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Key Role of Phosphorus in the Formation of the Iron Oxides in Mars Soils?
Icarus 145, 645–647 (2000) doi:10.1006/icar.2000.6408, available online at http://www.idealibrary.com on
NOTE Key Role of Phosphorus in the Formation of the Iron Oxides in Mars Soils? Jos´e Torrent and Vidal Barr´on Departamento de Ciencias y Recursos Agr´ıcolas y Forestales, Universidad de C´ordoba, Apdo. 3048, 14080 C´ordoba, Spain E-mail: [email protected] Received December 20, 1999; revised March 24, 2000
Abundance of phosphorus (P) in martian rocks might be crucial in the formation of iron oxides, hematite (α-Fe2 O3 ) and maghemite (γ -Fe2 O3 ), thought to occur in martian soils. This hypothesis is supported by laboratory experiments showing that the thermal transformation of P-doped lepidocrocites and magnetites results in hematite and maghemite with magnetic and spectral properties consistent with those of martian soils. °c 2000 Academic Press Key Words: Mars; surface; mineralogy; cosmochemistry.
The Viking (1976) and Pathfinder (1997) missions have provided indications that martian soil and dust particles are composites (mostly silicates) and magnetic, with a saturation magnetization (Js ) of 4 ± 2 A m2 kg−1 as the most likely value (Hviid et al. 1997). This excludes the possibility of the magnetism resulting solely from superparamagnetic (nanophase) hematite, which is weakly magnetic. The preferred interpretation of Morris et al. (2000) is that the strongly magnetic phase is lithogenic titanomagnetite (Fe3−x Tix O4 ), and/or their titanomaghemite oxidation products. Hviid et al. (1997) and Madsen et al. (1999) did consider this hypothesis, but even if the TiO2 content of Mars soils (∼1% in weight; Rieder et al. 1997) were assigned solely to titanomagnetite, the calculated Js values occur near or below the low end of the 4 ± 2 A m2 kg−1 range. Therefore, these authors favored the idea that maghemite, in intimate association with silicate particles in at least 6% weight, gives rise to the magnetic properties of the martian soil. Thermal conversion of maghemite, probably through meteoritic impact, might then partly explain the likely presence of (red) pigmentary hematite in Martian soils (Morris and Golden 1998, Morris et al. 1998). On Earth soils, maghemite is formed via (i) thermal transformation of goethite in the presence of organic matter, (ii) oxidation of magnetite (Fe3 O4 ), and (iii) dehydroxylation of lepidocrocite (Schwertmann and Taylor 1989). The abundance of maghemite in Martian soils can be explained neither by path (i) owing to the lack of organic matter nor by path (ii) because magnetite (and titanomagnetite) are minor components of Mars bedrocks. Hence, dehydroxylation of precursor lepidocrocite is the most plausible hypothesis (Hargraves et al. 1977, Posey-Dowty et al. 1986, Schwertmann and Taylor 1989, Banin et al. 1993, Morris et al. 1998). We conducted laboratory experiments aimed at understanding why lepidocrocite and maghemite seem to be formed in martian environments, in contrast with the predominance of the two other main crystalline Fe oxides, goethite and hematite, on the earth’s surface. Specifically, we tested the hypothesis that this contrast is due to the high P content in Martian bedrocks relative to most Earth bedrocks. The X-ray fluorescence analysis by the Viking landers (Clark et al. 1982) and the data collected by the alpha proton X-ray spectrometer (APXS) of the Pathfinder (Rieder et al. 1997) indicate that Martian rocks possess an andesitic composition with a mafic component. W¨anke and Dreibus (1988)
