Abstract
Various members of a series of metastable solid solutions with a quartz-type structure and with compositions between SiO2 and MgAl2O4 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 SiO2 the birefringence is zero. The refractive indices of the solid solutions increase with decreasing SiO2 content. Members with less than about 92 weight percent SiO2 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 (SiO2) as a result of the presence of Mg+2 and Al+3 in the structure.
