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This paper reports the results of hydrothermal synthesis in the system Na 2 O-MgO-FeO-Fe 2 O 3 -SiO 2 - H 2 O. Four samples of stoichiometric magnesioriebeckite composition, ideally ◻Na 2 Mg 3 Fe 2  3+ Si 8 O 22 (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 f O₂ . Powder XRD and SEM-EDX show a high (&gt;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 [NaNa 2 Mg 4 Fe 3+ Si 8 O 22 (OH) 2 ]. Powder XRD patterns were indexed in the space group C2/m; refined cell-parameters reß ect variations in the amphibole composition, and the cell volume is correlated linearly with the A-site occupancy. Mössbauer spectra show that in all samples, Fe 3+  is completely ordered at M2, whereas Fe 2+  occurs at the M1, M3, and M4 sites. The Fe 3+ /Fe 2+  ratio is a function of f O₂ : for increasing oxidation conditions, there is significant increase in M2Fe 3+  and decrease in Fe 2+ , notably in M 4 Fe 2+ . Mössbauer spectra also show significant variation in  M1 Fe 2+  and  M3 Fe 2+  quadrupole splitting as a function of the Fe 3+  content in the amphibole. IR spectra in the OH-stretching region show a well-resolved quadruplet at frequencies &lt;3680 cm -1 , assigned to octahedral  M1,3 (Mg, Fe 2+ )-OH-A◻ configurations, and a broad band consisting of four overlapping components related to  M1,3 (Mg, Fe 2+ ) configurations associated with occupied A-sites. Quantitative evaluation of the relative band intensities suggests a linear increase of A-site occupancy with decreasing f O₂  of synthesis. The composition of the amphiboles synthesized, can be best described by a combination of the  C (Mg,Fe 2+ ) 1  B (Mg,Fe 2+ ) 1  CFe 3+  -1  B Na -1  and the  A Na 1  C (Mg,Fe 2+ )1  A ◻ -1  CFe 3+  -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 f O₂  during crystallization

Synthesis and crystal-chemistry of alkali amphiboles in the system Na 2 O-MgO-FeO-Fe 2 O 3 - SiO 2 -H 2 O as a function of f O 2