estimated the bulk chemistry of Mars from SNC meteorites data and emphasized that the enrichment of P in the Martian mantle is by about a factor of 10 relative to the Earth mantle. Recently, Dreibus et al. (1999), with a refined method of searching for small peaks in the APXS spectra, found P content in the Martian rocks and soils at Pathfinder landing site to be ∼3 g kg−1 . Indeed, values of 1–4 g P kg−1 have been reported for similar Earth rocks (Bishop et al. 1998); with P occurring mainly in the form of apatite and other metal phosphates. With a typical Fe content of 100 g kg−1 for Martian rocks (Rieder et al. 1997), the ∼3 g kg−1 figure would yield a P/Fe atomic ratio of ∼0.05. Ratios of this order were used in our experiments involving the synthesis of Fe oxides. Different Fe oxides are obtained by hydrolysis and oxidation of Fe(II) salts depending on the pH, the oxidation rate, and the concentrations of Fe(II) and foreign ions (Cornell and Schwertmann 1996). Recent laboratory experiments (Cumplido et al., Clays Clay Miner., submitted for publication) show that oxidation at room temperature of an Fe(II) salt in the presence of phosphate at a high HCO3 concentration yields a mixture of goethite and lepidocrocite at P/Fe < 0.03, but exclusively lepidocrocite above this ratio. So the antilepidocrocitic effect of HCO3 (Cornell and Schwertmann 1996) seems to be offset by the pro-lepidocrocitic effect of P. This supports the idea that lepidocrocite rather than goethite might result from the alteration of mafic, P-rich Martian rocks even though the Mars atmosphere is rich in CO2 . This is consistent with the observation that, on the earth’s surface, lepidocrocite is formed in hydromorphic, CO2 -rich soils but preferably away from roots (Schwertmann and Fitzpatrick 1977), i.e., in zones where P has not been depleted by root uptake. We obtained lepidocrocite crystals from Fe(II) salts at P/Fe > 0.05 in the presence of HCO3 . These crystals contained phosphate in an occluded (nonalkali-desorbable) form and were small, as suggested by the broad lines in the X-ray diffractograms (bottom X-ray diffractogram of Fig. 1A). On being heated, these nanophase lepidocrocites were transformed first into maghemite (at 200– 250◦ C) and then into hematite (at 600–700◦ C) (Fig. 1A). Thus, the maghemites formed from “P-lepidocrocites” were stable over a temperature range of about 400◦ C, in contrast with one of only about 200◦ C for the maghemites obtained from highly crystalline pure lepidocrocite (Morris et al. 1998). These results are consistent with the finding that maghemites containing adsorbed phosphate become hematites at higher temperature than do pure maghemites (Tronc and Jolivet 1986). As a consequence, the products obtained in our heating experiments had a Js of >20 A m2 kg−1 for heating temperatures up to above 600◦ C (Fig. 2). Combination of these Js values with the Fe2 O3 content of the soils examined by the Pathfinder (14–18%) gives new Js values that are consistent with the above-mentioned average value (4 ± 2 A m2 kg−1 ). Even at 700◦ C, Js > 12 A m2 kg−1 , probably because part of the resulting hematite (Fig. 1A) was superparamagnetic and contained residual maghemite. The hematite formed at 700◦ C was bright red (suggesting a small particle size), probably because the presence of phosphate hindered sintering, a process that results in the formation of purplish red, large hematite particles. By combining the reflectance spectra for the initial products and the products obtained at 300, 500, and 700◦ C, which
645 0019-1035/00 $35.00 c 2000 by Academic Press Copyright ° All rights of reproduction in any form reserved.
