Recent analytical models for the Poisson’s ratios () of tetrahedral frameworks are applied to <?format ?>α-cristobalite and α-quartz for the first time. Rotation and dilation of the tetrahedral subunits are considered. Each mechanism leads to negative values, whereas negative and positive values are possible when they act concurrently. The concurrent model is in excellent agreement with experiment and explains the dichotomy between negative and positive values in -cristobalite and -quartz, respectively. The predicted strain-dependent trends confirm those from molecular modeling.