Magnetite solution

Table 1 Shift of the blocking temperature with increasing applied magnetic field to lower temperatures. The size of the infiltrated particles is 8 nm. The concentration of the magnetite solution was fixed at 8 mg Fe/ml...

Duncan and Frankenthal report on the effect of pH on the corrosion rate of gold in sulphate solutions in terms of the polarization curves. It was found that the rate of anodic dissolution is independent of pH in such solutions and that the rate controlling mechanism for anodic film formation and oxygen evolution are the same. For the open circuit behaviour of ferric oxide films on a gold substrate in sodium chloride solutions containing low iron concentration it is found that the film oxide is readily transformed to a lower oxidation state with a Fe /Fe ratio corresponding to that of magnetite . 

Probably, iron of biogenic magnetite was originated from hydrothermal solution. It is considered that ferric iron of hydrothermal solution was oxidized by iron oxidizing bacteria to form magnetite. 

Wood, S.A., Crerar, D.A. and Borcsik, M.P. (1987) Solubility of the assemblage pyrite-pyrrhotite-magnetite-sphalerite-galena-gold-stibnite-bismuthinite-argentite-molybdenite in H20-NaCl-C02 solutions from 200°C to 350°C. Econ. Geol, 82, 1864-1887. 

The H2S concentration of hydrothermal solution is plotted in Fig. 2.33. Based on these data, we can estimate the temperature of hydrothermal solution buffered by alteration mineral assemblages such as anhydrite-pyrite-calcite-magnetite and pyrite-pyrrhotite-magnetite for Okinawa fluids. 

For example, assuming anhydrite-magnetite-calcite-pyrite-pyrrhotite buffers redox in sub-seafloor reaction zones and a pressure of 500 bars, dissolved H2Saq concentrations of 21 °N EPR fluid indicate a temperature of 370-385°C. However, the estimated temperatures are higher than those of the measurement. This difference could be explained by adiabatic ascension and probably conductive heat loss during ascension of hydrothermal solution from deeper parts where chemical compositions of hydrothermal solutions are buffered by these assemblages. 

It is well known that anhydrite, pyrite, magnetite, and epidote are widespread in the country rocks in the Kuroko mine area. Therefore, it is likely that this assemblage controls the chemistry of the hydrothermal solution. 

Fig. 2.44. logaoi-pH range.s for Kuroko ore fluid.s and midoceanic ridge hydrothermal. solution. I Kuroko 2 Axial Explorer 3 2I°N, Southern Juan de Fuca 4 2I°N, Endeavour 5 Guaymas. Temperature = 250°C, ESr (total reduced sulfur concentration) = 6.6 x 10 m. HM hematite, MT magnetite, PY pyrite, PO pyrrhotite. Dotted line Au solubility (ppm) (Shikazono, 1988). 

The main alteration minerals surrounding Kuroko ore body are K-mica, K-feldspar, kaolinite, albite, chlorite, quartz, gypsum, anhydrite, and carbonates (dolomite, calcite, magnesite-siderite solid solution), hematite, pyrite and magnetite. Epidote is rarely found in the altered basalt (Shikazono et al., 1995). It contains higher amounts of ferrous iron (Fe203 content) than that from midoceanic ridges (Shikazono, 1984). 

It is likely that the minerals controlling /CO2 of hydrothermal solution at back-arc basins are dolomite, siderite, calcite, hematite, magnetite, graphite, K-mica and kaolinite. Most of these minerals are not found in altered ridge basalt. 

Dang, F., Kamada, K., Enomoto, N., Hojo, J. and Enpuku, K. (2007) Sonochemical synthesis of the magnetite nanopartides in aqueous solution. Journal of the Ceramic Society of Japan, 115 (1348), 867-872. 

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