The electronic structures of Fe3+ coordination sites in iron oxides: Applications to spectra, bonding, and magnetism

Sherman, David M.

doi:10.1007/bf00308210

Cited by 232 publications

(163 citation statements)

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“…The experimental results show that temperature variations in the reflectivity spectra of natural and synthetic samples containing red hematite are consistent with reductions in the width of the five ferric bands (four bands resulting from single-electron transitions near 430, 630, and 860 nm [Morris et al, 1985;Sherman, 1985] and one resulting from a pair transition near 500 nm [Sherman and Waite, 1985]) with decreasing temperature without detectable changes in their positions (+6 nm for the 860-nm minimum). spectral properties behave the same way as those for wellcrystalline and chemically pure red hematites produced in controlled industrial processes.…”

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confidence: 68%

Low‐temperature reflectivity spectra of red hematite and the color of Mars

Morris¹,

Golden²,

Bell³

1997

J. Geophys. Res.

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Abstract. Reflectivity spectra (visible and near IR) were measured near 141, 210, and 300 K for four red and well-crystalline powders of hematite (red hematite) used as commercial pigments, two samples of volcanic tephra from Mauna Kea volcano that contain red hematite as their dominant pigment, three samples of palagonitic tephra from the same location that contain nanophase ferric oxide as their dominant pigment, and two mixtures of the two types of pigmenting phases. Relative proportions of red hematite and nanophase ferric oxide were determined by M6ssbauer spectroscopy. For samples containing red hematite as the dominant pigment, the positions of the ferric electronic transitions near 430, 500, 630, and 860 nm are essentially independent of temperature, but their widths decrease with decreasing temperature.This decrease results in a well-defined minimum for the band at 630 nm at low temperatures and in significant increases in reflectivity in spectral regions near 1050 and 600 nm. For example, the reflectivity ratios g6oo/g53 o and g6oo/g86o both increase by a factor as large as -1.4 between 300 and 140 K. The spectral features from nanophase ferric oxide in samples of palagonitic tephra are nearly independent of temperature. Spectral data of Martian bright regions that are characterized by a shallow band minimum near 860 nm, a reflectivity maximum near 740 nm, a distinct bend near 600 nm, and a shallow absorption edge from-400 to 740 nm are attributed to the presence of nanophase ferric oxide plus subordinate amounts of red hematite. The 600-, 740-, and 860-nm features are associated with red hematite. Because the reflectivity of red hematite at 600 nm is strongly dependent on temperature and because this wavelength is in the red part of the visible spectrum, the color of the Martian surface may vary as a function of its temperature. A conservative upper limit for the red hematite content of the optical surface of Mars is 5%.

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confidence: 68%

Low‐temperature reflectivity spectra of red hematite and the color of Mars

Morris¹,

Golden²,

Bell³

1997

J. Geophys. Res.

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“…The L-glasses are slightly brighter than the JSC Mars-1 only at wavelengths shorter than 0.5 µm, most likely due to the reduction of Fe 3+ to Fe 2+ in the glasses. The JCS Mars-1 samples are very dark in this spectral region due to very strong overlapping absorption bands caused by O 2-→Fe 3+ charge transfer transitions and Fe 3+ crystal field transitions (e.g., Morris et al, 1985;Sherman, 1982Sherman, , 1985.…”

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confidence: 99%

Spectral properties of simulated impact glasses produced from martian soil analogue JSC Mars-1

Moroz

Basilevsky

Hiroi

et al. 2009

Icarus

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To simulate the formation of impact glasses on Mars, an analogue of Martian bright soil (altered volcanic soil JSC Mars-1) was melted at relevant oxygen fugacities using a pulsed laser and a resistance furnace. Reduction of Fe 3+ to Fe 2+ and in some cases formation of nanophase Fe spatial resolution and wavelength coverage may detect impact glasses at certain locations, e.g., in the vicinity of fresh impact craters. Such dark materials are usually interpreted as accumulations of mafic volcanic sand, but the possibility of an impact melt origin of such materials also should be considered. In addition, our data suggest that high contents of feldspars or zeolites are not necessary to produce the transparency feature at 12.1 µm typical of Martian dust spectra.

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“…Experimental transmission spectra of hematite single crystals and thin films (Marusak et aL, 1980), ligand field calculations (Orgel, 1952;Lever, 1984), and molecular orbital calculations (Tossell and Vaughan, 1974;Sherman, 1985;Sherman and Waite, 1985) revealed, however, at least six 3d transitions between 250-950 nm, in addition to charge transfer bands in the UV (Bums, 1993). In contrast to transmission spectra of oriented crystals, DRS bands are broader due to random orientation of crystallographic axes and scattering effects of the powder samples, leading to strongly overlapping and merging bands.…”

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confidence: 99%

Diffuse Reflectance Spectra of Al Substituted Goethite: A Ligand Field Approach

Scheinost

Schulze

Schwertmann

1999

Clays and clay miner.

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Abstract--Previous investigations of goethite revealed a substantial variation of color and diffuse reflectance spectxa (DRS) in the extended visible range (350-2200 nm). To better understand the causes of this variability and to assess the potential of DRS as a mineralogical tool, we investigated the DRS of pure and Al-substituted goethite, c~-Fej_~AI~OOH with x from 0 to 0.33, and mean crystal lengths (MCL) from 170 to 1800 nm. The strongly overlapping ligand field bands were extracted by fitting the single-electron transitions 6A l --~ 4Tt, 6A 1 ----) 4T2, 6A 1 ~ (4E; hA1), and 6A I --~ 4E(~D) as functions of the ligand field splitting energy, 10 Dq, and the interelectronic repulsion parameters, Racah-B and -C. With x increasing from 0 to 0.33, 6A 1 ---)' aT / decreased from 10,590 to 10,150 cm -1 (944 to 958 nm), and 6A l ~ 4T 2 decreased from 15,310 to 14,880 cm 1 (653 to 672 nm), while 10 Dq increased from 15,770 to 16,220 cm -1. From the change of 10 Dq we calculated a decrease of the Fe-(O,OH) distances from 202.0 to 200.9 pm (-0.5%). This decrease is smaller than the average decrease of all (A1,Fe)-(O,OH) distances (-1.8%) calculated from the change of the unit-cell lengths (UCL). That is, there remains a substantial difference in size between the larger Fe-and the smaller Al-occupied octahedra in the solid solution which may indicate the existence of diaspore clusters within the goethite structure. The increasing strain in the crystal structure due to the size mismatch and limited contractibility of the oxygen cage around Fe may be the primary reason for AI substitution being restricted to x < 0.33. The bands 6A l ~ (4E; 4Aa) and 6A 1 ~ 4E(4D) did not shift, indicating a constant covalency of the Fe-(O,OH) bonds with B = 628 cm ~ and C = 5.5B. Whereas variation of band energies could be explained in terms of the Fe-(O,OH) ligand field, the variation of color and band intensities was mainly determined by crystal size. Although our study confirmed the potential of DRS for mineralogical investigations, there is still a gap between the fundamental theory and the explanation of some spectral features.

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The electronic structures of Fe3+ coordination sites in iron oxides: Applications to spectra, bonding, and magnetism

Cited by 232 publications

References 28 publications

Low‐temperature reflectivity spectra of red hematite and the color of Mars

Low‐temperature reflectivity spectra of red hematite and the color of Mars

Spectral properties of simulated impact glasses produced from martian soil analogue JSC Mars-1

Diffuse Reflectance Spectra of Al Substituted Goethite: A Ligand Field Approach

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