Weathering of iron-rich phases in simulated Martian atmospheres
Vincent Chevrier*
 Centre Européen de Recherche et d’Enseignement en Géosciences de l9Environnement,
Pierre Rochette*
 Europôle de l9Arbois, BP 80, 13545 Aix-en-Provence, cedex 04, France
Pierre-Etienne Mathé* 
Olivier Grauby* Centre de Recherche en Matière Condensée et Nanosciences, Centre National de la Recherche Scientifique,
Campus de Luminy, case 913, 13288 Marseille cedex 13, France
ABSTRACT
In order to simulate the weathering of primary phases likely
to occur on the Martian surface, metallic iron a-Fe, magnetite, and
pyrrhotite were aged in CO2 1 H2O or CO2 1 H2O2 atmospheres
at room temperature for 1 yr. Only the magnetite remained stable
during experiments; thus any magnetite on Mars is likely to be
inherited from primary bedrock, whereas any metallic and most
sulfide iron minerals are provided by meteoritic accretion. Metastable siderite, neoformed from a-Fe, as well as sulfates and sulfur
from pyrrhotite, account for various Martian in situ observations.
Stepwise color changes are related either to changes in the relative
proportions of neoformed phases or to atmosphere-related changes
in crystallinity, rather than to fundamental mineralogical variations of iron phases. Goethite is the main crystalline iron-bearing
end product, eventually associated with ferrihydrite. If hematite is
the actual dominant iron oxide that colors the Red Planet, our
results imply strong changes in water activities of the primary CO2
and H2O rich atmosphere (i.e., evolution toward anhydrous conditions), or long-term evolution, for goethite to further convert into
hematite. Our experiments suggest that iron weathering may have
been active until recent times and would not have required bodies
of liquid water.
Keywords: carbonates, weathering, iron, oxides, (oxy)hydroxides,
Mars, peroxide, regolith, sulfates.
INTRODUCTION
The geologic processes that resulted in the spectroscopic and magnetic properties of the Martian regolith—i.e., colored iron oxides and
(oxy)hydroxides with strongly magnetic phases (Madsen et al., 1999;
Morris et al., 2000)—remain a puzzle. Attempts to model the formation
of these phases through surface-weathering processes—e.g., from nonoxidized precursors—have focused on the weathering of titanomagnetite inherited from magmatic rocks (Gunnlaugsson et al., 2002) and
neoformed phases—mainly hematite and goethite—precipitated from
Fe21 aqueous solutions (Christensen and Ruff, 2004). However, previous studies have not considered the possible occurrence of other primary minerals or the fundamental differences in the atmospheres of
Earth and Mars, with replacement of O2 by CO2 in the latter. For
example, magnetic sulfides like pyrrhotite are thought to be responsible
for the large magnetic crustal anomalies of Mars (Rochette et al.,
2001), and, according to mass-balance estimations, as much as 10–30
wt% of the regolith matter could be due to meteoritic bombardment
(Flynn and McKay, 1990; Bland and Smith, 2000). Meteorites and
interplanetary dust particles being usually far richer than Martian rocks
in iron-bearing minerals (10%–20% metal, sulfide, or less commonly
magnetite, vs. 1%–2% in Martian rocks), such extra-Martian materials
may significantly contribute to the superficial stock of primary ironrich minerals. Therefore, the aim of this research was to investigate the
weathering of such phases under controlled CO2 atmospheres, in ex*E-mails: [email protected]; [email protected]; [email protected]; grauby@
crmc2.univ-mrs.fr.
perimental conditions that reasonably mimic the past or present Martian climate.
