Mineral 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 red: Correlation of optical absorption intensities 3 and magnetic moments of Fe + minerals 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 + ions. A comparison is presented between the intensity of the lowest energy crystal- 3 field 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 IRON (Fe3+) is associated with the red units. They found that the Fe3+ -OH bond was more color which is commonly observed in many soils, ionic and had a smaller spin-polarization than the sedimentary rocks and weathering 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 pigment. 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 leads 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 garnet, 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 oxides 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 oxide 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 goethite 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 density 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 violet 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 magnetic moment, 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 reds 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.