Single crystals of hydrous ferroan ringwoodites with compositions (Mg0.97Fe1.00)1.97Si0.98H0.13O4 and (Mg1.21Fe0.662+Fe0.13+)1.97Si0.97H0.06O4 have been synthesized. Their lattice parameters at ambient conditions are a = 8.1597(6) Å and V = 543.28(13) Å3 and a = 8.1384(3) Å and V = 539.03(7) Å3, respectively. The unit-cell lattice parameters were measured at different pressures up to about 9 GPa by means of X-ray single-crystal diffraction. The P-V data were fitted with a second-order Birch-Murnaghan equation of state (K′ fixed to the value of 4) refining to the following equation of state parameters: V0 = 543.32(7) Å3 and KT0 = 186.5(9) GPa for (Mg0.97Fe1.00)1.97Si0.98H0.13O4 and V0 = 539.01(5) Å3 and KT0 = 184.1(7) GPa for (Mg1.21Fe0.662+Fe0.13+)1.97Si0.97H0.06O4. Structural refinements indicate the presence of significant octahedral vacancies in sample (Mg1.21Fe0.662+Fe0.13+)1.97Si0.97H0.06O4 due to the oxidation of 0.1 apfu of ferric iron. Correlation between the O-O distances of the octahedral edges and the shift of the OH-stretching frequency suggest that the H incorporation occurs at the octahedral site.

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