HIGH-QUARTZ SOLID SOLUTION PHASES FROM THERMALLY CRYSTALLIZED GLASSES OF COMPOSITIONS (LI2O,MGO).AL2O3.NSIO2

Thermally crystallized glasses of compositions (Li2,O2, MgO).Al2O3.nSiO2 were studied by X‐ray powder diffraction methods. High‐quartz solid solution phases developed at relatively low temperatures and, for n 3.5, transformed at higher temperatures to keatite solid solution phases. Associated phases, if present, were Mg spinel and/or cordierite, or a few other trace phases. The a crystallographic axis (a0) of high‐quartz solid solutions decreased with increase of MgO and/or SiO2. The c crystallographic axis (c0) decreased with increasing MgO; it also decreased with increasing SiO2, but only when MgO content was low. X‐ray diffraction photographs of single crystals of high‐quartz solid solutions of compositions LiaO.Al2O3.nSiO3 demonstrated that the maintenance of a basic high‐quartz structure is the basis of the solid solution relation. Three modifications of the high‐quartz structure were recognized in the Li2O‐Al2O3−SiO3 system. These modifications were based on the occurrence and positions of superlattice reflections. The high‐quartz solid solution from Li2O Al2O3−2SiO2, showing streaky reflections in its precession photographs, suggested a defective structure. The term “high‐quartz solid solution,” with or without additional prefixes specifying the compositional series and modification, was considered the preferred nomenclature for these solid solution phases. Copyright © 1968, Wiley Blackwell. All rights reserved