On the goethite to hematite phase transformation

Twin formation in hematite during dehydration was investigated using X-ray diffraction, electron diffraction, and high-resolution transmission electron microscopy (TEM).When synthetic goethite was heated at different temperatures between 100 and 800 °C, a phase transformation occurred at temperatures above 250 °C. The electron diffraction patterns showed that the single-crystalline goethite with a growth direction of [001] G was transformed into hematite with a growth direction of [100] H . Two non-equivalent structures emerged in hematite after dehydration, with twin boundaries at the interface between the two variants. As the temperature was increased, crystal growth occurred. At 800 °C, the majority of the twin boundaries disappeared; however, some hematite particles remained in the twinned variant. The electron diffraction patterns and high-resolution TEM observations indicated that the twin boundaries consisted of crystallographically equivalent prismatic (100), (010), and (11 � 0) planes. According to the total energy calculations based on spin-polarized density functional theory, the twin boundary of prismatic (100) screw had small interfacial energy (0.24 J/m 2 ). Owing to this low interfacial energy, the prismatic (100) screw interface remained after higher-temperature treatment at 800 °C.

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“…[12,31,32]. Figure 2 shows a TEM image of the slit-like pores generated at 250 °C, with heating provided by an electric furnace.…”

Section: Resultsmentioning

confidence: 99%

Twin formation in hematite during dehydration of goethite

et al. 2016

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“…Gualtieri and Venturelli (1999) found that decomposition of pure synthetic goethite started at 200°C to form a non-stoichiometric hematite, or protohematite (Fe 2Àx/3 (OH) 2 O 3Àx ) at ca.270°C. Gialanella et al (2010) using impure goethite (ca.35% Fe) with Mg, Si, Ca and Al, found that its transformation to hematite occurred at ca.300°C. Under water-saturated conditions the transformation can begin to occur at temperatures of <100°C, e.g.…”

Section: Implications From Experimental Resultsmentioning

confidence: 99%

Thermal origin of continental red beds in SE China: An experiment study

Jiang

Chen

Grapes

et al. 2015

Journal of Asian Earth Sciences

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“…It could be indicative of the different sites of the samples. Even if it is well known that a variety of different shades of yellow and red color could be obtained simply by heating goethite, which turns into hematite at temperatures in the range of ~260–280 °C, a previous work demonstrated that Raman spectroscopy cannot be used to differentiate heated goethite from natural hematite, in spite of being effective in making a distinction between ordered hematite from the disordered one. As a consequence of that, an interesting issue regards the feature that contributes to a great degree to PC2, arising from the loadings in the range 636–687 cm –1 .…”

Section: Resultsmentioning

confidence: 99%

The first spectroscopic analysis of Ethiopian prehistoric rock painting

Lofrumento

Ricci

Bachechi

et al. 2011

J Raman Spectroscopy

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An extensive micro-Raman spectroscopic study of prehistoric rock paintings found in Hararghe region, Ethiopia, was carried out, with the aim to evaluate the production skill of the local artist and the period of production of the discovered paintings. Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) Spectroscopy and Laser Induced Breakdown Spectroscopy (LIBS) were used as auxiliary techniques. Micro sampling were carried out on parts of red, white, black painting figures representing domestic and wild animals. The pigments used by artists were hematite for red color, calcite or gypsum for white color, and carbonaceous material for black coloration. A green pigment was also investigated; it resulted made of green earth. A consistent amount of Ca-oxalate was found particularly on red samples as well as on the white ones. Former studies attributed oxalates origin to a biological substrate attack, whereas in the present case Ca-oxalate is ascribed to the use of an organic stuff to spread properly the pigments on the substrate.Principal Component Analysis was performed on the hematite spectra; it evinced that the spectral features could be indicative of different sites and of the relative age.1These novel evaluations put into new perspective the knowledge about rock art pictorial technology of the Horn of Africa.

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On the goethite to hematite phase transformation

Cited by 162 publications

References 20 publications

Twin formation in hematite during dehydration of goethite

Twin formation in hematite during dehydration of goethite

Thermal origin of continental red beds in SE China: An experiment study

The first spectroscopic analysis of Ethiopian prehistoric rock painting

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