The formation of hard hematite in steam generators with relatively high levels (5 – 10 μ g/L) of dissolved oxygen at temperatures around 280 – 290°C and pressures around 6 – 8 MPa can serve as an analog for the formation of hard hematite in sedimentary processes. Furthermore, in steam generators, as well as in nature, hematite is an effective cementing agent, capable of incorporating as much as twice its own weight of other solids to form a hard composite material. Laboratory simulations showed ferrihydrite to be the likely starting material for the formation of hard, dense hematite at temperatures much lower than those required for sintering of anhydrous hematite. These laboratory simulations, performed at temperatures around 260°C and pressures of ~500 MPa, resulted in the formation of hard hematite or hematite-based composite solids over periods of 3–5 h, compared with several months in steam generators and many years in nature. The amount of water present during the synthesis (10 – 15% of the weight of dry ferrihydrite) and the gradual removal of water proved to be key parameters in the formation of hard, dense hematite. The mechanism, studied by means of X-ray diffractometry, Mössbauer spectroscopy and infrared spectroscopy, appeared to involve build-up, then gradual condensation of OH bridges, leading to the conversion of ferrihydrite to hydrohematite with approximately 4 – 5% of residual water. The presence of other solids, such as copper and its oxides, alumina and silica, in large quantities, resulted in smaller grain size of the hydrohematite product but did not affect its mechanical properties. On the other hand, the use of hydrazine to provide a reducing environment produced goethite during the precursor synthesis stage and soft magnetite during the pressing stage. However, whenever hematite was produced, it could not be subsequently reduced to magnetite by hydrazine under the reaction conditions specified above. The mechanical properties as well as the spectroscopic characteristics of the product of pressing agreed with observations on sedimentary hematite-cemented rocks.

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