646
´ TORRENT AND BARRON mimic the range of thermal transformation products expected from meteoritic impact on Mars, we obtained a spectrum similar to that for the Pathfinder bright soil (Fig. 3). Oxidation of magnetite is a common pathway to maghemite. Fine-grained magnetite can be produced in the laboratory by subjecting Fe(II) salts to hydrothermal conditions (Cornell and Schwertmann 1996) and can be oxidized readily to maghemite. Because of the small Fe content in the fluids of Martian hydrothermal systems (Newsom et al. 1999), this pathway seems unlikely on Mars; so it is more likely that magnetite can form by igneous processes, as in glassy flow or impact melt. In any case, if this hydrothermal pathway were active, it would be strongly affected by phosphate. We found the presence of phosphate during the synthesis of magnetite from FeSO4 at about 100◦ C and in the presence of HCO3 to give rise to (i) the occlusion of P in this mineral and in the maghemite resulting from its thermal transformation, (ii) an increase in the temperature at which maghemite was transformed into hematite from 200– 300◦ C for a P-free system to about 500–600◦ C for systems with P/Fe = 0.05 (Fig. 1B), (iii) large Js values for the thermal transformation products obtained at temperatures up to above 600◦ C (Fig. 2), and (iv) the combined spectrum for the initial products and the products formed at 300, 500 and 700◦ C being similar to that for the Pathfinder bright soil (Fig. 3). Lepidocrocite in Earth soils is believed to be formed from Fe(II) only (Schwertmann and Taylor 1989). For this reason, the lepidocrocite pathway to maghemite seems prima facie of little significance in Mars soils because the oxidizing conditions on the surface of the planet imply that chemical alteration processes involve Fe(III) rather than Fe(II). Recent laboratory experiments, however, have shown that ferrihydrite obtained by fast hydrolysis of Fe(III) salts partly crystallizes to lepidocrocite at 25◦ C at pH 7 provided P/Fe > 0.025 in the initial solution (G´alvez et al. 1999). We found that lepidocrocite in such a mixture evolved after heating to poorly crystalline maghemite and, eventually, to small hematite (Fig. 1C). The Js for the thermally treated mixture (Fig. 2) is
FIG. 1. X-ray diffractograms for the thermal transformation products of (A) lepidocrocite produced at P/Fe = 0.05, (B) magnetite synthesized by boiling a 0.05-M FeSO4 solution containg phosphate (P/Fe = 0.05), and (C) a mixture of ferrihydrite and lepidocrocite obtained by neutralizing a Fe(NO3 )3 –phosphate solution (P/Fe = 0.027) to pH 7 and storing the suspension at 25◦ C for 2 Yr. The initial products (lowest diffractogram in each box) were treated for 3 h at the temperatures stated on the diffractograms. The positions of the most characteristic ferrihydrite (F), lepidocrocite (L), magnetite (M), and maghemite (Mh) reflections are shown on representative diffractograms. Note the broad lines (indicating small particle size) for hematite and, particularly, maghemite.
FIG. 2. Saturation magnetization (Js ) of the thermal transformation products of lepidocrocite, magnetite, and lepidocrocite–ferrihydrite shown in Figs. 1A, 1B, and 1C, respectively, as a function of temperature. In those samples for which saturation was not reached, the magnetization value at 1.8 T (smaller than Js ) was plotted (open symbols). Measurements were carried out with a pendulum-type magnetometer (MANICS DSM-8) at room temperature.
NOTE
FIG. 3. Combination reflectance spectra for the initial and thermal transformation products of lepidocrocite, magnetite, and lepidocrocite–ferrihydrite shown in Figs. 1A, 1B, and 1C, respectively, compared with the reflectance spectrum for the Pathfinder Mars Bright II soil (Bell et al. 2000). The combination spectra were calculated using the Kubelka–Munk formalism (Barr´on and Torrent 1986) for 1 : 1 : 1 : 1 mixtures of the initial products and the products obtained by heating at 300, 500, and 700◦ C. smaller than that for the maghemite obtained from pure lepidocrocite since the ferrihydrite in the mixture is directly converted into hematite. Such a Js value, however, is still consistent with those of Martian soils if their Fe2 O3 content is considered. As with the previous pathways, the combined spectrum for the initial products and the products formed at 300, 500, and 700◦ C was similar to that for the Pathfinder bright soil (Fig. 3). It is worth noting that we successfully synthesized lepidocrocite from phosphated ferrihydrite using different combinations of pH, P/Fe, and temperature (25–100◦ C). In summary, maghemites and hematites seemingly comparable to those in martian soils can be produced from Fe salts through several pathways provided phosphate is present during the synthesis. The potential existence of various pathways leading to similar assemblage of Fe oxides is somewhat unfortunate as it makes it difficult to determine which of the proposed alteration mechanisms for Martian rocks (e.g., surface weathering, hydrothermal alteration, and acid fog weathering) (Newsom et al. 1999) predominates.
ACKNOWLEDGMENTS This work was partly supported by the Spanish Comisi´on Interministerial de Ciencia y Tecnolog´ıa (Projects OLI96–2183 and PB98–1015). We thank N. G´alvez and J. Cumplido for carrying out the synthesis of iron oxides and the Servei de Magnetoqu´ımica (Universitat de Barcelona, Spain) for the magnetic measurements.
