Cristallochemical characterization of synthetic Zn-substituted maghemites (g-Fe2-xZn xO3)

Inoue

et al. 2016

Self Cite

ABSTRACT:Burning is a common practice in tropical areas and related changes in mineralogy might affect the chemical and physical behavior of soils. Maghemite is a common iron oxide in soils formed from basic rocks in tropical regions. This mineral and hematite are the main pigments in these soils and exhibit high magnetization stemming from the precursor magnetite formed during the weathering process of primary minerals. The objective of the present study was to analyze changes in color, magnetic suceptibility values, Fourier transform infrared spectroscopy (FTIR) spectra, and available Fe and Al contents extracted with 1 mol L -1 KCl during the process of thermal transformation of synthetic Al-maghemites into Al-hematites. Synthetic substituted maghemites with different degrees of Al-substitution (0.0, 1.0, 2.0, 2.9, 3.8, 5.6, 6.7, 10.0, 12.0, and 17.1 mol% Al) were subjected to a temperature of 500 ± 10 °C for 0, 5, 10, 16, 64, 128, 192, 360, 720, 2160, 3600, 5040 and 6480 min. The color of the samples was analyzed by a Munsell system in a colorimeter. Mass-specific magnetic susceptibility (χ LF ) was measured at low-frequency. Available Fe and Al contents were estimated by a 1 mol L -1 KCl solution. Fourier transform infrared spectroscopy (FTIR) spectra were obtained through use of a Bruker Vertex 70X FTIR spectrophotometer at a spectral resolution of 4 cm -1 . Contents of Fe and Al extracted by 1 mol L -1 KCl in Al-hematites were not detected. All samples analyzed exhibited YR hue. Hue proportion decreased with increased heating time, and color changed from brown to red. The increase in isomorphic substitution (IS) led to increased hue values from maghemite to hematite, and the latter then became yellower. The χ LF values decreased with an increase in heating time, indicating transformation from a ferrimagnetic phase (maghemite) to an antiferrimagnetic phase (hematite). With increasing IS, the maghemite χ LF values decreased. Bands of the initial members (time 0) in the FTIR spectra were indexed as maghemites. The end members after completion of the heat treatment were identified as hematites. The IS of Fe by Al in maghemite influenced the thermal transformation to hematite, as well as the color and χ LF of the minerals. The χ LF proved to be very efficient in detecting maghemites remaining after thermal processing. Fe and/or Al were not ejected from the hematite crystalline structure after heat treatment.

Section: Colorsupporting

confidence: 67%

“…Many authors have observed the effect of isomorphic substitution on magnetic susceptibility in synthetic (Batista et al, 2008; and natural minerals (Costa et al, 1999;Silva et al, 2010). In general, increasing isomorphic substitution (IS) (i.e.…”

Section: Introductionmentioning

confidence: 99%

Thermal Transformation and Characterization of Synthetic Al-Substituted Maghemites (γ-Fe2-xAlxO3)

Nonaka

Inoue

et al. 2016

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“…(Schwertmann and Fechter, 1984). In synthesized maghemites, the degrees of substitution of Fe 3+ for Zn 2+ can reach a maximum of 0.34 mol mol -1 (Batista et al, 2008). The metallogenetic associations Fe/Mn and Fe/Zn are usually found in mineral deposits (Courtin-Nomade et al, 2016).…”

Section: Areamentioning

confidence: 99%

Identification of Heavy Metals in Crystals of Sand and Silt Fractions of Soils by Scanning Electron Microscopy (SEM EDS/WD-EPMA)

Melo

Gilkes

et al. 2018

Studies of heavy metals are concentrate on clay fractions, but coarser fractions of the soil can constitute significant sources of structural forms of heavy metals. The aim of this study was to evaluate the occurrence of heavy metals in the structure of minerals of the sand and silt fractions of soils from three different parent materials (metamorphic rocks and granite) in southern Brazil using SEM/EDS -Scanning Electron Microscopy with Energy Dispersive Spectroscopy and with WD-EPMA -with Wavelength Dispersive-Electron Probe Microanalysis. We sampled soils from two areas naturally rich in heavy metals, with high mineral deposits (galena -PbS) hosted in carbonate rocks and phyllite/mica schist. The main form of Ba in the sand and silt fractions was as barite (BaSO 4 ). The precipitation of Ba and S from the soil solution occurred on the surface of silicate mineral particles. Due to the proximity of ionic radius of Ba-Pb, there was isomorphic substitution of Ba for Pb in the barite structure. The only primary mineral source of Pb in the coarse soil fractions was trioctahedral mica. Several secondary minerals in the silt and sand are sources of structural Pb: plumbogummite, plumboferrite, magnetoplumbite, and cerussite. There was a strong geochemical association of Pb-Fe-Mn. Zinc was also associated with Fe. The SEM/EDS/WD-EPMA techniques are important analyses to complement standard procedures, such as X-ray diffraction and total chemical digestion, in geochemical studies.

“…Pereira (2009) found MCD values of 34 and 64 nm for two natural rhombohedral hematites with the probable presence of Al (samples also contained kaolinite and gibbsite with residual Al content). Batista et al (2008) studied Zn-substituted maghemites and observed a linear relationship between MCD and IS. In their study, MCD values decreased with increasing substitution.…”

Section: Ray Diffraction (Xrd)mentioning

confidence: 99%

“…Isomorphic substitution and thermal transformation may change some mineralogical properties in iron oxides: unit cell dimensions, specific surface area, solubility in different environments (Ruan and Gilkes, 1995;Gasser et al, 1996;Batista et al, 2008;Batista et al, 2011), and temperature of transformation (Sahoo et al, 2010). Some soil minerals are affected by heating in natural and managed fires (Yusiharni and Gilkes, 2012).…”

Section: Introductionmentioning

confidence: 99%

Kinetics of Thermal Transformation of Synthetic Al-Maghemites into Al-Hematites

Nonaka

Costa

et al. 2017

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) may largely influence this process. We analyzed the kinetics of thermal transformation of Al-maghemites into Al-hematites and some of its mineralogical aspects. Synthetic substituted maghemites with different degrees of Al-substitution (0.0, 1.0, 2.0, 2.9, 3.8, 5.6, 6.7, 10.0, 12.0, and 17.1 mol% Al) were subjected to a temperature of 500±10 °C for 0, 5, 10, 16, 64, 128, 192, 360, 720, 2160, 3600, 5040, and 6480 min. After thermal treatment, samples were characterized by X ray diffraction (XRD), differential thermal analysis (DTA), specific surface area (SSA) measurement, and total chemical analysis. XRD patterns were analyzed by Rietveld refinement, and maghemite and hematite contents were calculated using Rietveld refinement and the method proposed by Sidhu. Isomorphic substitution of Fe by Al increased the critical temperature of transformation and the time necessary for maghemite to hematite transformation. Rietveld refinement data showed a better fit than the data adjusted by the Sidhu method. Increasing isomorphic substitution also decreased lattice parameters and mean crystallite dimension (MCD) values in maghemite; but only c-dimension and MCD decreased with increasing Al-substitution in hematite. For maghemite, the SSA increased with isomorphic substitution, rising up to 5.9 mol% Al; for hematite, SSA increased linearly. SSA decreased with heating time, regardless of isomorphic substitution.