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

Abstract

Enthalpies of drop-solution (∆Hdrop-soln) of a suite of stuffed quartz-derivative phases with the composition Li1-xAl1-xSi1+xO4 (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 ≅ 0.65, which corresponds to a compositionally induced α-β quartz transition with a small enthalpy of transformation.