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Barr´on, V., and J. Torrent 1986. Use of the Kubelka–Munk theory to study the influence of iron oxides on soil colour. J. Soil Sci. 37, 499–510. Bell, J. F., and 23 colleagues 2000. Mineralogic and compositional properties of martian soil and dust: Results from Mars Pathfinder. J. Geophys. Res. 105, 1721–1755. Bishop, J. L., H. Fr¨oschl, and R. C. Mancinelli 1998. Alteration processes in volcanic soils and identification of exobiologically important weathering products on Mars using remote sensing. J. Geophys. Res. 103, 31,457–31,476. Clark, B. C., A. K. Baird, R. J. Weldon, D. M. Tsusaki, L. Schnabel, and M. P. Candelaria 1982. Chemical composition of martian fines. J. Geophys. Res. 87, 10,059–10,067. Cornell, R. M., and U. Schwertmann 1996. The Iron Oxides. VCH, Weinheim, Germany. Dreibus, G., J. Brueckner, G. W. Lugmair, R. Rieder, and H. Waenke 1999. Possible indication of sedimentary products at the Mars Pathfinder landing site: Phosphorus in APXS X-ray spectra. Suppl. EOS Trans. AGU 80, 46, P32A–06. G´alvez, N., V. Barr´on, and J. Torrent 1999. Effect of phosphate on the crystallization of hematite, goethite, and lepidocrocite from ferrihydrite. Clays Clay Miner. 47, 304–311. Hargraves, R. B., D. W. Collinson, R. E. Arvidson, and C. R. Spitzer 1977. The Viking magnetic properties experiment: Primary mission results. J. Geophys. Res. 82, 4547–4558. Hviid, S. F., and 12 colleagues 1997. Magnetic properties experiments on the Mars Pathfinder Lander: Preliminary results. Science 278, 1768–1770. Madsen, M. B., S. F. Hviid, H. P. Gunnlaugsson, J. M. Knudsen, W. Goetz, C. T. Pedersen, A. R. Dinesen, C. T. Mogensen, M. Olsen, and R. B. Hargraves 1999. The magnetic properties experiments on Mars Pathfinder. J. Geophys. Res. 104, 8761–8779. Morris, R. V., and D. C. Golden 1998. Goldenrod pigments and the occurrence of hematite and possibly goethite in the Olympus–Amazonis region of Mars. Icarus 134, 1–10. Morris, R. V., and 11 colleagues 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. J. Geophys. Res. 105, 1757–1817. Morris, R. V., D. C. Golden, T. D. Shelfer, and J. H. V. Lauer 1998. Lepidocrocite to maghemite to hematite: A pathway to magnetic and hematitic martian soil. Meteorit. Planet. Sci. 33, 743–751. Newsom, H. E., J. J. Hagerty, and F. Goff 1999. Mixed hydrothermal fluids and the origin of the martian soil. J. Geophys. Res. 104, 8717–8728. Posey-Dowty, J., B. Moskowitz, D. Crerar, R. Hargraves, L. Tanenbaum, and E. Dowty 1986. Iron oxide and hydroxide precipitation from ferrous solutions and its relevance to martian surface mineralogy. Icarus 66, 105–116. Rieder, R., T. Economou, H. W¨anke, A. Turkevich, J. Crisp, J. Br¨uckner, G. Dreibus, and H. Y. McSween Jr. 1997. The chemical composition of martian soil and rocks returned by the mobile alpha proton X-ray spectrometer: Preliminary results from the X-ray mode. Science 278, 1771–1774. Schwertmann, U., and R. W. Fitzpatrick 1977. Occurrence of lepidocrocite and its association with goethite in Natal soils. Soil Sci. Soc. Am. J. 41, 1013– 1018. Schwertmann, U., and R. M. Taylor 1989. Iron oxides. In Minerals in Soil Environments (J. B. Dixon and S. B. Weed, Eds.), 2nd ed., pp. 379–438. Soil Sci. Soc. Am., Madison, WI.
REFERENCES
Tronc, E., and J. P. Jolivet 1986. Surface effects on magnetically coupled “γ -Fe2 O3 ” colloids. Hyperfine Interact. 28, 525–528.
Banin, A., T. Ben-Shlomo, L. Margulies, D. F. Blake, R. L. Mancinelli, and A. U. Gehring 1993. The nanophase iron mineral(s) in Mars soil. J. Geophys. Res. 98, 20,831–20,853.
