Dehydroxylation Of Goethite Research Articles

The effect of pre-roasting on atmospheric sulfuric acid leaching of saprolitic laterites

The effect of pre-roasting on the atmospheric sulfuric acid leaching behavior of saprolitic laterites, obtained from Indonesia, was investigated. The phase transformation of laterite minerals roasted at different temperatures was investigated with X-ray powder diffraction (XRD). The TG-DTA analysis showed that there were two phase transformation processes for that dehydroxylation of goethite (FeO(OH)) and lizardite (Mg3(Si2O5)(OH)) at roasting temperatures of 260.8 °C and 602.2 °C, respectively. Roasting at temperatures up to 600 °C improved the metal extraction, because roasting at that temperature transforms the main mineral structures of lizardite and goethite, allowing a rapid interaction between the leachant and the entrapped nickel species during leaching. The transformation of the mineral structures caused by roasting is the decisive factor influencing the metal extraction, followed by changes to the specific surface area. These results contribute to the development of more efficient atmospheric acid leaching process for laterites.

Study of the Effects of Heating on Organic Matter and Mineral Phases in Limestones

Color-induced changes in stones due to heating can be useful in determining the degree of degradation generated in their components and structure. We evaluate the effect of heating in three types of limestones, useful in building and restoration, corresponding to biomicrite (BC), biosparite (BS), and intramicrite (IM), in the range of temperature from 100°C to 600°C. Our analyses indicate that the strongest changes in reflectance occur in the samples of intramicrite, followed by biomicrite, being biosparite showing the smallest ones. Additionally, IM samples suffer the largest modifications in effective porosity due to the thermal treatment; in contrast, BS and BC samples show smaller temperature-induced alterations associated with smaller changes in reflectance. Moreover, we reveal that most of the thermally induced variations are associated, in this range of temperature, to the dehydroxylation of goethite that transforms into hematite, as well as by the burning and partial release of the organic matter present in the rock. Colorimetry analyses show that the sequence of thermally induced changes can be followed by the evolution of the three coordinates, L∗(lightness), a∗(red-green), and b∗(yellow-blue), of the CIE 1976 L∗a∗b∗space.

Mineralogical, Geochemical and Geotechnical Study of BCV 2017 Bentonite—The Initial State and the State following Thermal Treatment at 200 °C

Bentonites are considered to be the most suitable materials for the multibarrier system of high-level radioactive waste repositories. Since BCV bentonite has been proved to be an ideal representative of Czech Ca-Mg bentonites in this respect, it has been included in the Czech Radioactive Waste Repository Authority (SÚRAO) buffer and backfill R&D programme. Detailed knowledge of processes in the material induced by thermal loading provides invaluable assistance regarding the evolution of the material under repository conditions. Samples of both original BCV 2017 bentonite and the same material thermally treated at 200 °C were characterised by means of chemical analysis, powder X-ray diffraction, infrared spectroscopy, thermal analysis, cation exchange capacity, specific surface area (BET) measurements, the determination of the swell index, the liquid limit, the swelling pressure and water retention curves. The smectite in BCV 2017 bentonite comprises Ca-Mg montmorillonite with a significant degree of Fe3+ substitution in the octahedral sheet. Two main transformation processes were observed following heating at 200 °C over 27 months, the first of which comprised the dehydration of the montmorillonite and the subsequent reduction of the 001 basal distance from 14.5 Å (the original BCV 2017) to 9.8 Å, thus indicating the absence of water molecules in the interlayer space. The second concerned the dehydration and partial dehydroxylation of goethite. With the exception of the dehydration of the interlayer space, the PXRD and FTIR study revealed the crystallochemical stability of the montmorillonite in BCV 2017 bentonite under the selected experimental conditions. The geotechnical tests indicated no major changes in the mechanical properties of the thermally treated BCV 2017 bentonite, as demonstrated by the similar swelling pressure values. However, the variation in the swell index and the gradual increase in the liquid limit with the wetting time indicated a lower hydration rate. The retention curves consistently showed the lower retention capacity of the thermally treated samples, thus indicating the incomplete re-hydration of the thermally treated BCV 2017 exposed to air humidity and the difference in its behaviour compared to the material exposed to liquid water.

