Kinetics of the magnetite–maghemite–hematite transformation, with special reference to hydrothermal systems

Abstract

. Maghemite, prepared in the usual way (but with exclusion of silica, e.g., from glassware) by precipitation of non-stoichiometric magnetite Fe3-,O,-, in aqueous MOH (M = Na, K) followed by air oxidation, picks up moisture from the air to reach the limiting composition MsH,-sFe50,. where 6 -0.02-0.03 for fresh material, but changes under hydrothermal conditions because of ion exchange. Despite the role of absorbed moisture in stabilizing maghernite, formation of the latter from Fe3-,O,-, is markedly retarded, and its decomposition to u-Fe,03 greatly accelerated, under hydrothermal conditions relative to the rates of the corresponding reactions of the dry solids. The rate of hydrothermal decomposition of maghemite is strongly retarded by silica. Over the range 160-187°C at least, silica-free maghemite decomposes in water according to the empirical equation -In ( I -a ) = (kt)", whereu is the fractional extent of decomposition. and 11 -2.5 for neutral water (with k = 1.3 x s-I at 160°C and 3.1 x 10-5s-' at 175°C) but approaches unity, without major effects on the overall time-scale of reaction, at high [MOH]. The mechanistic significance of these and previous results are considered; the hydrothermal conversion of maghemite to hematite evidently proceeds by ii dissolution-reprecipitation sequence. THOMAS WILSON SWADDLE et P~~I L L I P~L T M A N N .Can. J . Chem. 58, 1763 (1980). La maghemite. preparee de manitre habituelle (mais avec elimination de la silice par exemple celle de la verrerie) par precipitation de la magnetite non stoechiometrique Fe,-,O,_, dans du MOH aqueux (M = Na, K) suivie d'une oxydation a I'air, absorbe I'humiditC de I'air pour atteindre une composition limite de MsHI+,Fe5O, ou 6 -0.02-0.03 pour le produit fr-ais, mais il se produit des changements dans des conditions hydrothermiques pa; suite de I'echange d'ions. En depit du r6le de I'humidite absorbee en tant que stabilisatrice de la maghernite, la formation de cette derniere B partir de Fe3+,0,_, est fortement retiirdee et sa dCcomposition en Fe203 u est grandement accClerCe dans des conditions hydrothermiques s'apparentant aux vitesses des reactions correspondantes des solides secs. La vitesse de decomposition hydrothermique de la maghemite est fortement retardee par la silice. Au moins dans I'intervalle de 160-187"C, la maghemite depourvue de silice se decompose dans I'eau selon I'equation empirique In (1 -a ) =

Oxidation pathways for formic acid under low temperature hydrothermal conditions: Implications for the chemical and isotopic evolution of organics on Mars

“…This is consistent with the relatively sluggish kinetics of Fe 3 O 4 oxidation to Fe 2 O 3 at low-temperature hydrothermal conditions (Swaddle and Oltmann, 1980;Otake et al, 2010), and likely corresponds to surface oxidation effects similar to those identified on magnetite grains after reaction with H 2 O 2 -enriched aqueous solutions at ambient conditions (Costa et al, 2006). Formation of maghemite (cFe 2 O 3 ) was also not observed, however, cannot be excluded, as it appears to be a metastable intermediate phase during low temperature magnetite alteration to hematite (Colombo et al, 1968;Feitknecht and Gallagher, 1970;Swaddle and Oltmann, 1980;Otake et al, 2010). However, Mö ßbauer analysis suggest that magnetite coexisting with H 2 O 2 -HCOOH-bearing fluids develops active Fe 2+ sites during reaction progress as indicated by the decrease in the Fe 3+ /RFe ratio ($0.71) relative to the starting Fe 3+ / RFe composition ($0.84) ( Table 2 and Fig.…”

Mechanisms of iron oxide transformations in hydrothermal systems

“…Maghemite, which has a spinel structure and is often formed as a result of oxidation of magnetite (Swaddle and Oltmann, 1980), is thermodynamically unstable with respect to hematite. However, because transformation of maghemite to hematite is very slow at room temperature, maghemite is a common mineral in nature, including weathered volcanic rocks and some soils (Singer and Fine, 1989;Da Costa et al, 1999;).…”

Mechanism and kinetics of magnetite oxidation under hydrothermal conditions

“…Indeed, in nuclear power plants with pressurized water reactors (PWRs), the corrosion of carbon steel generates primarily magnetite in the secondary water vapour circuit due to the reducing conditions in this region. 26 Given that the steam generators (SGs) in such plants transform water into vapour, the corrosion products become concentrated in the circuit, which may eventually lead to fouling and clogging. [27][28][29][30] For this reason, deposited corrosion products must be periodically removed to ensure the integrity and heat exchange capacity of the SGs.…”