W¨anke, H., and G. Dreibus 1988. Chemical composition and accretion history of terrestrial planets. Phil. Trans. R. Soc. London Ser. A 325, 545– 557.
NOTE Key Role of Phosphorus in the Formation of the Iron Oxides in Mars Soils? Jos´e Torrent and Vidal Barr´on Departamento de Ciencias y Recursos Agr´ıcolas y Forestales, Universidad de C´ordoba, Apdo. 3048, 14080 C´ordoba, Spain E-mail: [email protected] Received December 20, 1999; revised March 24, 2000
Abundance of phosphorus (P) in martian rocks might be crucial in the formation of iron oxides, hematite (α-Fe2 O3 ) and maghemite (γ -Fe2 O3 ), thought to occur in martian soils. This hypothesis is supported by laboratory experiments showing that the thermal transformation of P-doped lepidocrocites and magnetites results in hematite and maghemite with magnetic and spectral properties consistent with those of martian soils. °c 2000 Academic Press Key Words: Mars; surface; mineralogy; cosmochemistry.
The Viking (1976) and Pathfinder (1997) missions have provided indications that martian soil and dust particles are composites (mostly silicates) and magnetic, with a saturation magnetization (Js ) of 4 ± 2 A m2 kg−1 as the most likely value (Hviid et al. 1997). This excludes the possibility of the magnetism resulting solely from superparamagnetic (nanophase) hematite, which is weakly magnetic. The preferred interpretation of Morris et al. (2000) is that the strongly magnetic phase is lithogenic titanomagnetite (Fe3−x Tix O4 ), and/or their titanomaghemite oxidation products. Hviid et al. (1997) and Madsen et al. (1999) did consider this hypothesis, but even if the TiO2 content of Mars soils (∼1% in weight; Rieder et al. 1997) were assigned solely to titanomagnetite, the calculated Js values occur near or below the low end of the 4 ± 2 A m2 kg−1 range. Therefore, these authors favored the idea that maghemite, in intimate association with silicate particles in at least 6% weight, gives rise to the magnetic properties of the martian soil. Thermal conversion of maghemite, probably through meteoritic impact, might then partly explain the likely presence of (red) pigmentary hematite in Martian soils (Morris and Golden 1998, Morris et al. 1998). On Earth soils, maghemite is formed via (i) thermal transformation of goethite in the presence of organic matter, (ii) oxidation of magnetite (Fe3 O4 ), and (iii) dehydroxylation of lepidocrocite (Schwertmann and Taylor 1989). The abundance of maghemite in Martian soils can be explained neither by path (i) owing to the lack of organic matter nor by path (ii) because magnetite (and titanomagnetite) are minor components of Mars bedrocks. Hence, dehydroxylation of precursor lepidocrocite is the most plausible hypothesis (Hargraves et al. 1977, Posey-Dowty et al. 1986, Schwertmann and Taylor 1989, Banin et al. 1993, Morris et al. 1998). We conducted laboratory experiments aimed at understanding why lepidocrocite and maghemite seem to be formed in martian environments, in contrast with the predominance of the two other main crystalline Fe oxides, goethite and hematite, on the earth’s surface. Specifically, we tested the hypothesis that this contrast is due to the high P content in Martian bedrocks relative to most Earth bedrocks. The X-ray fluorescence analysis by the Viking landers (Clark et al. 1982) and the data collected by the alpha proton X-ray spectrometer (APXS) of the Pathfinder (Rieder et al. 1997) indicate that Martian rocks possess an andesitic composition with a mafic component. W¨anke and Dreibus (1988)
