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Spectroscopy: A Tribute to Roger G. Bums © The Geochemical Society, Special Publication No.5, 1996 Editors: M. D. Dyar, C. McCammon and M. W. Schaefer

Why hematite is : Correlation of optical absorption intensities 3 and magnetic moments of Fe +

GEORGE R. ROSSMAN Division of Geological and Planetary Sciences. California Institute of Technology, Pasadena, California 91125, U.S.A.

3 Abstract-Structures with Fe + shared through oxo- or hydroxyl-groups have antiferromagnetic interactions. Such interactions result in enhanced intensity of the Fe3+ optical absorption bands which in some systems can be as great as a factor of 100 compared to isolated, octahedrally 3 coordinated Fe + . A comparison is presented between the intensity of the lowest energy crystal- 3 band of Fe + minerals and their magnetic moments which demonstrates the dependence of optical absorption intensity and antiferromagnetic interactions in the host phase. Hematite, which is usually responsible for the red color of geological materials, owes its intense color to these magnetic interactions.

INTRODUCTION calculations on clusters with Fe3+-0 and Fe3+ -OH OXIDIZED (Fe3+) is associated with the red units. They found that the Fe3+ -OH bond was more color which is commonly observed in many , ionic and had a smaller spin-polarization than the sedimentary rocks and products (BLOD. Fe3+-0 bond. This gave rise to much weaker mag- GETTet al., 1993). In these materials, Fe3+, primar- netic exchange (superexchange) between hydroxyl- 3 ily in the form of finely disseminated hematite bridged Fe + cations compared to oxo-bridged Fe3+ (Fe203), is an intense . The prevalence of cations. In addition to the effects discussed by hematite as a red pigment in geological materials Sherman on the internal magnetic hyperfine fields to the common association of red color with observed in the Mossbauer spectra, superexchange oxidized iron in general. interactions produce the major intensifications of 3 On the other hand, a variety of other Fe3+ the Fe + ligand field transitions in the optical spec- minerals and chemical compounds are pale col- tra (ROSSMAN, 1975, 1976a,b). ored. Many examples exist of pale colored Fe3+ This paper presents empirical correlations be- minerals including light greenish-yellow sili- tween the intensities of optical absorption bands cates such as andradite , and lavender and the magnetic susceptibility of Fe3+ minerals. phosphates and sulfates such as strengite and These observations provide experimental support coquimbite. A problem has been to reconcile the for the concepts developed by SHERMAN (1985). intense color of hematite and related hydrous As part of these correlations, the special case of iron with the pale color of many other the color of hematite and other hydrous iron oxides 3 Fe + minerals and compounds. is considered. ROSSMAN (1975, 1976a,b) noted that the in- tensity of color per unit of Fe3+ ions in iron EXPERIMENTAL

sulfates increases dramatically when the iron Magnetic and optical data were taken from the litera- ions are joined through shared and hy- ture or were determined by the methods described in droxide ions (shared edges or vertices of coordi- ROSSMAN (1975). Both new and reviewed data are pre- sented in Table 1. nation polyhedra). He observed that such sys- tems are antiferromagnetic and suggested that, in these systems, the intensity of color was re- RESULTS AND DISCUSSION lated to the extent of magnetic interaction. Fur- The optical absorption spectrum of Fe3+ in an thermore, ferric sulfate systems joined through octahedral site isolated from other Fe3+' octahedra shared oxide ions often show much greater mag- consists of two broad bands at lower energies (la- netic interaction and stronger color than those beled 4T1g and 4T2g in order of increasing energy) joined through hydroxide ions. and a pair of bands, often overlapping near 440 SHERMAN (1985) and SHERMAN and WAITE nm, labeled (4A1g,4Eg). The light not absorbed by (1985) discussed the results of molecular orbital these bands determines the color of Fe3+ minerals.

