Goethite crystal structure

As in dissolution, a chemical and structural change can occur from hydrolysis as the ions replaced by or OH may be of a different size so that the crystal structure is stressed and weakened. An example of this is the weathering of feldspar or goethite by H ... 

Goethite crystals produced by oxidation of Fe solutions at ambient temperature in neutral solution (Fig. 4.7 right) - a process likely to occur in nature - are usually much less developed and the crystals are smaller (MCLb 10 nm) than those obtained in alkaline Fe " solutions. If Al is taken up in the structure, these crystals become extremely small (MCL 5 nm) and show almost no particular habit. At higher pH (-12) the crystals are again acicular (MCL -30 nm) despite containing structural Al (Al/(Al-i-Fe) -0.3) they show internal disorder, however, and stars are frequent. This morphology is also observed for soil goethites (see Chap. 16). 

The commonest habits for hematite crystals are rhombohedral, platy and rounded (Fig. 4.19). The plates vary in thickness and can be round, hexagonal or of irregular shape. Under hydrothermal conditions, these three morphologies predominate successively as the temperature decreases (Rosier, 1983). The principal forms are given in Table 4.1. Hematite twins on the 001 and the 102 planes. The crystal structure of hematite has a less directional effect on crystal habit than does that of goethite and for this reason, the habit of hematite is readily modified. A variety of morphologies has been synthesized, but in most cases, the crystal faces that enclose the crystals have not been identified. 

Equally often, goethite and hematite have been used as model adsorbents because they have a well defined crystal structure, are widespread in nature and can be synthesized readily in the laboratory. 

From these data it follows that when iron is precipitated in acid and neutral environments the first products should be X-ray-amorphous highly dispersed iron hydroxides, which in the course of time acquire the crystal structure of goethite or hematite. The mechanism of this process depends on kinetic factors (rate of oxidation of Fe " ), form of migration of the iron (ionic or colloidal), and acidity of the parent solution. In neutral environments ferrihydrite possibly is formed as an intermediate metastable phase, especially if the iron migrates in colloidal form or in the form of the Fe ion. The products of diagenesis of such a sediment may be both goethite (in the case of low Eh values typical of the Precambrian iron-ore process) and dispersed hematite (in the case of deposition of the oxide facies of BIF). 

However, these results must be carefully evaluated since contamination of the systems with molecular oxygen cannot be excluded, especially in the early studies. Thus, the experimental conditions and systems reported in this literature are not directly comparable to our study where predominantly non mixed-valence iron oxides were investigated under strict exclusion of molecular oxygen. Direct electron transfer between adsorbed Fe(II) and structural Fe(III) at the goethite surface seems unlikely as Fe(III) within the crystal structure is stabilized by neighbouring oxygen ligands. 

Ferrihydrite (Fe50Hg.4H20) is common in young soil deposits but readily transforms to more stable iron hydroxides and oxides such as goethite and hematite (qq.v.). Ferrihydrite is also called amorphous iron oxide or hydrous ferric oxide, due to its disordered crystal structure the precise structure or formula of this mineral is not yet known (Schwertmann and Cornell, 2000). Ferrihydrite may be present in samples of naturally occurring raw ochres and sieimas (qq.v.) but identifications in works of art have not yet been made. 

FIG. 12 Goethite sxuface structure for the (110) crystal plane. The different functional groups occurring on this plane are shown with the lUPAC terms (see also Fig. 13). 

---