METHODS
Three powdered samples (10 g each) were used as primary phases:
(1) synthetic iron a-Fe, (2) synthetic magnetite Fe3O4, and (3) natural
hexagonal pyrrhotite (HPo) Fe9S10. Initial grain sizes were 5–10, ,25,
and ,100 mm, respectively. HPo contained silicate traces, as shown
by X-ray diffraction (XRD), bulk analyses with 1.3 wt% CaO and 0.1
wt% K2O, and transmission electron microscope–energy-dispersive
spectrometry (TEM-EDS) identification of a K-bearing mica. Samples
were placed in two desiccators, the bottoms of which were filled either
with liquid H2O (water atmosphere [w.a.]) or with H2O 1 33% H2O2
(hydrogen peroxide atmosphere [h.p.a.]), in order to maintain vapor
saturation. Both atmospheres were equilibrated with CO2 at the initial
pressure of 0.8 bar and kept between 15 and 20 8C throughout the
experiment. Such high pressure and temperature and water-saturated
conditions, although differing from the present-day atmospheric environment on Mars, were selected in order to induce significant weathering rates and to mimic a thicker and warmer atmosphere as is thought
to have prevailed on early Mars (Forget and Pierrehumbert, 1997),
when weathering processes were strongly active. The use of H2O2 vapor was justified by its suggested key role in Martian weathering (Yen
et al., 2000; Clancy et al., 2004). Equilibration between CO2 and H2O
or H2O2 is relatively quick (a few hours) and affects neither the partial
pressure of CO2 nor the weathering processes, whose kinetics are much
slower. The weathered powders were subsampled after 5, 19, 40, 75,
117, 173, and 259 days and were characterized by XRD, scanning
electron microscope (SEM) coupled with EDS probe, TEM, and saturation magnetization (MS) studies.
RESULTS
The color of neoformed products at different steps of weathering
is shown in Data Repository Figures DR1 and DR2.1 Magnetite remains unweathered in both atmospheres, as no change in structure or
composition has been observed. In w.a., a-Fe acquires an olive color
(Munsell chart 5Y 4/3) after 19 days, which turns progressively to deep
dusky red (2.5YR 2/2) at 75 days and then to a dark yellowish brown
(10YR 3/4), whereas a-Fe remains unchanged after 1 yr in h.p.a. The
progressive decrease in MS values indicates transformation of 75 wt%
of ferromagnetic a-Fe into neoformed paramagnetic and antiferromagnetic phases after 259 days in w.a. (Fig. 1). Siderite is identified on
XRD patterns after 40 days (Fig. 2) and in SEM observations as
micrometer-sized rhombohedral crystals (Fig. 3A). After 117 days, siderite crystals exhibit features interpreted as dissolution pits (Fig. 3B),
whereas the XRD pattern shows broad peaks attributed to poorly crystalline goethite (Fig. 2). The nanometric crystallite size of goethite is
confirmed by TEM observations (Fig. 4A). Full width at half maximum
(FWHM) estimated from XRD main peak of goethite evolves from 2u
1GSA Data Repository item 2004163, Figure DR1, colors of raw powders
at various weathering durations, and Figure DR2, colors of neoformed products
on a-Fe metal at various stages, is available online at www.geosociety.org/pubs/
ft2004.htm, or on request from [email protected] or Documents Secretary,
GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.
q 2004 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
Geology; December 2004; v. 32; no. 12; p. 1033–1036; doi: 10.1130/G21078.1; 4 figures; Data Repository item 2004163.
1033
Figure 1. Saturation magnetization MS of a-Fe weathered in water atmosphere. Right scale shows equivalent remaining mass
of a-Fe (m) normalized to initial mass (m0).
5 1.58568 at 75 days to 0.81948 and 0.64448 at 173 and 259 days,
respectively, indicating an increased crystallinity.
HPo turns to yellowish red (5YR 4/8) after only 5 days in h.p.a.,
whereas it takes ;100 days in w.a. to turn dark brownish red (2.5YR
2/4). Whatever the atmosphere, the neoformed minerals are virtually
identical: goethite, elemental sulfur (Fig. 3G), and minor amounts of
iron sulfates (Fig. 3H). Jarosite [KFe331 (SO4)2(OH)6] (Fig. 4B) and
gypsum (Fig. 3C) also occur as a result of silicate leaching. According
Figure 3. Scanning electron microscope pictures of weathered products. A: Aggregates of siderite crystals developed on a-Fe after 40
days in water atmosphere (w.a). B: Dissolution features on siderite
crystals after 259 days in w.a. C: Neoformed phases developed on
hexagonal pyrrhotite (HPo) after 173 days in w.a.; elongated crystals
of gypsum (Gy) associated with rounded crystals of elemental sulfur
(S) on goethite (Gt) nanocrystals in background. D: Products developed on HPo after 173 days in hydrogen peroxide atmosphere
(h.p.a.); mixture of goethite crusts (Gt) and sulfur crusts (S). E: Higher magnification of goethite crystals developed on HPo after 173
days in w.a. F: Higher magnification of goethite crusts formed on
HPo after 173 days in h.p.a.; no crystal can be distinguished, particularly when compared to E. G: Rounded crystal of sulfur embedding
fragments of goethite crust after 173 days in w.a. H: Aggregates of
tabular crystals of iron sulfates developed on HPo after 259 days in
w.a.
to SEM observations, the main difference between the w.a. and h.p.a.
lies in morphology and grain size of neoformed crystals (cf. Figs. 3C
and 3D, as well as Figs. 4C and 4D). In w.a., HPo is covered with
moss-like small goethite crystals (,200 nm; Fig. 3E) in contact with
iron gels (Fig. 4D). In contrast, in h.p.a., SEM pictures reveal a crust
of mixed goethite and sulfur, with no visible crystals (Fig. 3F). TEM
observations are consistent with a goethite of lower crystallinity and
an amorphous phase, possibly ferrihydrite (Fig. 4C).
