Goethite to hematite

The kinetic data for the dehydroxylation process, i. e. the degree of transformation, a, as a function of time could be fitted to both a random nudeation model, i. e. 

The rearrangement of the Fe atoms during the transformation was inferred from differential broadening of the XRD peaks of hematite all reflections except 110, 113 and 300, for which the structure factor depends on the position of the Fe atom, are broadened. This suggests that the peak broadening results from cation disorder ( Po-mies et al. 1998 1999). 

Hematite obtained at low temperatures retains the acicular morphology of the goethite precursor crystals, but at temperatures 600 °C, a sintering process leads to irregular particles of hematite. 

1) The existence of a superstructure was revealed by satellite spots in the XRD single crystal diffraction pattern of partly dehydrated goethite. The superstructure was considered to be an in- 

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During conversion of goethite to hematite only small fractionation effects seem to occur, because most of the oxygen remains in the solid (Yapp 1987). Thus, in principle it should be possible to reconstruct the sedimentary environment of iron oxides from Precambrian banded iron formations (BIF). By analyzing the least metamorphosed BlFs, Hoefs (1992) concluded, however, that the situation is not so simple. Infiltration of external fluids during diagenesis and/or low temperature metamor-... 

Fig. 14.3 High resolution electron micrographs of the thermal transformation of goethite to hematite showing (Gt[001]//[Hm[210] orientation. Upper Gradual development (a d) of slit pores along Hm[001]. Lower Largely transformed region along the (Gt[001]//[Hm[210] orientation. Electron diffraction patterns in the in-...

Study of the mechano-chemical transformation of goethite to hematite by... 

The conditions of metamorphism of BIF of different original composition are examined on the basis of thermodynamic calculations and some experimental data. In oxide iron-formations the conversion of goethite to hematite was accomplished even before the beginning of regional metamorphism the P-T curve of the dehydration reaction falls within 110° and... 

From a critical review of the experimental data (Mel nik, 1972b) it was concluded that the temperature of equilibrium conversion of goethite to hematite at the pressure of a vapor-saturated aqueous solution varies from 70 to 125°C. Experimental investigations at high pressures have been limited to up to 3kbar (Smith and Kidd, 1949 Schmalz, 1959 Wefers, 1966), and their results are contradictory the discrepancies in determinations of the goethite hematite equilibrium temperature reached 50-60°C, and the slope of the P-T curve was not established. 

ABSTRACT The kinetics and mechanisms of the phase transformation of 2-line ferrihydrite to goethite and hematite are being assessed as a function of pH, temperature and Fe/As, Fe/Se, Fe/Mo molar ratios using batch experiments, BET analyses, XRD, and XANES. Initial results from XRD analyses show that ferrihydrite is stable at high pH ( 10) for up to seven days at 25°C, but considerable crystallization occurs at elevated temperatures. Specifically, XRD data show that ferrihydrite is transformed to a mixture of hematite and goethite at 50°C (-85% hematite and -15% goethite) and 75°C (-95% hematite and -5% goethite) after 24 hours and these ratios remain constant to the end of the experiments (seven days). 

Measured surface areas (11-point BET analyses) for pure phases such as ferrihydrite, goethite and hematite are in the range as proposed by Cornell Schwertmann (2003) (Table 1). Preliminary XRD analyses showed that temperature impacts the kinetics of phase transformation of ferrihydrite. Data indicated that after seven days, the rate of transformation from ferrihydrite to more crystalline forms, if it was occurring, was too slow to be measured at 25°C (Fig. 1). In contrast to the 25°C experiment, significant, transformations were observed at 50 (Fig. 2) and 75°C (Fig. 3) after 24... 

A common method of synthesizing M-substituted oxides, particularly goethite and hematite is to add base to mixed M-Fe salt solutions to precipitate M-associated ferrihydrite. Most ions do not change their oxidation state, but incorporation of Mn and Co in goethite is preceded by oxidation of these ions to the trivalent state (Giovanoli Cornell, 1992). An indication of whether isomorphous substitution has occurred can be obtained from changes in the unit cell dimensions of the Fe oxides... 

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).

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