Goethite cation substitution

Table 3.3 Cations substituting for Fe " in goethite. Maximum substitution and corres ponding unit...

Scheinost A. C. and Schwertmann U. (1997) VIS-NIR reflectance spectra of goethite (a-FeOOH) as a function of particle size, luiit-cell size, and cation substitution. 28 Limar and Planetary Science Conference, March 17-21, Houston, TX. 

In relation to goethite, the room temperature magnetic hyperfine fields are always below 38.0 T, which is the reference value of well-crystallized large-particle goethite [21,22]. The low hyperfine field values may be related to the small grain sizes as well as the alteration of the Fe neighboring due to anion and/or cation substitutions. 

Fig. 3.7 Top Relationship between the unit cell edge length a of synthetic goethites and various structurally incorporated metals. Bottom Rate of change of a per mol of substituted metal (= slope of the upper curves) vs. ionic radius of the respective metal cations (Gerth, 1990, with permission).

The band positions of Fe oxides are also influenced by the substitution for Fe by other cations in the structure, as indicated partly by their colour. Scheinost et al. (1999) noticed a linear shift in the position of the Ai " Ti transition from 943 to 985 nm and that of the Ai " T2 transition from 653 to 671 nm for 47 synthetic goethites whose Al-substitution (Al/(Al-i-Fe) ranged between 0 and 0.33 mol mol (R = 0.92 for both). Mn "-substituted goethites showed bands arising from Mn " near 454 and 596 nm. The overall reflectivity in the visible range decreased as structural Mn increased from 0 to 0.20 mol mol (Vempati et al., 1995). The same effect has been observed for V "-substituted goethites (Schwertmann Pfab, 1994). The position of the EPT band of Mn "-substituted hematite shifted to 545 nm and that of the Ai " T2 transition to 700 nm (Vempati et al., 1995). The position of the same transition shifted from ca. 600 to 592 nm as the Al-substitution in hematite rose from 0 to 0.125 mol mol (Kosmas et al., 1986). Crystal size and crystal shape also have an effect on diffuse reflectance, as shown for hematite (see Fig. 6.12). As the crystals become smaller, reflectance increases and needles also reflect more than cubes, i. e. the colour becomes more vivid. 

Ainsworth et al. (1994) observed that oxide aging did not cause hysteresis of trace element cation sorption-desorption. Aging the hydrous ferric oxide with trace elements cations resulted in hysteresis with Cd and Cu, but little hysteresis was observed with Pb. The extent of reversibility with aging for Co, Cd, and Pb was inversely proportional to the ionic radius of the ions (i.e., Co < Cd < Pb). The authors attibuted the hysteresis to Co and Cd incorporation into a recrystallizing solid (probably goethite) via isomorphic substitution, not to micropore diffusion. 

Influence of Trivalent Cations Due to the common presence of Al-substituted a-FeOOH in natural samples (well oxidized soils), the Impact of Al-substitution on the properties of natural and synthetic goethite was... 

---