23 24 G. R. Rossman

Table 1. Spectral and magnetic data

Sample Formula ",(TIs) 10 ",(A Ig,4Eg) 10 µ Regular Octahedra 1 andradite Ca3FeiSi04)3 854 0.08 440 1.5 Distorted Octahedra 2.0 5.87 2 coquimbite FeiS04h9H2O 778 0.18 427 0.34 423 6.5 3 phosposiderite FeP04·2H2O 746 Dimer 36 4.70 4 magnesiocopiapite MgFe4(S04)6{OH)2'20H20 864 1.7 430 Chains 33 3.8 5 butlerite Fe(S04)(OH)'2H2O 920 2.4 424 426 28 3.3 6 parabutlerite Fe(S04)(OH)'2H2O 912 2.5 428 55 7 stewartite MnFeiP04h(OH)2·8H20 880 2.3 423 11 3.72 8 fibroferrite Fe(S04)(OH)'5H2O 840 1.9 432 96 3.97 9 botryogen MgFe(S04h(OH)'7H2O 939 3.4 Clusters 3.42 10 metavo1tine K2NlI{\FeFe6(S04)1202'18H20 855 5.5 464 66 90 2.53 11 amarantite Fe(S04)(OH)'3H2O 866 12.7 442 8.4 5.13 12 1eucophosphite KFe2(P04h(OH)'2H2O 800 0.78 441 446 19 4.71 13 pharmocosiderite KFeiAs04)J(OH)4'7H20 820 0.5 Extended Structures 2.06 14 hematite Fe203 855 15.3 3.23 15 FeO(OH) 915 7.0 - 3.01 16 1epidocrocite FeO(OH) 918 13.5 1.95 17 bernalite Fe(OH)3 885 0.12 431

Wavelengths of absorption bands ("') in nm; molar absorption coefficients (c) in l/mol- ern"; effective magnetic moments (µ) in Bohr magnetons at approximately 295°C. Sources of data: 1 (MAo and BELL, 1974); 2-9 (ROSSMAN,1975, 1976a,b); 10 (previously unpublished); 11,12 (ROSSMAN,1976b); 13 (previously unpublished); 14 (BAILEY,1960; HOFERet al., 1946); 15,16 (MAo et al. 1974, HOFERet al., 1946); 17 (MCCAMMONet al., 1995)

Andradite garnet, Ca3Fe2(Si04)3, is a mineral con- (0001) plane) is compared to the spectrum of an- 3 taining such isolated octahedral Fe + sites. The dradite. The spectra are normalized for and general features of the spectrum of andradite (Fig. iron concentration such that they are presented for 3 1) are typical for Fe3+ in octahedral coordination in equal amounts of Fe + in the sample path. From a variety of silicate, sulfate and phosphate minerals. this comparison, it can be seen that hematite has a The two broad bands at longer wavelengths (4TIg much greater absorption intensity than andradite. and 4T2g) have low intensity and do not produce The intense, deep red color of hematite is deter- strong colors in a thickness of a few millimeters mined by the narrow transmission window near and the more intense, sharp band near 440 nm is 750 nm. so far in the that it contributes little to the The absolute intensity of the bands is measured color of the mineral. in terms of the molar absorption coefficient, E, de- In hematite, all Fe3+ sites are adjacent to other fined by absorbance = E (llmol X cm ") X path Fe3+ sites. In Fig. 1 the hematite spectrum (in the (em) X concentration (moles/liter). The E value for Why hematite is red 25

1.2 Thus it is necessary to consider the correlation Hematite between intensity of absorption and the strength 1.0 of the magnetic interaction. For many minerals,