Figure 2. X-ray diffraction patterns of a-Fe weathered in water
atmosphere at different steps (time, t 5 0, 40, and 259 days),
showing progressive development of neoformed phases. Abbreviations: Fe—a-Fe, Sd—siderite, Gt—goethite.
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DISCUSSION
The stability of magnetite during experiments indicates that it may
be inherited from bedrock through weathering processes, as suggested
by terrestrial analogs (Gunnlaugsson et al., 2002). Goethite is the dominant neoformed iron-bearing phase under relatively high temperatures
and partial pressures of water and CO2. Its formation is also favored
GEOLOGY, December 2004
Figure 4. Transmission electron microscope pictures of
weathered powders after 259 days. A: Aggregates of goethite crystals developed around amorphous precursor (aFe in water atmosphere [w.a.]). B: Single crystal of jarosite with electron-diffraction pattern (hexagonal pyrrhotite,
HPo, in hydrogen peroxide atmosphere [h.p.a.]). C: Aggregate of iron (oxy)hydroxide with electron-diffraction
pattern (HPo in h.p.a.). D: Acicular crystals of goethite
developed around aggregate of amorphous iron gel (HPo
in w.a.).
by the low pH (3.95) of the water at the bottom of the desiccator
(Schwertmann, 1985). Goethite crystallizes from ferrihydrite, suspected
to be present in the Martian regolith (Banin, 1996). However, despite
the possible presence of goethite (Bell et al., 2000; Morris et al., 2000),
hematite remains the major invoked colored iron phase in the Martian
regolith (Morris et al., 2000; Christensen et al., 2000). Goethite can
evolve into hematite under a wide range of Eh-pH conditions as evidenced by experimental studies and in natural systems (FernandezRemolar et al., 2004). On Earth, the transformation of goethite into
hematite is favored by low water activity, through climate change by
reequilibration of pedogenic systems in tropical lateritic profiles (Tardy
and Nahon, 1985), and also by dry polar climate (Bender Koch et al.,
1995). In lateritic profiles from Burkina-Faso, the hematite/(goethite 1
hematite) ratio has been shown to have changed from 0 at 0.1 Ma to
1 at 10 Ma (Tardy et al., 1988). Therefore, goethite may have formed
on Mars in conditions of a warmer and CO2 and H2O rich atmosphere
(Burns and Fisher, 1990, 1993) and later aged into hematite because
of lower water-vapor activity (Gooding, 1978). Hematite results either
from direct dehydroxylation of goethite, possibly promoted by ultraviolet radiation or surface heating (Glotch et al., 2004), or by different
evolution of its ferrihydrite precursor (Schwertmann, 1985). Thus, goethite could be a relict from ancient weathering processes on early Mars
and could be covered by a layer of hematite-bearing regolith corresponding to the evolution of weathering conditions, as suggested by
Banin (1996).
GEOLOGY, December 2004
According to our observations, the crystalline state of neoformed
goethite depends on atmospheric composition. The stronger oxidizing
character of H2O2 implies initially faster kinetics, as testified by the
development of weathering crusts only after 5 days, and thus poorly
crystalline and smaller grains. Alternatively, powders weathered in w.a.
have higher H2O contents than those in h.p.a., implying better developed fluid films on particles, which improves crystal growth. The fluid
available for chemical reaction depends on water partial pressure, as
weak chemical bonds generated by dissolved species like H2O2 decrease the availability of water to evaporate. Thus, the very low water
pressures prevailing today on Mars may generate poorly crystalline
nanophases. Low partial pressure as well as the catalytic deoxygenation
of H2O2 by the metallic grain surface may also be responsible for the
stability of a-Fe in h.p.a.