estimated the bulk chemistry of Mars from SNC meteorites data and emphasized that the enrichment of P in the Martian mantle is by about a factor of 10 relative to the Earth mantle. Recently, Dreibus et al. (1999), with a refined method of searching for small peaks in the APXS spectra, found P content in the Martian rocks and soils at Pathfinder landing site to be ∼3 g kg−1 . Indeed, values of 1–4 g P kg−1 have been reported for similar Earth rocks (Bishop et al. 1998); with P occurring mainly in the form of apatite and other metal phosphates. With a typical Fe content of 100 g kg−1 for Martian rocks (Rieder et al. 1997), the ∼3 g kg−1 figure would yield a P/Fe atomic ratio of ∼0.05. Ratios of this order were used in our experiments involving the synthesis of Fe oxides. Different Fe oxides are obtained by hydrolysis and oxidation of Fe(II) salts depending on the pH, the oxidation rate, and the concentrations of Fe(II) and foreign ions (Cornell and Schwertmann 1996). Recent laboratory experiments (Cumplido et al., Clays Clay Miner., submitted for publication) show that oxidation at room temperature of an Fe(II) salt in the presence of phosphate at a high HCO3 concentration yields a mixture of goethite and lepidocrocite at P/Fe < 0.03, but exclusively lepidocrocite above this ratio. So the antilepidocrocitic effect of HCO3 (Cornell and Schwertmann 1996) seems to be offset by the pro-lepidocrocitic effect of P. This supports the idea that lepidocrocite rather than goethite might result from the alteration of mafic, P-rich Martian rocks even though the Mars atmosphere is rich in CO2 . This is consistent with the observation that, on the earth’s surface, lepidocrocite is formed in hydromorphic, CO2 -rich soils but preferably away from roots (Schwertmann and Fitzpatrick 1977), i.e., in zones where P has not been depleted by root uptake. We obtained lepidocrocite crystals from Fe(II) salts at P/Fe > 0.05 in the presence of HCO3 . These crystals contained phosphate in an occluded (nonalkali-desorbable) form and were small, as suggested by the broad lines in the X-ray diffractograms (bottom X-ray diffractogram of Fig. 1A). On being heated, these nanophase lepidocrocites were transformed first into maghemite (at 200– 250◦ C) and then into hematite (at 600–700◦ C) (Fig. 1A). Thus, the maghemites formed from “P-lepidocrocites” were stable over a temperature range of about 400◦ C, in contrast with one of only about 200◦ C for the maghemites obtained from highly crystalline pure lepidocrocite (Morris et al. 1998). These results are consistent with the finding that maghemites containing adsorbed phosphate become hematites at higher temperature than do pure maghemites (Tronc and Jolivet 1986). As a consequence, the products obtained in our heating experiments had a Js of >20 A m2 kg−1 for heating temperatures up to above 600◦ C (Fig. 2). Combination of these Js values with the Fe2 O3 content of the soils examined by the Pathfinder (14–18%) gives new Js values that are consistent with the above-mentioned average value (4 ± 2 A m2 kg−1 ). Even at 700◦ C, Js > 12 A m2 kg−1 , probably because part of the resulting hematite (Fig. 1A) was superparamagnetic and contained residual maghemite. The hematite formed at 700◦ C was bright red (suggesting a small particle size), probably because the presence of phosphate hindered sintering, a process that results in the formation of purplish red, large hematite particles. By combining the reflectance spectra for the initial products and the products obtained at 300, 500, and 700◦ C, which
645 0019-1035/00 $35.00 c 2000 by Academic Press Copyright ° All rights of reproduction in any form reserved.