the intensity of the (4AIg,4Eg) band is often dif- ~ 0.8 ficult to measure due to overlap with the tail of c ttl intense absorption bands in the ultraviolet. In -e 0.6 nearly all cases, the 4TIg band near 800 nm is o C/) unaffected by this tail. Its intensity is therefore .0 « 0.4 more amenable to quantification. The strength of the magnetic interaction can be measured 0.2 through the , derived from Andradite measurements of the mineral's bulk magnetic susceptibility. The effective magnetic moment 400 600 800 1000 1200 of Fe3+ ions isolated from anti-ferromagnetic interactions is about 5.9 Bohr magnetons. Wavelength, nm Strong antiferromagnetic interactions decrease this value to about 2.0 in the case of pairs of 3 FIG. 1. Comparison of the optical absorption spectra Fe + bridged by a nearly linear oxo-bridge of hematite and andradite garnet. The spectra have been et al., Ig normalized for density and Fe-concentration so that they (SCHUGAR 1972). The intensity of the 4T are presented for identical amounts of Fe in the sample band increases markedly as the magnetic mo- 3 path. ment per Fe + decreases due to magnetic ex- change interactions (Fig. 3). Although the con- cept is well established that absorption band intensification is related to magnetic interac- hematite is 15.3 in the 850 nm region compared to tions, Fig. 3 provides a quantitative demonstra- 0.08 for andradite. This increase of intensity of tion of this relationship and illustrates that there absorption by over two orders of magnitude cou- is some variation from a smooth trend, undoubt- pled with increased intensity of the higher energy edly caused by variations in the structural de- bands is responsible for the intense color of tails of the interacting Fe3+ sites. hematite. A visible manifestation of the importance of A general observation which follows from these magnetic interactions is the intensity of color these studies is that intense color and high ab- 3 of the host phases. If Fe + in the host mineral is sorption band intensity is associated with miner- free of magnetic interactions the mineral is usually als which have chains, clusters or extended net- pale yellow-green (typically silicates) or pale lav- 3 works of Fe + cations. This observation is ender (phosphates and sulfates). The and reinforced by Fig. 2 which illustrates that the browns usually associated with Fe3+are observed intensity of the 4TIg band is generally much greater for various clusterings of Fe3+ cations than for Fe3+in isolated, symmetrical or moder- ately distorted octahedra. Similar behavior is 16 14 observed for the often sharp (4AIg,4Eg) band near 440 nm (Table 1). In the case of anisotropic 12 absorption, the data are presented for the polar- 'bb 10 ization direction with the greatest intensity. i 8 Comparable intensification is not observed for to 6 the 4T2g band in many of these systems, so this band is not further considered. The apparent ex- ceptions to this generalization which involve clusters and extended structures based on hy- Isolated Distorted Dimer Chain Cluster Extended droxyl units as the shared are rationalized FIG.2. Bar graph illustrating the range of molar absorp- on the basis of the magnetic interactions in such tion coefficients for the -r., band in the 750 - 950 nm units. range in a variety of minerals with FeH in different distor- Magnetic interactions usually accompany tions and types of polyhedral linkages. In all cases, FeH absorption intensity is much greater when the Fe3+ ions 3 clustering or polymerization of the Fe + cations. are magnetically coupled through shared polyhedra. 26 G. R. Rossman

6 t Excited State t. SZ t t • •• •t --- t2g '8; 5 -r• • ~ • •• t j_ • • e i '. • • • Ground State - - g ~ 3- • t t t 0 • • • • t,g 3 ~ • Fe3 .. Fe \ o 2 - :OJ Q) C FIG. 4. Orbital energy diagram for a pair of adjacent g> 1 - magnetically coupled Fe3+ ions before (bottom) and after ~ (top) an electronic transition illustrating the coupled elec- tronic promotion and spin orientation changes which re- +---._.----.----,-,...---r-r---1 o sults in no net electronic spin change for the coupled o 2 4 6 8 10 12 14 16 system.

Intensity, I> (4T19)