The role of thermal analysis in the development of high-iron-content kaolinite-based photocatalysts

Dynamic and controlled-rate thermogravimetric analyses have been carried out on acid-treated (11 and 5.8 M HCl), high-iron-content kaolinites as potential photocatalysts. The mineral contaminants were determined by XRD, while the defect sites of reduced coordination number obtained by surface treatments were identified with 27Al MAS NMR spectroscopy. Upon heating, water is evolved from the surface-treated samples in three main stages: (1) removal of adsorbed water up to ~ 200 °C, (2) goethite dehydroxylation between 200 and 350 °C and (3) dehydroxylation of the clay in the 300–700 °C temperature range. Identification of water released from the above mass loss steps is difficult due to the significant overlap of steps 2 and 3, as well as to the presence of coordinated water at broken edges and defect sites (–OH2+ groups). As a result, the thermal behavior of surface-treated kaolinites should be taken into account both in the preparation of hybrids/composites and in the acid–base characterization of the catalytic surface.

Effects of Cu doping on the microstructural, thermal, optical and photocatalytic properties of α-FeOOH and α-Fe2O3 1D nanoparticles

Nanostructural morphology and cation (divalent or tetravalent) doping are known to be important factors for photocatalytic and photoelectrochemical activities of iron oxides. Modifications of iron oxides in an appropriate way could significantly improve these activities. In the present work the effects of Cu doping on the microstructural, thermal, optical and photocatalytic properties of goethite (α-FeOOH) and hematite (α-Fe2O3) 1D nanoparticles were investigated. Goethite and Cu-doped goethite 1D nanoparticles were synthesized by precipitation in a highly alkaline medium. Hematite and Cu-doped hematite 1D nanoparticles were prepared by calcination of goethite samples at 500 °C. Cu2+-for-Fe3+ substitution in goethite and hematite was confirmed by determining the change in unit cell size using X-ray powder diffraction and by measuring the hyperfine magnetic field reduction using Mössbauer spectroscopy. An increased Cu2+-for-Fe3+ substitution in goethite caused a gradual elongation and narrowing of nanorods with the formation of nanoneedles at 3 or 4 mol% Cu. The shape and size of nanoparticles were not changed by thermal transformation of goethite to hematite. Thermal analysis showed a lower temperature of goethite dehydroxylation upon increased Cu doping. Cu2+-for-Fe3+ substitution in goethite and hematite caused increased absorption of visible and near-IR radiation beyond the absorption edge, as well as a slight decrease in direct and indirect optical band gaps compared with pure phases. The visible light photocatalytic activities of Cu-doped goethite and hematite nanoneedles were significantly improved compared with undoped goethite and hematite nanorods, which could be attributed to a combined effect of nanoneedle morphology and Cu doping.

Investigation into coal-based magnetizing roasting of an iron-rich rare earth ore and the associated mineralogical transformations

The factors influencing goethite to magnetite conversion during magnetizing roasting of an iron-rich rare earth ore using coal as the reductant were explored in the present paper. The effect of temperature, time and reductant addition on magnetite conversion; and the optimal conditions which resulted in complete conversion of goethite to magnetite were determined through the use of satmagan and quantitative X-ray diffraction. For this particular ore, optimal roasting conditions were determined to be a roasting temperature of 600–650°C with the addition of 10–20wt% coal in the mixture and a roasting time of 90min. The mineralogical changes occurring during magnetizing roasting were also investigated using X-ray diffraction. Magnetite formation was found to proceed by the reduction of hematite formed from the dehydroxylation of goethite. X-ray diffraction showed that further reaction of magnetite to form wüstite and fayalite had taken place under highly reducing conditions and temperatures exceeding 650°C. The primary rare earth mineral in this ore, monazite was stable at all roasting conditions whilst florencite underwent thermal decomposition resulting in an increase in the monazite content of the roasted ore.

