Hematite colour

Plate 16.111 Highway on a hematite-coloured soil betwen Concepcion and San Ignacio de Velasco, Bolivia (Courtesy Dr. P. Schad). 

I 6 Electronic, electrical and magnetic properties and colour 6.3.4.6 Hematite... 

Fig. 6.10 Left Placement of various synthetic goethites (G), lepidocrocites (L) and hematites (H) in CIE L a b colour space. Right Development of a and b in the CIE L a b colour space during the transformation of ferrihydrite (common starting point) to goethite or hematite, respectively (Nagano et al., 1994, with permission).

Iron oxides are responsible for the vivid colours of many rocks and sods. The typical yello v-red to purple red colours of the so-called red beds are due to hematite (Torrent Schwertmann, 1987). The strong influence of hematite on soil colour is referred to in various languages and appears in the terms red earths, terra rossa and krasno-zems (see Chap. 15 16). 

Fig. 6.11 Munsell colours of synthetic hematite, ferrihydrite, lepidocrocite, and goethite samples (Courtesy A.C.Scheinost Scheffer Schachtschabel,...

Red hematite can be converted to purple hematite by increasing the size sufficiently, for example, by heating to >800 °C (von Steinwehr, 1969). There is some evidence that associations of small, platy, hematite crystals into orientated aggregates (e. g. in red beds) also cause a colour shift towards purple (Torrent Schwertmann, 1987). 

The band positions of Fe oxides are also influenced by the substitution for Fe by other cations in the structure, as indicated partly by their colour. Scheinost et al. (1999) noticed a linear shift in the position of the Ai " Ti transition from 943 to 985 nm and that of the Ai " T2 transition from 653 to 671 nm for 47 synthetic goethites whose Al-substitution (Al/(Al-i-Fe) ranged between 0 and 0.33 mol mol (R = 0.92 for both). Mn "-substituted goethites showed bands arising from Mn " near 454 and 596 nm. The overall reflectivity in the visible range decreased as structural Mn increased from 0 to 0.20 mol mol (Vempati et al., 1995). The same effect has been observed for V "-substituted goethites (Schwertmann Pfab, 1994). The position of the EPT band of Mn "-substituted hematite shifted to 545 nm and that of the Ai " T2 transition to 700 nm (Vempati et al., 1995). The position of the same transition shifted from ca. 600 to 592 nm as the Al-substitution in hematite rose from 0 to 0.125 mol mol (Kosmas et al., 1986). Crystal size and crystal shape also have an effect on diffuse reflectance, as shown for hematite (see Fig. 6.12). As the crystals become smaller, reflectance increases and needles also reflect more than cubes, i. e. the colour becomes more vivid. 

In terms of the Munsell hue (see Chap 6) the colour of red beds varies usually between 5YR-2.5YR (reddish-brown to red), but may also extend into 10RP-7.5RP (red-purple). A more detailed colour measurement using the CIE D a b system places the red beds within a space encircled by a range of synthetic hematites of different crystal sizes, as seen in Figure 15.2. This makes it likely that the colour of red beds is determined by hematite. 

Fig. 15.2 Position of 16 red beds in the CIE L a b colour space as compared with 8 synthetic hematites of different colour between yellow-red and purple in a 3% hematite - 97% kaolinite mixture (Torrent Schwertmann, 1987, with permission).

Fig. 16.3 Munsell colour notation and goethite/(goethite + hematite) ratio [Gt/(Gt-FHm)] in three soil toposequences, two from Brazil (upper, middle) and one from Malawi (lower) (Data from Curi, Franzmeier, 1984 a Santana, 1984 Karim Adams, 1984 with permission).

This reflects the fact that the colour is one of the easiest ways of distinguishing soils. Even in 1937 Alexander et al. noticed that very red soils owe their colour to the presence of hematite . An objective notation of soil colour is needed to describe... 

Oxide films can be stripped off iron by using bromine in methanol followed by heating at 300 °C in N2 to remove FeBr2 (Mayne and Ridgeway, 1971). The thickness of such films can be measured by weighing, by cathodic reduction and from the interference colours of the films the latter technique can also be applied to measurement of film thickness in situ. The first order interference colours of hematite films on iron are yellow/brown, mauve, blue and silver grey and the second order colours are pinky-blue, blue and greenish-blue (Evans, 1963). 

Another early use of iron oxides was as a cosmetic. The cosmetic boxes (cockleshells) found in the Royal Cemetery in the ancient Sumerian city of Ur contained a range of different colours. XRD analysis by the Research Department of the British Museum showed that the principal components of the red and yellow colours were hematite and goethite, respectively (Bimson, 1980). One box also contained a purple powder consisting of a mixture of quartz grains and large crystals of hematite. 

Dehydroxylation of goethite produces the ferrite reds - extremely colour fast and pure hematite. With low temperature calcination the acicular shape of the goethite precursor is retained, whereas high temperatures lead to a sintered product. 

Barron, V. Torrent, J. (1984) Influence of aluminum substitution on the color of synthetic hematites. Clays Clay Min. 32 157-158 Barron, V. Torrent, J. (1986) Use of the Ku-belka-Munk theory to study the influence of iron oxides on soil colour. Soil Sci. 37 499-510... 

Torrent, J. Schwertmann, U. Fechter, H. A1-ferez, F. (1983) Quantitative relationships between soil colour and hematite content. Soil Sci. 136 354-358... 

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