No abstract

The crystal-chemical mechanisms by which transition metals are associated with goethite are fundamental to our understanding of the solubility and bioavailability of micronutrients and heavy metals in soils, and in the formation of laterite ore deposits. Transition metals such as Cr, Mn and Ni may sorb onto goethite by forming surface precipitates, surface complexes or by replacing Fe3+ in the goethite structure. In the work reported here, we investigated the local coordination environment of Cr, Mn and Ni in synthetic goethite using EXAFS spectroscopy. We demonstrate the isomorphous substitution for Fe3+ by Cr3+ (up to 8 mol.%), Mn3+ (up to 15 mol.%) and Ni2+ (up to 5 mol.%). We find, however, that the next-nearest-neighbour coordination environment changes with composition. The perturbations are likely to be responsible for limiting the accommodation of Cr3+, Mn3+ or Ni2+ in the FeOOH structure.

Mineral and isothermal magnetic properties of Al‐, Mn‐ and Ni‐substituted haematites were characterized and their relationships evaluated in order to interpret better the results of magnetic analyses of soils and recent sediments. Aluminium, manganese and nickel haematites generally behaved as single‐domain (SD) particles. The influence of incorporated Al on the magnetic behaviour of haematite was consistent with Al acting as a paramagnetic dilutent. Mass magnetic susceptibility (χ) and SIRM800 decreased as the level of Al substitution increased. Incorporation of Mn and Ni increased χ, which could be associated with enhancement of the spin canting effect of haematite. The stability of SIRM800 to demagnetization for Al‐haematite appears to be related to a defect mechanism associated with the development of smaller crystallites arising from Al substitution. Magnetic domain rotation or flipping was probably inhibited, being blocked by structural defects during magnetization and demagnetization, and resulted in a low but stable partial SIRM (SIRM800 ). %IRM/SIRM800 demagnetization curves and estimated (Bo )CR values of ≤100 mT for Mn‐haematite indicate pseudo‐single‐domain/multidomain‐like behaviour despite Mn‐haematite having particle and crystallite dimensions similar to Ni‐haematite, which did not show this behaviour. Data indicate that parameters involving unsaturated, partial SIRM should be used with caution in magnetic studies of soils and sediments.

Mineralogical and thermal characteristics of synthetic Al-, Cr-, Mn-, Ni- and Ti-bearing goethites, synthesized via alkaline hydrolysis of metal-ferrihydrite gels, were investigated by powder X-ray diffraction and differential thermal analysis. Shifts in unit-cell dimensions were consistent with size of substituent metal ions and confirmed the incorporation of Al3+, Cr3+, Mn3+, Ni2+ and Ti4+ in the goethite structure. A weight loss of 6.2 wt.% for goethite containing 12.2 mol.% Ti, being significantly less than for stoichiometric goethite, is consistent with the replacement of Fe by Ti in the goethite structure coupled with the substitution of O2− ions for OH− (i.e. proton loss). These data provide the first confirmation of the direct replacement of Fe by Ti within goethite. Formation of multiple dehydroxylation endotherms for goethite containing 4.5 mol.% Al, 15.3 mol.% Mn and 12.2 mol.% Ti was not attributed to the decomposition of surface OH groups or related simply to the crystallinity of precursor goethite (‘high-a’ vs. ‘low-a’) as defined by the magnitude of a. Instead, endotherm doublet formation was associated with weight loss due to the dehydroxylation of goethite remaining after initial phase transformation to protohematite and to the evolution of OH− associated with the rapid increase in crystallite size of protohematite directed primarily along the a direction. Development of the first endotherm is due to initial dehydroxylation and transformation to protohematite. With continued heating of well ordered goethite or goethite containing moderate to high levels of substituent cations, domain growth along the a direction is delayed or inhibited to a critical point that provides enough thermal energy to enable goethite transformation to proceed to completion and for proto-hematite domain growth to occur. This results in the formation of a second endotherm. For less well ordered goethite and/or goethite containing only low levels of foreign metal cations, protohematite domain growth is not inhibited and proceeds continuously with heating to give only a single endotherm.