R. Villalba

Study of the mechano-chemical transformation of goethite to hematite by TEM and XRD

A study of the mechano-chemical transformation of goethite to hematite was carried out using transmission electron microscopy (TEM) and X-ray diffraction (XRD). Synthetic goethite was dry-ground in air for different times up to 104 h and characterized with the aim of understanding the mechanisms taking part in the transformation, the phases formed, and particle distribution, morphology and size. It was found that the transformation is topotactic, without the formation of intermediate phases, and takes place by fragmentation of crystals, creation of twins, dehydration of goethite particles, and formation of voids. The voids are arranged in almost parallel striations, and are associated with the loss of hydroxyl groups and formation of hematite in small domains of equiaxial crystallites. The hematite crystal size is about 10 nm at the initial stages and after prolonged periods of grinding remains constant at about 19 nm.

Processings of iron oxides at room temperature. I. Solid state conversion of pure α-FeOOH into distinctive α-Fe2O3

Abstract A distinct α-Fe 2 O 3 variety is formed when pure synthetic α-FeOOH is dry ground at ambient temperature in a mechanically driven mortar at > 40 hr periods. XRD and IR spectroscopy indicate that its structural order is distinct from protohematite (obtained by heating α-FeOOH to ∼450°C) and from well crystallized hematite. According to the changes observed in the WHH (width at half height) and So (surface area) values vs. grinding time, the broadening of α-Fe 2 O 3 diffraction peaks is attributed mainly to decreasing particle size. Preferential disruption of some lattice planes occurs during the irreversible solid state conversion of α-FeOOH → defect α-Fe 2 O 3 . Variations in size, color and eventually in water content of the progressively formed α-Fe 2 O 3 particles are observed.

Processings of iron oxides at room temperature. II. Mechanochemical reaction effects on the structure and surface of pure, synthetic lepidocrocite

Abstract The gradual transformation, at ambient temperature, of synthetic crystalline γ-FeOOH during several dry grinding periods in an automatic mortar, has been followed by IR, XRD and surface area analysis. This study shows that as grinding proceeds, parallel to the progressive disappearence of γ-FeOOH, a distinct α-Fe 2 O 3 phase is developing. This mechanochemical reaction is completed at 32 hr grinding, when γ-FeOOH is no further detected by IR spectroscopy nor by X-ray powder diffraction. The surface area (So) measurements indicate a decrease of the γ-FeOOH particles size during the early stages of grinding (24 hr); thereafter the So profile is ruled by the surface energy of the formed α-Fe 2 O 3 particles. XRD suggests that broadening affects in a non uniform way the peaks describing the α-Fe 2 O 3 diffraction planes, the typical 024 plane being the most affected. By analogy with α-FeOOH dry grinding, variations in size, color and eventually in water content are gradually observed during this γ-FeOOH → defect α-Fe 2 O 3 mechanochemical decomposition, The latters mechanism is discussed.

Synthesis and characteristics of Fe-Mn glycerate (alkoxides), new precursors of ferrite structures

Realisation de la reaction de glyceration sur des melanges de gœthite synthetique (FeOOH) et de pyrolusite (MnO 2 )

High temperature synthesis and characteristics of Al-substituted iron alkoxide

Abstract A crystalline solid identified as Al-substituted iron alkoxide is obtained from the reaction at 246° C of glycerol with synthetic Al-bearing goethite, when the latters composition is: Fe 0.904 Al 0.096 OOH. X-ray powder diffractograms (XRD) of this solid show a significant shift of the d spacings in comparison to a previously reported pure iron glycerate complex derived from synthetic α-FeOOH. The composition of the metallic alkoxide, calculated from chemical analysis data is: [(C 3 H 5 O 3 Fe 0.98 Al 0.02 )] n . When this powder is calcined at 1000°C, well-crystallized Al-substituted hematite is produced, whose XRD patterns indicate a shift of the characteristic hematite lines to lower values. The partial (isomorphic) substitution of Fe by Al may also be ascertained from the hydrolysis of the [(C 3 H 5 O 3 Fe 0.98 Al 0.02 )] n complex by boiling water; then the XRD and chemical analyses of the resulting product show limited formation of a spinel solid solution. In contrast, hydrolysis of pure iron alkoxide yields high amounts of ferrimagnetic spinel (mainly FeO·Fe 2 O 3 ). Some characteristics of the Al-substituted iron alkoxide and of its hydrolysis and calcination products are briefly examined.

Selective destruction and differentiation of clay minerals from natural diaspore admixture by mortar grinding

Abstract A dry grinding process at several periods has been used successfully in order to differentiate among the different minerals forming a natural diaspore clay. This is achieved by selective and incongruent destruction of the clay minerals of this admixture, on the basis of their disparate structural stability, hardness and relative amount present. Kaolin clays (represented mainly by kaolinite + halloysite) are destroyed after >3 hr grinding, anatase after ∼40 hr, whereas the structure of α-AlOOH is still preserved after >250 hr, as ascertained from XRD. From IR spectroscopy analysis it is inferred that as a consequence of grinding prototropy takes place in some of the clay hydroxyls. This is supported by water losses results at 600°C.