Millions of years of meteorite accumulation may provide the main
source of strongly magnetic iron in the Martian regolith and could
partially explain its strong Fe enrichment. Exogenic iron may constitute
a major Fe31 source in the Martian regolith via the weathering processes identified here. Moreover, the MS data (Fig. 1) indicate that the
conversion rate of a-Fe slows down with time (t), following a Fick’s
type law in 1/t½. This progressive slowing suggests that weathering
rates are controlled by diffusion of reactants through layers of neoformed products, which act as a protective coating on the particles. Thus,
composite particles of exogenic iron cores and weathering-induced
coatings should be preserved for longer times on Mars, where lower
temperatures and gas pressures prevail. As meteoritic material is
strongly Ni enriched compared to magmatic bedrock, the detection by
Mars Exploration Rover (MER) Spirit of increasing Ni content from
fresh rock to regolith confirms the exogenic contribution (Gellert et al.,
2004). The transitory character of siderite in our experiments does not
fit with the identification of surficial carbonates by both the thermal
emission spectrometer (TES) onboard Mars Global Surveyor (Bandfield et al., 2003) and the MER Spirit’s mini-TES. The TES findings
imply the stabilization of carbonate phases under Martian conditions.
As opposed to the well-documented siderite to goethite transformation,
under O2 (McMillan and Schwertmann, 1998) or CO2-rich atmosphere
(Catling, 1999), the siderite to goethite reaction from a-Fe under w.a.
requires a complementary reaction:
Fe 0 1 H2O 1 CO2 5 Fe 21 CO3 1 H2
2Fe 21 CO3 1 2H2O 5 2Fe 31 O(OH) 1 2CO2 1 H2 .
(1)
This chemical pathway introduces both a deficit in oxygen, trapped
during regolith weathering (Lammer et al., 2003) either in solid or
aqueous phases, and a release of hydrogen, which can escape more
easily than H2O from the planet through thermal losses. Therefore,
added to the well-admitted consumption of CO2 by sulfides and silicates during weathering, the incorporation of significant amounts of aFe in the regolith would contribute via weathering processes to the
observed budget of oxygen and hydrogen on Mars.
Considering the sulfide-rich sources on Mars, i.e., primary bedrock and meteorites, the indurated crusts of elemental sulfur that segregates from HPo during weathering (Figs. 3D, 3G) may also account
for the strong sulfur enrichment of the Martian regolith, up to 8% SO3
(Bell et al., 2000). Acting like a cement between individual grains, the
occurrence of such neoformed material in the Martian regolith would
also explain its rather cohesive properties, as suggested by MER
observations.
Sulfide weathering may favor the alteration of other phases like
silicates (Burns and Fisher, 1993) by releasing Fe31 oxidizing ions and
H2SO4. The presence of gypsum and jarosite indicates that Ca21 and
K1 have been mobilized from the silicates present in our samples. Both
sulfates are usually produced by the weathering of sulfide deposits
1035
(Burns and Fisher, 1990) and by acid drainage of ore mine waste (Sracek et al., 2004). Thus, the presence of sulfates recently evidenced by
MER Opportunity Lander in the Meridiani Planum region may result
from intensive weathering of silicates in the presence of abundant
sulfides.
The Viking Lander 1 Gas Exchange Experiment and Labeled Exchange Experiment showed the Martian surface to be extremely oxidizing (Ballou et al., 1978), possibly due to H2O2 and derived species
(Yen et al., 2000). Our study confirms a previous hypothesis about the
trapping of H2O2 by the regolith via different mechanisms, including
chemical reactions with minerals. The rapidity of the H2O2 reaction
with sulfides suggests its very short lifetime in the Martian atmosphere.
CONCLUSIONS
Our 1 yr weathering experiments in a Martian-like atmosphere
showed that, whereas magnetite remained stable in both atmospheres,
metallic iron and iron sulfide were rapidly weathered into siderite, elemental sulfur, sulfates, and goethite. This result provides a possible
mechanism for the origin of these phases in the Martian regolith, without involving features absent today: e.g., oxygen, acid vapor, and extensive surface liquid water. The presence of peroxide prevents metal
alteration and does not affect the mineralogy of sulfide alteration products: it is thus not required to explain iron oxidation. Only the water
partial pressure affects the weathering process and the crystallinity of
neoformed phases.
Considering our experimental conditions, the lower temperatures
on Mars should lead to slower reaction kinetics. Even in conditions of
frozen water, Antarctic analogues of Martian soils evidence weathering
via thin liquid films (Dickinson and Rosen, 2003). Nevertheless, considering that Mars had a past warmer climate leading to goethite formation, the evolution toward an anhydrous atmosphere may have promoted the formation of hematite at the expense of goethite, through
dehydroxylation processes.
