Hematite cubic crystals

Under hydrothermal conditions (150-180 °C) maghemite transforms to hematite via solution probably by a dissolution/reprecipitation mechanism (Swaddle Olt-mann, 1980 Blesa Matijevic, 1989). In water, the small, cubic crystals of maghemite were replaced by much larger hematite rhombohedra (up to 0.3 Lim across). Large hematite plates up to 5 Lim across were produced in KOH. The reaction conditions influenced both the extent of nucleation and crystal morphology. The transformation curve was sigmoidal and the kinetic data in water and in KOH fitted a first order, random nucleation model (Avrami-Erofejev), i.e. 

Light-yellow powder cubic crystal occurs in both amorphous and crystalline forms pale-yellow amorphous form converts to crystalline form on heating at higher temperatures isomorphous with hematite, Fe203 density 7.18 g/cm3 melts around 2,000°C insoluble in water amorphous form dissolves readily in mineral acids crystalline form has low solubility in acids. 

Fe(OH)2 exists as hexagonal plates as do the green rusts (Feitknecht Keller, 1950 Bernal et ak, 1959). The basic morphology of wiistite is cubic, but this compound is frequently obtained as very irregular particles. It is formed as irregular rounded crystals 20-100 (xm across by reduction of hematite with H2/H2O at 800 °C (Moukassi et al., 1984). 

An infrared spectrum is a plot of percent radiation absorbed versus the frequency of the incident radiation given in wavenumbers (cm ) or in wave length ( xm). A variation of this method, diffuse reflectance spectroscopy, is used for samples with poor transmittance, e.g. cubic hematite crystals. Increased resolution and sensitivity as well as more rapid collection of data is provided by Fourier-transform-IR (FTIR), which averages a large number of spectra. Another IR technique makes use of attenuated total reflectance FTIR (ATR-FTIR) often using a cylindrical internal reflectance cell (CIR) (e.g. Tejedor-Tejedor Anderson, 1986). ATR enables wet systems and adsorbing species to be studied in situ. 

Cubic and platelet hematite particles are produced by transformation from other particles in aqueous solutions. In this procedure, preformed particles are recrystallized from other precipitates into their hnal forms. Large cubic-like hematite particles were produced through conversion of previously generated a-FeOOH in acidic solutions of HCl at 373 K. Plate-like hematite particles were formed from a-FeOOH or Fe(OFl)3 under strong basic conditions at elevated temperatures. " There was a critical temperature for each alkaline concentration in the hydrothermal transformation. The platelets produced were single crystals with (001) flat planes. ... 

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