Hematite purple

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). 

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). 

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).

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. 

The scissile, purple Hematite, found in many places, notably in the mines of Hassia. 

Purple hematite. Hematite crystals several j,m in size have a purple color. Such crystals can be produced by holding ferrihydrite under very strongly alkaline condition (5 M NaOH) at 70 °C for 8 days. The crystals are idiomorpliic. Some goethite is usually associated with the hematite. 

More recent applications of the term (Bikiaris et ai, 1999 Daniila et al., 2002 Oliveira et al., 2002) appear to refer to naturally occurring and heat treated hematite-rich pigments producing a violet colour used in Roman, Byzantine and post-Byzantine art. Here, and perhaps due to the Latin name, caput mortuum is directly related to the purple pigments documented by the Roman authors Pliny (77 AD) and Vitruvius (first century BC), although the name was never used by either and in fact is unknown from other classical sources. However, it has been used synonymously with Pliny s and Vitruvius usta and ostrum although the link between these materials is, al best, tenuous. The pigments of these names described by Vitruvius refer specifically to T)rian purple and Pliny s usta refers to burnt cerussa (red lead qq.v.). 

In a recent paper, Oliveira et al. (2002) have Raman spectroscopy to characterise various modem purple iron oxide pigments called caput mortuum, supplied by Kremer. They note that variation in colour in these samples is related to particle size the darker hues have the largest particle size. However, thq consider real caput mortuum to be that used in Roman contexts, in this case (Rushton Roman villa, UK) the pigment is identified as hematite mixed with calcium carbonate and kaohnite qq.v.), and again is unrelated to wet chemically precipitated compounds. 

Iron oxides and hydroxides group Hematite Iron(lll) oxide, hematite type Kadinite Burnt vitriol Cerussa Colcothar Copperas Red lead Spanish brown Tyrian purple Venetian red... 

Iron oxides and hydroxides gronp Goethite Hematite Iron(III) oxide, hematite type Caput mortuum Mineral purple Purple ochre Field (1835) 137-138 Helwig (1998) Salter(1869) 299 Vibert (1892) Weber (1923) 89... 

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