REFERENCES CITED
Ballou, E.V., Wood, P.C., Wydeven, T., Lehwalt, M.E., and Mack, R.E., 1978, Chemical interpretation of Viking Lander 1 life detection experiment: Nature, v. 271,
p. 644–645.
Bandfield, J.L., Glotch, T.D., and Christensen, P.R., 2003, Spectroscopic identification
of carbonate minerals in the Martian dust: Science, v. 301, no. 5636,
p. 1084–1087.
Banin, A., 1996, The missing crystalline minerals in Mars soils: Advanced Space
Research, v. 18, no. 12, p. 12,233–12,240.
Bell, J.F., III, McSween, H.Y., Jr., Crisp, J.A., Morris, R.V., Murchie, S.L., Bridges,
N.T., Johnson, J.R., Britt, D.T., Golombek, M.P., Moore, H.J., Ghosh, A., Bishop, J.L., Anderson, R.C., Brückner, J., Economou, T., Greenwood, J.P., Gunnlaugsson, H.P., Hargraves, R.M., Hviid, S., Knudsen, J.M., Madsen, M.B., Reid,
R., Rieder, R., and Soderblom, L., 2000, Mineralogic and compositional properties of Martian soil and dust: Results from Mars Pathfinder: Journal of Geophysical Research, v. 105, no. E1, p. 1721–1755.
Bender Koch, C., Morup, S., Madsen, M.B., and Vistisen, L., 1995, Iron-containing
products of basalt in a cold-dry climate: Chemical Geology, v. 122, p. 109–119.
Bland, P.A., and Smith, T.B., 2000, Meteorite accumulation on Mars: Icarus, v. 144,
p. 21–26.
Burns, R.G., and Fisher, D.S., 1990, Iron-sulfur mineralogy of Mars: Magmatic evolution and chemical weathering products: Journal of Geophysical Research,
v. 95, no. B9, p. 14,415–14,421.
Burns, R.G., and Fisher, D.S., 1993, Rates of oxidative weathering on the surface of
Mars: Journal of Geophysical Research, v. 98, no. E2, p. 3365–3372.
Catling, D.C., 1999, A chemical model for evaporites on early Mars: Possible sedimentary tracers of the early climate and implications for exploration: Journal
of Geophysical Research, v. 104, no. E7, p. 16,453–16,469.
Christensen, P.R., and Ruff, S.W., 2004, Formation of the hematite-bearing units in
Meridiani Planum: Evidence for deposition in standing water: Journal of Geophysical Research, v. 109, no. E8, E08003, doi: 10.1029/2003JE002233.
Christensen, P.R., Banfield, J.L., Clark, R.N., Edgett, K.S., Hamilton, V.E., Hoefen,
1036
T., Kieffer, H.H., Kuzmin, R.O., Lane, M.D., Malin, M.C., Morris, R.V., Pearl,
J.C., Pearson, R., Roush, T.L., Ruff, S.W., and Smith, M.D., 2000, Detection
of crystalline hematite mineralization on Mars by the thermal emission spectrometer: Evidence for near-surface water: Journal of Geophysical Research,
v. 105, no. E4, p. 9623–9642.
Clancy, R.T., Sandor, B.J., and Moriarty-Schieven, G.H., 2004, A measurement of
the 362 GHz absorption line of Mars atmospheric H2O2: Icarus, v. 168,
p. 116–121.
Dickinson, W.W., and Rosen, M.R., 2003, Antarctic permafrost: An analogue for
water and diagenetic minerals on Mars: Geology, v. 31, p. 199–202.
Fernandez-Remolar, D., Gomez-Elvira, J., Gomez, F., Sebastian, E., Martiin, J., Manfredi, J.A., Torres, J., Gonzalez Kesler, C., and Amils, R., 2004, The Tinto
River, an extreme acidic environment under control of iron, as an analog of the
Terra Meridiani hematite site of Mars: Planetary and Space Science, v. 52,
p. 239–248.
Flynn, G.J., and McKay, D.S., 1990, An assessment of the meteorite contribution to
the Martian soil: Journal of Geophysical Research, v. 95, p. 14,497–14,509.
Forget, F., and Pierrehumbert, R.T., 1997, Warming early Mars with carbon dioxide
clouds that scatter infrared radiation: Science, v. 278, p. 1273–1276.
