The Influence of Aluminum on Iron Oxides: XIV. Al-Substituted Magnetite Synthesized at Ambient Temperatures

Abstract

Abstract--Mixtures of magnetite and goethite were formed by the slow oxidation of mixed FeCI2-A1C13 solutions in an alkaline environment at room temperature. The compositions of the products ranged from almost exclusively magnetite in Al-free systems to goethite only at AI/(A1 + Fe) ~ 0.3. The magnetic phase consisted of a partly oxidized (Fe2+/Fe 3+ < 0.5), Al-substituted magnetite. The unit-cell edge length a of the magnetite decreased with increasing A1 (Al = 0-0.37 per formula unit, corresponding to 0-14 mole % A1) and decreasing Fe 2+ in the structure as described by the empirical relationship a(A) = 8.3455 + 0.0693 Fe 2~ -0.0789 A1. A correlation between the experimentally determined a and that calculated from the unit-cell edge lengths of end-member magnetite, maghemite, and hercynite was highly significant (r = .96) although shifted by about 0.01 A. M6ssbauer spectra showed A1 to have entered preferentially the tetrahedral rather than the octahedral sites at low A1 substitutions (<0.15 per formula unit), perhaps because of steric reasons. With increasing A1 substitution the crystal size of magnetite decreased and structural strain increased, indicating that the structure had a limited capability to incorporate Al under these synthesis conditions. The capacity of the goethite structure to tolerate more Al may explain why goethite replaced magnetite at higher A1 concentrations.

Acidified Oxalate and Dithionite Solubility and Color of Synthetic, Partially Oxidized Al-Magnetites and Their Thermal Oxidation Products

“…XRD data for the powder MT-RT-00 with Als = 0.001 along with those of its heated products is shown in Figure 3. Goethite content increased with increase in Als for the MT-RT series in agreement with a similar effect reported by Schwertmann and Murad (1990). Mean crystallite dimensions (MCDs) for magnetite (unheated powders) and hematite (500~ powders) were calculated using the Scherrer equation (Zussmann, 1977).…”

“…The magnetites are, therefore, strongly cation deficient (partially oxidized). This is apparently the general case for magnetite prepared by wet chemical methods (Schwertmann and Murad, 1990;Sidhu et aL, 1981). Upon heating the magnetite powders to 400~ for 4 hr in the air, the Fe 2 § was completely oxidized to Fe 3 § ( Figure 2).…”

The Influence of Aluminum on Iron Oxides. XV. Al-for-Fe Substitution in Synthetic Lepidocrocite

“…Because the rate of oxidation of Fe 2+ increases with increasing pH (Stumm and Lee, 1961), a pH increase from 7 to 8 might have favored lepidocrocite. At the same time, the formation of magnetite-maghemite, which is very likely at this pH, might have been suppressed by A1 (Schwertmann and Murad, 1989;Taylor and Schwertmann, 1978).…”

Silicon-substituted magnetite and accompanying iron oxides and hydroxides from the Kumano mine, Yamaguchi Prefecture, Japan: Reexamination of the so-called maghemite (γ-Fe2O3)

“…In the inverse spinel structure, the trivalent cations occupy both tetrahedral and octahedral sites. The Si 4+ ion prefers a tetrahedral site (Newberry et al, 1982), as does the Al 3+ ion (Schwertmann and Murad, 1990). Although De Sitter et al (1977) concluded that the Ca 2+ ion prefers a tetrahedral site, we have followed recent studies (e.g., Shiga, 1989;Westendorp et al, 1991), in which the Ca 2+ ion is assigned to an octahedral site.…”