FeIII-substituted brucite: Hydrothermal synthesis from (Mg0.8FeII0.2)-brucite, crystal chemistry and relevance to the alteration of ultramafic rocks

Ferroan brucite, (Mg,Fe)(OH)2, is a common mineral product of serpentinization reactions. The alteration of ferroan brucite under subsurface conditions is expected to form magnetite and hydrogen (H2). Ferroan brucite, (Mg0.8Fe0.2)(OH)2, with a grain size of 20 – 100 nm, was synthesized by precipitation of iron salts in the presence of NaOH under sub-anoxic conditions. Both composition and grain size of the synthetic product are similar to those of ferroan brucite found in serpentinized peridotites collected during the Oman Drilling Project. Synthetic ferroan brucite was then reacted in aqueous solutions at 378 and 403 K during 1 to 36 days either in PTFE-lined reactors or in gold capsules placed in externally heated pressure vessels. In gold capsules, ferroan brucite barely reacted and minor magnetite and H2 were produced. In PTFE-lined reactors, reaction progress over 75% could be achieved with reaction products composed of magnetite, pyroaurite [Mg6Fe2(OH)16(CO3)•4.5H2O], and a new phase, identified as ferrian brucite [(Mg0.8FeIII 0.2)O0.2(OH)1.8] based on the Rietveld refinement of its X-ray powder diffraction data, FeIII/Fetot colorimetric determination and thermogravimetric analysis. Ferrian brucite is isostructural to ferroan brucite and displays the same iron/magnesium ratio. However, all iron is trivalent. X-ray diffraction and thermogravimetric data support deprotonation as the mechanism responsible for charge compensating FeII oxidation (Fe2+ + OH- => Fe3+ + O2-). The difference in reaction products obtained with the two types of reactors is attributed to the higher permeability to H2 (and/or O2) of the PTFE-lined reactors compared to gold capsules. Ferroan brucite conversion into ferrian brucite was unexpectedly fast and proceeded in a few days. The thermodynamic stability of ferrian brucite is discussed as a function of oxygen fugacity and pH.