Gellert, R., Rieder, R., Anderson, R.C., Brückner, J., Clark, B.C., Dreibus, G., Economou, T., Klingelhöfer, G., Lugmair, G.W., Ming, D.W., Squyres, S.W., d’Uston,
C., Wänke, H., Yen, A., and Zipfel, J., 2004, Chemistry of rocks and soils in
Gusev Crater from the Alpha Particle X-ray Spectrometer: Science, v. 305,
no. 5685, p. 829–832.
Glotch, T.D., Morris, R.V., Christensen, P.R., and Sharp, T.G., 2004, Effect of precursor mineralogy on the thermal infrared emission spectra of hematite: Application to Martian hematite mineralization: Journal of Geophysical Research,
v.109, no. E7, E07003, doi: 10.1029/2003JE002224.
Gooding, J.L., 1978, Chemical weathering on Mars. Thermodynamic stabilities of
primary minerals (and their alteration products) from mafic igneous rocks: Icarus, v. 33, p. 483–513.
Gunnlaugsson, H.P., Weyer, G., and Helgason, Ö., 2002, Titanomaghemite in Icelandic basalt: Possible clues for the strongly magnetic phase in Martian soil and
dust: Planetary and Space Science, v. 50, p. 157–161.
Lammer, H., Lichtenegger, H.I.M., Kolb, C., Ribas, I., Guinan, E.F., Abart, R., and
Bauer, S.J., 2003, Loss of water from Mars: Implications for the oxidation of
the soil: Icarus, v. 165, p. 9–25.
Madsen, M.B., Hviid, S.F., Gunnlaugsson, H.P., Knudsen, J.M., Goetz, W., Pedersen,
C.T., Dinesen, A.R., Mogensen, C.T., and Olsen, M., 1999, The magnetic properties experiment on Mars Pathfinder: Journal of Geophysical Research, v. 104,
no. E4, p. 8761–8779.
McMillan, S.G., and Schwertmann, U., 1998, Morphological and genetic relations
between siderite, calcite and goethite in a low moor peat from southern Germany: European Journal of Soil Science, v. 49, p. 283–293.
Morris, R.V., Golden, D.C., Bell, J.F., III, Shelfer, T.D., Scheinost, A.C., Hinman,
N.W., Furniss, G., Mertzman, S.A., Bishop, J.L., Ming, D.W., Allen, C.C., and
Britt, T., 2000, Mineralogy, composition, and alteration of Mars Pathfinder
rocks and soils: Evidence from multispectral, elemental, and magnetic data on
terrestrial analogue, SNC meteorite, and Pathfinder samples: Journal of Geophysical Research, v. 105, no. E1, p. 1757–1817.
Rochette, P., Lorand, J.-P., Fillion, G., and Sautter, V., 2001, Pyrrhotite and the remanent magnetization of SNC meteorites: A changing perpective on Martian
magnetism: Earth and Planetary Science Letters, v. 190, p. 1–12.
Schwertmann, U., 1985, The effect of pedogenic environment on iron oxide minerals:
Advances in Soil Science, v. 1, p. 171–200.
Sracek, O., Choquette, M., Gélinas, P., Lefebvre, R., and Nicholson, R.V., 2004,
Geochemical characterization of acid mine drainage from a waste rock pile,
Mine Doyon, Québec, Canada: Journal of Contaminant Hydrology, v. 69,
p. 45–71.
Tardy, Y., and Nahon, D., 1985, Geochemistry of laterites, stability of Al-goethite,
Al-hematite and Fe31 kaolinites in bauxites and ferricretes: American Journal
of Science, v. 285, p. 865–903.
Tardy, Y., Mazaltarim, D., Boeglin, J.L., Roquin, C., Pion, J.C., Paquet, H., and
Millot, G., 1988, Lithodependence and homogenization of mineralogical and
chemical-composition in lateritic ferricretes: Paris, Académie des Sciences
Comptes Rendus, Sciences de la Terre, sér. II, v. 307, no. 16, p. 1765–1772.
Yen, A.S., Kim, S.S., Hecht, M.H., Frant, M.S., and Murray, B., 2000, Evidence that
the reactivity of the Martian soils is due to superoxide ions: Science, v. 289,
no. 5486, p. 1909–1912.
Manuscript received 12 August 2004
Revised manuscript received 24 August 2004
Manuscript accepted 31 August 2004
Printed in USA
GEOLOGY, December 2004