Al-goethite

Fig. 8.4 Stability fields of Al-goethite and Al-hematite as a function of Al substitution.

Fig. 12.17 Dissolution morphology of synthetic 210 faces, c) Monodomainic Al-goethite (Al/ goethite crystals after partial dissolution in 6 M (Fe-tAI) = 0.097 mol mol" ) with cavernous dis-HCI at 25 °C. a) Pure goethite with dissolution solution at crystal edges (Schwertmann, 1984 a, along domain boundaries, b) Pure goethite with with permission), diamond-shaped dissolution holes bounded by...

Schwertmann, U. (1984a) The influence of aluminium on iron oxides. IX. Dissolution of Al-goethites in 6 M HCl. Clay Min. 19 9-19 Schwertmann, U. (1984b) Citation Classic, Current Contents, Agric. Biol. Envir. Sd. 15 No 22... 

As in the example of magnesian calcite, measurement of the IAP for Al-goethite as a function of the mole fraction of A1 would permit the calculation of the activities of the end-member solid phases, goethite and diaspore, in the mixed solid (Eq. 3.35). Alternatively, simply by way of an illustration, it may be assumed that the two solid phases mix to form an ideal solid solution, which, by definition,24 exhibits solid-phase activities always equal to the respective mole fractions of the components (FeOOH) = 1 - x and (AIOOH) = x. Under this simplifying assumption, Eq. 3.34 can be integrated directly, with the following result ... 

This equation also can be derived from Eq. 3.32 after incorporating the definition of ideal mixing. For an ideal Al-goethite, Eqs. 3.39 and 3.43 yield the solubility product constant ... 

The effect of Al-goethite on A1 solubility can be illustrated through a reconsideration of the activity-ratio diagram in Fig. 3.5, but with the system simplified to comprise only kaolinite and gibbsite in addition to ideal Al-goethite. For (H20) = 1.0, Eqs. 3.21b and 3.21c yield the activity ratios for kaolinite and gibbsite ... 

Y. Tardy and D. Nahon, Geochemistry of laterites, stability of Al-goethite, Al-hematite, and Fe3+-kaoIinite in bauxites and ferricretes An approach to the mechanism of concretion formation, Am. J. Sci. 285 865 (1985). 

C.A. Barreto, R.E. Vandenberghe, E. De Grave, A.L. Morales, H. Perez, The experimental nuclear quadrupole interaction in synthetic Al-goethites of various crystallinity. Hyp. Interact 148H49, 337-344 (2003)... 

---