While the end-member formula of oxy-dravite may be formalized as NaY(Al3)Z(Al4Mg2)Si6O18(BO3)3(OH)3O, the most representative structural formula is NaY(Al2Mg)Z(Al5Mg)Si6O18(BO3)3(OH)3O. The difference between these two formulas is solely in Al-Mg order-disorder, i.e., there is no difference in chemical composition. Although the Mg-Al disorder over the Y and Z sites is controlled by the short-range bond-valence requirements of O2− at the O1 (≡ W) site, the amount of Mg at the Z site is a function of the degree of cation size mismatch at Z.
The crystal structure of oxy-dravite was refined to statistical index R1 of 1.17% using 1586 equivalent reflections collected with MoKα X-radiation. Oxy-dravite is chemically related to dravite (and fluor-dravite), NaMg3Al6Si6O18(BO3)3(OH)3(OH,F), by the heterovalent substitution Al3+ + O2− → Mg2+ + (OH,F)1−.