Thermal and Mineral Properties of Al-, Cr-, Mn-, Ni- and Ti-Substituted Goethite

Abstract

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.

Mineral transformations and textural evolution during roasting of bog iron ores

“…Goethite disappears in all the samples at temperatures of 300°C (Figs. 4-7), which is consistent with literature datadehydroxylation endotherm of the oxyhydroxide usually occurs between 250 and 300°C [1,3,6,14,75,85,86]. The hematite 110, 104 and 116 peaks are distinct in the XRD patterns, except the GD sample (Fig.…”

“…The results of thermal analyses indirectly support this possibility. Isomorphic substitution of Mn for Fe in goethite complicates the progress of dehydroxylation [3,14,87], which can cause the formation of fully developed hematite at a temperature somewhat higher than during transformation of pure (non-substituted) goethite, despite the Mn-goethite itself starts to dehydroxylate at slightly lower temperature than the pure one.…”

“…Hematite (a-Fe 2 O 3 ) is usually the final product of such transformations. Although the overall course of the processes is widely known, the effects of various crystal-chemical and chemical factors on the transformation mechanisms and properties of the products are still under vigorous debate [1,3]. This type of research focuses usually on monomineral synthetic samples [e.g., [4][5][6][7][8][9][10] and does not fully reflect the complexity of the processes during thermal transformations of natural Fe-oxide-rich materials [e.g., [10][11][12][13][14][15][16][17], archeological findings [18,19] and composite industrial products or wastes [20][21][22].…”

“…Poorly crystalline hydrous manganese oxides, mainly vernadite (MnO 2 ÁnH 2 O) type [24,46], are also encountered. Under reducing conditions and at elevated content of organic matter, iron phosphates [predominately vivianite, Fe 3 (PO 4 ) 2 Á8H 2 O] and carbonates (siderite, FeCO 3 and various Mn-Ca-Fe carbonates) can be formed [24,31,37]. Pyrite (FeS 2 ) was reported in some areas as well [26,27,48,49].…”

Association of trace elements and dissolution rates of soil iron oxides

“…The initial greater release of these elements relative to Fe also suggests that some fraction of the element was present in a discrete solid or soluble phase or adsorbed on to Fe oxides. There are several reports indicating that these elements can be adsorbed by Fe oxides and/or incorporated into Fe-oxide structures (Quin et al 1988;Angove et al 1999;Christophi and Axe 2000;Manceau et al 2000;Wells et al 2006;Kaur et al 2009a;Marcussen et al 2009). The dissolution pattern for Ti was varied (Fig.…”