Natural hematite

Low levels of structural Ge" have also been observed in natural hematite from the Apex mine, Utah (Bernstein Waychunas, 1987) and to achieve charge balance, incorporation of two Fe for one Ge", i.e. similar to the two Fe" for one in ilme-nite, has been suggested. Synthetic, single crystals of Ge substituted hematite have also been grown by a chemical vapour transport method (Sieber et al. 1985). A range of elements including Zr, Ge, Hf, V, Nb, Ta, W and Pb has been used as low level dopants (2 10 - 0.2 g kg ) to improve the semiconductor behaviour of hematite anodes (Anderman Kermedy, 1988). The increase in unit cell c from 1.3760 to 1.3791 nm and in a from 0.50378 to 0.50433 nm indicated that Nd (as an inactive model for trivalent actinides of similar ionic size (Am r = 0.0983 nm Nd " r = 0.098 nm)) was incorporated in the structure (Nagano et al. 1999). 

Fig. 16.16 Adsorbed phosphate of natural hematitic material as a function of the aspect ratio width/thickness (Torrent et al., 1994 with permission).

Sunagawa, I. (1961) Step heights of spirals on natural hematite. Am Min. 46 1216-1226... 

Phosphate sorption by natural hematites. Eur. J. Soil Sd. 45 45—51 Torrent, J. Schwertmann, U. Schulze, D.G. (1980) Iron oxide mineralogy of some soils of two river terrace sequences in Spain. Geoderma 23 191-208... 

Thus, spiral step patterns served as excellent subjects, and many observations were reported using these new techniques [3], [4]. It was also around this time that the movement of spiral growth layers spreading on the (0001) face of Cdl growing in aqueous solution was first observed in situ. By using these optical techniques, spiral growth layers with monomolecular height (0.23 nm) were observed and measured on natural hematite crystals [5]. 

I. Sunagawa, Step height of spirals on natural hematite crystals. Am. Min., 46,1961, 1216-26... 

There appear to be two possible explanations for these central peaks in the natural hematite. One is that some of the ferric material is actually not hematite but is bound to clay that might be present. Other workers (11) obained similar results when they prepared samples of iron compounds absorbed on kaolinite or bentonite. The second reason concerns the effect of small crystal size on Mossbauer spectra and perhaps is better illustrated for goethite. 

Hematite is found in large quantities in the vicinity of Malaga in Spain (Spanish red) and near the Persian Gulf (Persian red). The Spanish reds have a brown undertone. Their water-soluble salt content is very low and their Fe203 content often exceeds 90 %. The Persian reds have a pure hue, but their water-soluble salt content is disadvantageous for some applications. Other natural hematite deposits are of only local importance. A special variety occurs in the form of platelets and is extracted in large quantities in Karnten (Austria). This micaceous iron oxide, is mainly used in corrosion protection coatings. 

Despite the simplicity of the method there are three reasons why thermal dehydration recipes are not given here (1) Hematite produced in this way does not consist of idiomorphic crystals at temperatures of up to ca. 500-600 °C pseudomorphs of the precursor result and above 600 °C coalesced crystals with ill-defined crystal faces are obtained and the surface area is low, (2) In natural environments hematite rarely forms at such high temperatures, and (3) natural hematites frequently consist of single, more or less idiomorphic crystals. 

The sorptive capability of solids for charged species is proportional to their surface area per weight and the density of charged sites on their surfaces (their surface-site density or exchange capacity). Explain this statement as it applies to the sorptive capacity of natural hematite versus that of ferrihydrate. 

FIG. 3.20 Distribution of PZC values reported for natural hematite in the literature. 

Das, M.R. et al.. Kinetics and adsorption of benzoate and salicylate at the natural hematite-water interface, Colloids Surf. A, 254. 49, 2005. 

Rabung, T. et al.. Sorption of Eufllt) on a natural hematite Apphcation of a surface complexation model, 7. Colloid Interf. Sci., 208, 153, 1998. 

Minerals are inorganic compounds that are found in nature and have both a well-defined composition and crystalline arrangement of atoms. Coal and petroleum hydrocarbons are organic and thus not minerals. Obsidian is not a mineral because it has neither crystalline structure nor a specific composition. Stones such as chert and flint, which are mainly silica, SiO, have a relatively precise composition but lack crystalline structure, so are not minerals. While copper is a mineral, brass and bronze do not occur in nature and do not have a fixed elemental composition, so they are not minerals. A synthetic material can be a mineral, however, as long as it is also found in nature. Hematite can be produced artificially by firing ceramics in an oxidizing environment, but it is still considered mineral because hematite can be found in nature. A synthetic ruby is likewise a mineral because rubies do occur in nature, but modem cubic zirconia is not. 

FIGURE 16,5-3 Zeia potential of natural hematite in sodium dodecy I sulfate solution. (Data from Shergold and MeUgren.74- 3)... 

Oxygen isotopic fractionations between various Fe(III)-oxides and water are even more poorly constrained (Fig. 19). In addition to equations based on isotopic analysis of natural hematite in low-temperature environments (Clayton and Epstein 1961 Clayton 1963), several investigators recently reported fractionation factors for hematite and goethite obtained from laboratory synthesis experiments. They show a large (>109 variation especially at temperatures below 40° C, depending on reaction pathways and solution compositions (e.g. pH). Mineral precipitates at low temperatures tend to be... 

Song, T Shen, L Zhang, H Gu, H Zhang, S Xiao and J. (2012) Chemical looping combustion of two bituminous coal / char with natural hematite as oxygen carrier in 1 kW th reactor. Proc. 2nd Int. Conf. Chem. Looping, Darmstadt, pp. 26-28. 

E. De Grave, R.E. Vandenberghe, Mossbauer effect study of the spin structure in natural hematites. Phys. Chem. Miner. 17, 344—352 (1990)... 

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