The milarite structure-type has an uncommonly large number of distinct mineral species (23 at present). Here we explore this structure type from the point of view of possible root-charge arrangements and endmember compositions. Enumeration shows that there are 34 distinct root-charge arrangements with Si = 12 apfu and 39 distinct root-charge arrangements with Si = 8–11 apfu. A priori bond-valence calculations for all root-charge arrangements allows evaluation of lattice-induced strain as a function of chemical composition for all arrangements for which the detailed atomic arrangement has been refined. Analysis of localized strain indicates that the B site has the highest amount of strain in the structure, and in accord with this finding, milarite-group minerals with vacancies at the B site are more common than milarite-group minerals with fully occupied B sites. The a priori bond-valence calculations suggest that many other compositions are possible for the milarite structure-type. Examination of synthesis results reveals 20 synthetic compounds with the milarite-type structure that have distinct (dominant) endmember compositions. Examination of ∼350 chemical analyses from the literature reveals 29 distinct endmember compositions, six of which deserve to be described as new mineral species. Two additional analyses could lead to new minerals, but require confirmation of site populations by structure refinement.