High‐Quartz Solid Solution Phases from Thermally Crystallized Glasses of Compositions Li ₂ O ₂ ,MgO).Al ₂ O ₃ ,nSiO ₂

Thermally crystallized glasses of compositions (Li 2 ,O 2 , MgO).Al 2 O 3 .nSiO 2 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 (a 0 ) of high‐quartz solid solutions decreased with increase of MgO and/or SiO 2 . The c crystallographic axis (c 0 ) decreased with increasing MgO; it also decreased with increasing SiO 2 , but only when MgO content was low. X‐ray diffraction photographs of single crystals of high‐quartz solid solutions of compositions LiaO.Al 2 O 3 .nSiO 3 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 Li 2 O‐Al 2 O 3 −SiO 3 system. These modifications were based on the occurrence and positions of superlattice reflections. The high‐quartz solid solution from Li 2 O Al 2 O 3 −2SiO 2 , 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.