This paper reports the results of hydrothermal synthesis in the system Na2O-MgO-FeO-Fe2O3-SiO2-H2O. Four samples of stoichiometric magnesioriebeckite composition, ideally □Na2Mg3Fe3+2 Si8O22(OH)2, were run at 700–800 °C, 0.4 GPa, and redox conditions varying from NNO (Nickel–Nickel Oxide) to NNO + 2.3 log fO2. Powder XRD and SEM-EDX show a high (>85%) amphibole yield for all samples; however, in no case was the end-member composition attained. EMP analyses show that the amphiboles obtained deviate strongly from nominal stoichiometry toward magnesio-arfvedsonite [NaNa2Mg4Fe3+Si8O22(OH)2]. Powder XRD patterns were indexed in the space group C2/m; refined cell-parameters reflect variations in the amphibole composition, and the cell volume is correlated linearly with the A-site occupancy. Mössbauer spectra show that in all samples, Fe3+ is completely ordered at M2, whereas Fe2+ occurs at the M1, M3, and M4 sites. The Fe3+/Fe2+ ratio is a function of fO2: for increasing oxidation conditions, there is significant increase in M2Fe3+ and decrease in Fe2+, notably in M4Fe2+. Mössbauer spectra also show significant variation in M1Fe2+ and M3Fe2+ quadrupole splitting as a function of the Fe3+ content in the amphibole. IR spectra in the OH-stretching region show a well-resolved quadruplet at frequencies <3680 cm−1, assigned to octahedral M1,3(Mg, Fe2+)-OH-A□ configurations, and a broad band consisting of four overlapping components related to M1,3(Mg, Fe2+) configurations associated with occupied A-sites. Quantitative evaluation of the relative band intensities suggests a linear increase of A-site occupancy with decreasing fO2 of synthesis. The composition of the amphiboles synthesized, can be best described by a combination of the C(Mg,Fe2+)1B(Mg,Fe2+)1CFe3+−1BNa−1 and the ANa1C(Mg,Fe2+)1A□−1CFe3+−1 exchange vectors. The experimental trend is in accord with the trend documented for natural amphiboles, and suggests that the amphibole composition can in fact be used to monitor changes in fO2 during crystallization.