646
´ TORRENT AND BARRON mimic the range of thermal transformation products expected from meteoritic impact on Mars, we obtained a spectrum similar to that for the Pathfinder bright soil (Fig. 3). Oxidation of magnetite is a common pathway to maghemite. Fine-grained magnetite can be produced in the laboratory by subjecting Fe(II) salts to hydrothermal conditions (Cornell and Schwertmann 1996) and can be oxidized readily to maghemite. Because of the small Fe content in the fluids of Martian hydrothermal systems (Newsom et al. 1999), this pathway seems unlikely on Mars; so it is more likely that magnetite can form by igneous processes, as in glassy flow or impact melt. In any case, if this hydrothermal pathway were active, it would be strongly affected by phosphate. We found the presence of phosphate during the synthesis of magnetite from FeSO4 at about 100◦ C and in the presence of HCO3 to give rise to (i) the occlusion of P in this mineral and in the maghemite resulting from its thermal transformation, (ii) an increase in the temperature at which maghemite was transformed into hematite from 200– 300◦ C for a P-free system to about 500–600◦ C for systems with P/Fe = 0.05 (Fig. 1B), (iii) large Js values for the thermal transformation products obtained at temperatures up to above 600◦ C (Fig. 2), and (iv) the combined spectrum for the initial products and the products formed at 300, 500 and 700◦ C being similar to that for the Pathfinder bright soil (Fig. 3). Lepidocrocite in Earth soils is believed to be formed from Fe(II) only (Schwertmann and Taylor 1989). For this reason, the lepidocrocite pathway to maghemite seems prima facie of little significance in Mars soils because the oxidizing conditions on the surface of the planet imply that chemical alteration processes involve Fe(III) rather than Fe(II). Recent laboratory experiments, however, have shown that ferrihydrite obtained by fast hydrolysis of Fe(III) salts partly crystallizes to lepidocrocite at 25◦ C at pH 7 provided P/Fe > 0.025 in the initial solution (G´alvez et al. 1999). We found that lepidocrocite in such a mixture evolved after heating to poorly crystalline maghemite and, eventually, to small hematite (Fig. 1C). The Js for the thermally treated mixture (Fig. 2) is
FIG. 1. X-ray diffractograms for the thermal transformation products of (A) lepidocrocite produced at P/Fe = 0.05, (B) magnetite synthesized by boiling a 0.05-M FeSO4 solution containg phosphate (P/Fe = 0.05), and (C) a mixture of ferrihydrite and lepidocrocite obtained by neutralizing a Fe(NO3 )3 –phosphate solution (P/Fe = 0.027) to pH 7 and storing the suspension at 25◦ C for 2 Yr. The initial products (lowest diffractogram in each box) were treated for 3 h at the temperatures stated on the diffractograms. The positions of the most characteristic ferrihydrite (F), lepidocrocite (L), magnetite (M), and maghemite (Mh) reflections are shown on representative diffractograms. Note the broad lines (indicating small particle size) for hematite and, particularly, maghemite.
FIG. 2. Saturation magnetization (Js ) of the thermal transformation products of lepidocrocite, magnetite, and lepidocrocite–ferrihydrite shown in Figs. 1A, 1B, and 1C, respectively, as a function of temperature. In those samples for which saturation was not reached, the magnetization value at 1.8 T (smaller than Js ) was plotted (open symbols). Measurements were carried out with a pendulum-type magnetometer (MANICS DSM-8) at room temperature.
NOTE
FIG. 3. Combination reflectance spectra for the initial and thermal transformation products of lepidocrocite, magnetite, and lepidocrocite–ferrihydrite shown in Figs. 1A, 1B, and 1C, respectively, compared with the reflectance spectrum for the Pathfinder Mars Bright II soil (Bell et al. 2000). The combination spectra were calculated using the Kubelka–Munk formalism (Barr´on and Torrent 1986) for 1 : 1 : 1 : 1 mixtures of the initial products and the products obtained by heating at 300, 500, and 700◦ C. smaller than that for the maghemite obtained from pure lepidocrocite since the ferrihydrite in the mixture is directly converted into hematite. Such a Js value, however, is still consistent with those of Martian soils if their Fe2 O3 content is considered. As with the previous pathways, the combined spectrum for the initial products and the products formed at 300, 500, and 700◦ C was similar to that for the Pathfinder bright soil (Fig. 3). It is worth noting that we successfully synthesized lepidocrocite from phosphated ferrihydrite using different combinations of pH, P/Fe, and temperature (25–100◦ C). In summary, maghemites and hematites seemingly comparable to those in martian soils can be produced from Fe salts through several pathways provided phosphate is present during the synthesis. The potential existence of various pathways leading to similar assemblage of Fe oxides is somewhat unfortunate as it makes it difficult to determine which of the proposed alteration mechanisms for Martian rocks (e.g., surface weathering, hydrothermal alteration, and acid fog weathering) (Newsom et al. 1999) predominates.
ACKNOWLEDGMENTS This work was partly supported by the Spanish Comisi´on Interministerial de Ciencia y Tecnolog´ıa (Projects OLI96–2183 and PB98–1015). We thank N. G´alvez and J. Cumplido for carrying out the synthesis of iron oxides and the Servei de Magnetoqu´ımica (Universitat de Barcelona, Spain) for the magnetic measurements.
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