Spinels and structurally related spinelloid polytypes have been synthesised in the MgO–FeO–Fe2O3–SiO2 system at 3.0–10.5 GPa and 1100–1200 °C. Here we report on the structural systematics of these phases as a function of composition, with particular emphasis on the structural changes induced by Mg incorporation. Analysis of the molar volume–composition relations in spinels suggests ideal or near ideal mixing behaviour. Spinelloids II, III and V are stable polytypes and their respective stability fields are confined in composition space, each related to specific structural constraints. A primary limitation to Si-Fe3+ substitution is the differing structural relations between the tetrahedral sites in the three polymorphs. Mg incorporation in the spinelloid polytypes is generally restricted. Maximum Mg2SiO4 contents follow the sequence spinelloid V > spinelloid III > spinelloid II, except for the additional stability field of the Mg2SiO4 endmember (wadsleyite), which has the spinelloid III structure. A general Mg-Fe3+ avoidance appears to limit Mg incorporation in the spinelloid structures. This effect is amplified by the preference of Mg for the octahedral site that is bonded to the bridging oxygen of the tetrahedral group, as determined by single-crystal refinements.