Interactions between the iron and the aluminum minerals during the heating of Venezuelan lateritic bauxites. II. X-ray diffraction study

Abstract Four samples of Venezuelan lateritic bauxites were heated to 300, 600 and 1000°C and the thermal reactions were studied by X-ray diffraction (XED) and by chemical extractability of silica and alumina. Gibbsite was converted to boehmite at 300°C, to an amorphous phase at 600°C and partly to corundum at 1000°C, with isomorphic substitution of Fe for some of the Al in the corundum structure. Goethite was converted to protohematite at 600°C and the hematite at 1000°C, with isomorphic substitution for Al for some of the Fe in both α-Fe 2 O 3 varieties. Ti contributed by ilmenite is also occluded by the hematites. The occlusion of Ti takes place at 1000°C during the decomposition of the ilmenite and concomitant recrystallization of α-Fe 2 O 3 .

Topotactic conversions of γ-FeOOH and comparative transformation methods of iron oxides in magnetic spinels

Abstract The DTA of a natural, well crystallized lepidocrocite sample containing small amounts of goethite, quartz and manganese shows that the conversion of γ-FeOOH to α-Fe2O3 (through the γ-Fe2O3 metastable phase) occurs at 440°C, whereas in pure, synthetic γ-FeOOH the exothermic peak appears at ~ 500°. Quartz (1.4% SiO2) is not affected, nor does it hamper the transformation of FeOOH into magnetic spinels, when the sample is reacted with glycerol at reflux temperature followed by hydrolysis of the resulting solid (iron alkoxide) by boiling H2O. Starting γ-FeOOH and final solid products are characterized by XRD and IR spectroscopy. This reaction is also discussed comparatively with the solid state transformations of iron oxides into magnetic materials by a combination of heating and grinding-pressing cycles of disks containing alkali halides as diluting agents.

A distinctive mechanochemical transformation of manganosite into manganite by mortar dry grinding

Abstract Synthetic manganosite was converted to manganite by progressive mortar dry grinding (in air). The grinding process which lasted 203 hours was followed by X-ray powder diffraction (XRD), infrared spectroscopy (IR) and surface area determinations. Both the XRD and IR results show that parallel to the gradual structural degradations of manganosite (starting material) and of hausmannite (intermediary phase), the nascent manganite phase undergoes further crystallization, reaching stability in the last stages of grinding. Such a mechanochemical transformation of manganosite into manganite by mortar dry grinding is peculiar, as it was not observed by other grinding methods employed before. Some hypothesis are proposed in order to explain this solid-state transformation.

Comparative thermal transformations of synthetic Fe−Mn glycerate (alkoxides)

Synthetic iron-manganese glycerates with compositions corresponding to different molar ratios of Fe: Mn, contain large amounts of H2O (up to 22%). Heating in air at ≈270°C produces a hydrated, disordered Mn-ferrite structure (jacobsite), as shown by XRD and IR spectroscopy. At this temperature no alkoxide groups are detected. TG curves show 45.6% to ≈54% weight losses at 290°C, with a sharp loss from 270° to 290°C for all samples, attributed mostly to the Curie transition of MnFe2O4. Further heating of each sample at ≈670°C results in a well-crystallized hematite and variable amounts of bixbyite. At this stage no H2O is left. Further calcination at ≈1050°C gives qualitatively the same products as at 670°C. Colour changes occur during the heating process. In admixtures of goethite with MnCO3 or pyrolusite the main difference from the counterpart alkoxide is shown after heating at 270°C, when the Fe−Mn mineral mixture produces mostly protohematite (disordered hematite) instead of disordered jacobsite resulting from the alkoxides.ZusammenfassungSynthetische Eisen-Manganglycerate mit Zusammensetzungen mit unterschiedlichen Fe:Mn-Verhältnissen enthalten eine große Menge Wasser (bis 22%). Wie Röntgendiffraktion und IR-Spektroskopie zeigen, entsteht beim Erhitzen an Luft bei etwa 270°C eine hydratierte, fehlgeordnete Mn-Ferrit-Struktur (Jacobsit). Bei dieser Temperatur werden keine Alkoxidgruppen festgestellt. Die TG-Kurven zeigen einen 45.6–54%-igen Gewichtsverlust bei 290°C, mit einem scharfen Verlustpeak zwischen 270° und 290°C für alle Proben, meist charakterisiert durch eine Curie-Umwandlung von MnFe2O4. Ein weiteres Erhitzen aller Proben ergibt bei etwa 670°C gutkristallisiertes Hämatit und verschiedene Mengen Bixbyit. An dieser Stelle wird kein Wasser zurückgelassen. Ein weiteres Kalzinieren bei etwa 1050°C liefert qualitativ die gleichen Produkte wie bei 670°C. Während des Erhitzungsvorganges verändern sich die Farben. In Gemischen von Goethit mit MnCO3 oder Pyrolusit zeigt sich der Hauptunterschied nach Erhitzen bei 270°C, wenn das Fe−Mn-Mineralgemisch anstelle des fehlgeordneten Jacobsit hauptsächlich Protohämatit (fehlgeordnetes Hämatit) ergibt.