/- 2.50 kJ/mol for beta-eucryptite, which are in good agreement with results previously de termined by HF solution calorimetry at 346.7 K (Barany and Adami 1966). The enthalpies of formation of other compositions are reported for the first t ime.