Metastable solid solutions with quartz-type structures on the join SiO2–MgAl2O4

Various members of a series of metastable solid solutions with a quartz-type structure and with compositions between SiO 2 and MgAl 2 O 4 have been synthesized from glass. Increasing amounts of Mg +2 and Al +3 in the quartz structure cause a gradual contraction parallel to, and a gradual expansion perpendicular to, the c axis. Siliceous members of the series are optically positive and less siliceous ones are negative; for a member with about 73 weight percent SiO 2 the birefringence is zero. The refractive indices of the solid solutions increase with decreasing SiO 2 content. Members with less than about 92 weight percent SiO 2 exhibit high-quartz structures even at room temperature, whereas more siliceous members go through an inversion to a low-quartz structure when quenched to room temperature. The temperature of this inversion is lower than that of pure quartz (SiO 2 ) as a result of the presence of Mg +2 and Al +3 in the structure.