FIG. 3. Comparison of the effective room temperature In a magnetically coupled system, the electrons 3 magnetic moment per Fe + and the intensity of the first in adjacent Fe3+ ions interact to circumvent the ligand field band (4T ). Absorption intensity is greatest 1g spin selection rules by acting collectively as a sys- for minerals with strong antiferromagnetic coupling be- 3 tween adjacent Fe + cations. tem. Fig. 4 illustrates the ground state of a pair of Fe3+ ions with strong magnetic interaction. When an electronic excitation (promotion of an electron to an orbital of higher energy) occurs on Fe~+, the only in minerals with strong magnetic interactions promoted electron must change its spin state when due to extensive polyhedral sharing. it pairs with the higher energy electron. If simulta- The importance of O-linkages between Fe3+ rather neously an electron on the adjacent Fe~+ changes than OH-linkages in providing the pathway for ab- its spin state without a corresponding promotion to sorption band enhancement has previously been a higher energy orbital, the net spin state of the noted (ROSSMAN, 1976b) in the tetrameric clusters of system remains unchanged. Thus, the simultaneous amarantite (deep red, oxo-bridged cluster) and leuco- electronic and magnetic state transition of the phosphite (pale , hydroxyl-bridged cluster). paired system does not represent a spin-forbidden The importance of oxo-linkages is further demon- process. The transition is spin-allowed; the inten- strated by the contrast between hematite, Fe203, sity of light absorption will be much higher, as which has oxo-linkages and a deep red color with will be the corresponding intensity of color. This 3 EB55 nm = 15.3, and bernalite, Fe(OH)3, which has concept, developed for a pair of interacting Fe + hydroxyl-linkages (BIRCH et al., 1993), a green color, ions, is also applicable to more extended inter- 3 and 1'885 nm = 0.12 calculated from the data presented acting clusters and networks of Fe + ions such as in MCCAMMON et al. (1995). hematite. The relationship between magnetic interactions and enhanced intensity of light absorption can be Acknowledgments-Support for optical spectroscopy of simply explained using crystal field theory by con- minerals has been provided by the White Rose Foundation sidering the electronic states of isolated Fe3+ ions and the National Science Foundation (USA). Contribution and magnetically coupled ions. In isolated octahe- number 5624 of the Division of Geological and Planetary drally coordinated Fe3+, all electronic transitions Sciences. require a change in the electronic spin state of the ion. As such, to first order, they are forbidden by REFERENCES the quantum mechanical laws which govern such BAILEY P. C. (1960) Absorption and reflectivity measure- transitions and will-happen, in practice, with low ments on some rare earth iron and a-Fe203' J. probability. Consequently, the intensity of light ab- Appl. Phys. 31, 39s-40s. sorption will be low as will be the corresponding BIRCH W. D., PRING A., RELLER A. and SCHMALLE intensity of color. H. W. (1993) Berna1ite, Fe(OH)3, a new mineral from Why hematite is red 27

Broken Hill, New South Wales-description and struc- ies of ferric iron hydroxy sulfates: intensification of ture. Amer. Mineral. 78, 827-834. color in ferric iron clusters bridged by a single hydrox- BLODGETT R. H., CRABAUGH J. P. and McBRIDE E. F. ide ion. Amer. Mineral. 60, 698-704. (1993) The color of red beds; a geologic perspective. ROSSMANG. R. (1976a) Spectroscopic and magnetic stud- In: Color, Soil Sci. Soc. Amer., Madison, WI, ies of ferric iron hydroxy sulfates: the series USA, Spec. Pub. 31, pp. 127-159. Fe(OH)SO. X nH20 and the jarosite. Amer. Mineral. HOFER L. J. E., PEEBLES W. C. and DIETER W. E. (1946) 61, 398-404. X-ray diffraction and magnetic studies of unreduced ROSSMAN G. R. (1976b) The optical spectroscopic ferric oxide Fischer- Tropsch catalysts. 1. Amer. Chem. comparison of ferric iron tetrameric clusters in Soc. 68, 1953-1956. amarantite and leucophosphite. Amer. Mineral. 61, MAO H. K. and BELL P. M. (1974) Crystal-field effects 933-938. of ferric iron in goethite and lepidocrocite: band assign- SCHUGARH. J, ROSSMANG. R., BARRACLOUGHC. G. and ment and geochemical applications at high pressure. GRAY H. B. (1972) Electronic structure of oxo-bridged Carnegie Inst. Wash. Year Book 73, 502-506. iron(III) dimers. 1. Amer. Chem. Soc. 94, 2683-2690. MCCAMMON C. A., PRING A., KEPPLER H. and SHARP SHERMAND. M. (1985) The electronic structures of Fe3+ coordination sites in iron oxides; Applications to spec- T. (1995) A study of bernalite, Fe(OH)3, using Moss- tra, bonding and magnetism. Phys. Chem. Mineral. 12, bauer spectroscopy, optical spectroscopy and trans- 161-175. mission electron microscopy. Phys. Chem. Mineral. SHERMAND. M. and WAITE T. D. (1985) Electronic spec- 22, 11-20. 3 tra of Fe + oxides and oxide hydroxides in the near IR ROSSMANG. R. (1975) Spectroscopic and magnetic stud- to near UV. Amer. Mineral. 70, 1262-1269.