Back Mount Taranaki Infographics by Robin Martin, RNZ, Reporter
Iron oxide deposits occur at the vents of Kokowai Springs and in the streambed downstream on Mt Egmont, New Zealand. The Maori people collected ochre from this site and used it for functional and ceremonial purposes. Isotope ratios, 180/160, D/H, and 13C/12C, indicate that the springwater is largely meteoric in origin and that the dissolved carbon dioxide has a volcanic source. Iron (40 g m-3 as Fe (II)) is the dominant cation in the springwater, followed by magnesium (33 g m-3); the dominant anion, by far, is bicarbonate.
This composition is accounted for by the preferential attack of the carbonated water on ferromagnesian minerals in the andesitic rock. The ferrous iron oxidises and precipitates rapidly on contact with air to form the ochre deposits.
The major components of the ochre are Fe (45-55%), Si (5-8%), H2O(+) (10-14%) (110½C-dry basis), and H2O(-) (16-23%) (air-dry basis); surface areas lie in the range 200-400 m2 g-1 and dellsities in the range 3.3-3.7 g cm-3. Munsell colours of air-dry material are 7.5YR 5/8 (strong brown) through 7.5YR 6/8 (reddish yellow) to 10YR 6/6 (brownish yellow). X-ray diffractograms,
Moessbauer spectra, electron micrographs, infrared and differential thermal analysis patterns indicate that the ochre is essentially siliceous ferrihydrite. On heating, the deposit material reddens in colour and by 750½C, crystalline hematite has formed and the colour 2.5 YR 4/8 (red) is obtained.
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