Crystal Nucleation and Growth in Hydrolysing Iron(III) Chloride Solutions

Abstraet--AI-substituted goethites were prepared by rapid oxidation of mixed FeCI2-AICI 3 solutions at pH 6.8 in the presence of CO2 at 25~ A combination of AI substitution and adsorption of CO2 reduced crystal size (except for an increase at small additions of AI) and produced unusual thin, porous particles. Product goethites had surface areas up to 283 mZ/g and unit-cell expansions induced by hydration. Substitution of AI for Fe reduced the 111 spacing and increased infrared OH-bending vibrational frequencies. A1 substitution split the goethite dehydroxylation endotherm during differential thermal analysis into a doublet and increased the temperature of all reactions. Both cold and hot alkali solutions dissolved AI from the goethite structure.After drying the product in vacuo at 110~ X-ray powder diffraction data indicated minimal deviation from Vegard's law for the goethite-diaspore solid solution up to about 30 mole % AI substitution. Goethite prepared in the presence of 40 mole % AI had a 111 spacing of 2.403/~ corresponding to 36 mole % structural AI if Vegard's law was obeyed. Rapid oxidation of mixed FeCI2-AICI 3 solutions appears to be conducive to a higher degree of AI substitution in goethite than alkaline aging of hydroxy-Fe(III)-Al coprecipitates.

Section: Physical Properties Of the Productsmentioning

confidence: 60%

Synthesis and Properties of Poorly Crystalline Hydrated Aluminous Goethites

Fey

1981

“…In acid media it changes to hematite (Atkinson et aL, 1977;Hamada and Matijevic, 1982), and in reducing, alkaline conditions, it goes to magnetite (Blesa et aL, 1986); these transformations all involve dissolution of akagan6ite as a preliminary step.…”

Section: Discussionmentioning

confidence: 99%

Transformation of Akaganéite Into Goethite and Hematite in Alkaline Media

Cornell¹,

Giovanoli

1990

Abstract--The conversion of akaganrite to goethite and/or hematite in alkaline media has been followed by X-ray powder diffraction and transmission electron microscopy (TEM). The rate of transformation fell and the amount of hematite in the product increased as the [OH-] decreased to < 1 M. Kinetic studies and TEM indicated that the transformation involved dissolution ofakaganrite followed by reprecipitation of goethite and/or hematite. The rate-determining step was the dissolution of akaganrite.Silicate species retarded the formation of goethite + hematite principally by inhibiting dissolution of akaganrite; to a lesser extent, they interfered with the nucleation of goethite. Silicate modified the morphology ofgoethite, but not hematite.Comparison of the transformation behavior ofakaganrite with that previously observed for ferrihydrite indicated that the composition of the reaction product depended strongly on the transformation conditions, i.e., pH and the presence of foreign species. The nature of the solid precursor was important insofar as its degree ofcrystallinity governed the dissolution kinetics and its surface properties influenced interaction with any foreign species in the system.

“…The well-developed needles and blades of goethite associated with the Fe-free kaolinite in the hexagonal plates differ from goethite and other oxides and oxyhydroxides that commonly occur as poorly developed coatings on mineral grains (e.g., those in the overlying Ardon Formation; . They resemble, however, goethite crystals grown by ageing of noncrystalline iron oxides (Atkinson et al, 1977). It is significant that the crystalline goethite and hematite occur exclusively on the authigenic, Fe-free kaolinite and not on the anhedral, Fe-rich kaolinite.…”

Section: Relations Between Mineral Phasesmentioning

confidence: 93%

“…The hexagonal plates and cubes are from a few hundred micrometers to somewhat more than one thousand micrometers across (Figures 1 and 2). They occur in about equal amount and constitute 10-25% of The acicular crystals resemble artificially precipitated goethite, whereas the small platelets resemble hematite (Atkinson et al, 1977). Neither was observed on the anhedral clay grains.…”

Section: Texture and Morphologymentioning

confidence: 98%

Transformation of Iron-Bearing Kaolinite to Iron-Free Kaolinite, Goethite, and Hematite

Rozenson

1982

Abstract--Ferruginous clay partings in limestones of the marine, largely evaporitic, Upper Triassic Mohila Formation (Makhtesh Ramon, Israel) contain hexagonal plates and cube-like bodies up to one millimeter across. Analyses by electron microscopy, infrared and Mrssbauer spectroscopy, and X-ray powder diffraction indicate that the matrix contains Fe-rich anhedral kaolinite, up to 100/zm in size; the hexagonal plates are composed of euhedral, Fe-free kaolinite covered with well-developed acicular goethite and platy hematite (0.5 to 2/zm in size), and the cubes consist of fine-grained goethite with minor amounts of kaolinite.The anhedral kaolinite appears to be detrital, the hexagonal plates to be authigenic, and the cubes to be pseudomorphs after pyrite. Oxidation appears to have altered Fe-rich kaolinite and pyrite to Fe-free kaolinite, goethite, and hematite and was accompanied by recrystallization and pseudomorphic replacement. The alteration process was slow and was probably induced by a small increase in pH and in the Al/Fe ratio, resulting from oxidation of reduced components (pyrite, ferrous carbonate, organic matter) in a semiclosed, sediment-mud system. Overlying kaolinitic flint clay deposits may be the final product of a similar process.