Mineralogy and geochemistry of laterites from the Morro dos Seis Lagos Nb (Ti, REE) deposit (Amazonas, Brazil)

The Morro dos Seis Lagos niobium deposit (2897.9Mt at 2.81wt% Nb2O5) is associated with laterites formed by the weathering of siderite carbonatite. This iron-rich lateritic profile (>100m in thickness) is divided into six textural and compositional types, which from the top to the base of the sequence is: (1) pisolitic laterite, (2) fragmented laterite, (3) mottled laterite, (4) purple laterite, (5) manganiferous laterite, and (6) brown laterite. All the laterites are composed mainly of goethite (predominant in the lower and upper varieties) and hematite (predominant in the intermediate types, formed from goethite dehydroxylation). The upper laterites were reworked, resulting in goethite formation. In the manganiferous laterite (10m thick), the manganese oxides (mainly hollandite, with associated cerianite) occur as veins or irregular masses, formed in a late event during the development of the lateritic profile, precipitated from a solution with higher oxidation potential than that for Fe oxides, closer to the water table. Siderite is the source for the Mn. The main Nb ore mineral is Nb-rich rutile (with 11.26–22.23wt% Nb2O5), which occurs in all of the laterites and formed at expense of a former secondary pyrochlore, together with Ce-pyrochlore (last pyrochore before final breakdown), Nb-rich goethite and minor cerianite. The paragenesis results of lateritization have been extremely intense. Minor Nb-rich brookite formed from Nb-rich rutile occurs as broken spherules with an “oolitic” (or Liesegang ring structure). Nb-rich rutile and Nb-rich brookite incorporate Nb following the [Fe3++(Nb, Ta) for 2Ti] substitution and both contain up to 2wt% WO3. The laterites have an average Nb2O5 content of 2.91wt% and average TiO2 5.00wt% in the upper parts of the sequence. Average CeO2 concentration increases with increasing depth, from 0.12wt% in the pisolitic type to 3.50wt% in the brown laterite. HREE concentration is very low.

Synthesis, Rietveld refinement and DSC analysis of Al-goethites to support mineralogical quantification of gibbsitic bauxites

This study describes a procedure to synthesize and characterize a series of Al-goethites (varieties with Al-for-Fe substitution ranging from 9 to 28%) to serve as heat-of-reaction (dehydroxylation) standards aiming the quantification of Al-goethite in gibbsitic bauxites using differential scanning calorimetry (DSC). The goal of this study is to establish the proposed procedure as a supporting method for the comparison and validation of old and new (different methods of) mineralogical quantification methods of gibbsitic bauxites, since the methods commonly used are rather questionable in terms of precision and accuracy. Although the application of DSC in an aluminum industry setting may be unlikely due to practicality and time-of-analysis constraints, the technique may prove to be useful for validating methods currently being developed. The increasing order of Al-for-Fe substitution in planned syntheses was confirmed by XRD, XRF, DSC and SEM results, which showed that a small database of goethite dehydroxylation standard enthalpies could be established in this study. The production of standards of multiple goethite varieties is so important than one goethite standard alone, because such mixtures are thermodynamically common in nature, and therefore very common in bauxites. The elevated dilution of the Al-goethite phase in the test bauxite did not allow DSC electronic signal, which implied the need for physical separation/concentration, in this case sieving and cyclone to concentrate the goethite in the fractions below 0.15 mm.

The effects of In3+ doping on the properties of precipitated goethite

Indium-doped goethite samples were synthesized by the precipitation method at pH 12 using an organic alkali tetramethylammonium hydroxide (TMAH) and investigated using different instrumental techniques. In3+-for-Fe3+ substitution in the goethite structure was confirmed by the measurement of the unit cell expansion by XRD and the reduction of the hyperfine magnetic field by Mössbauer spectroscopy. Maximum substitution was estimated at about 8 mol%. Indium substitution in goethite also caused an increase in the size of crystallites and particles, a shift in the position of bands in FT-IR and UV–Vis–NIR spectra and a rise in goethite dehydroxylation temperature (observed by TGA-DTA).

Preparation and Characterization of Porous Hematite through Thermal Decomposition of a Goethite Concentrate

In this work, the thermal decomposition of a goethite concentrate from an iron ore concentration plant was studied in order to develop an economic process of preparing porous hematite as an effective adsorbent for the treatment of wastewater containing heavy metals. This study was performed by means of the measurements of X-ray diffraction (XRD), physical adsorption, and thermal gravimetric analyzer (TGA) for the decomposition process and the characterization of the prepared porous hematite. The experimental results showed that the prepared porous hematite mainly consisted of micropores, and considerably increased the specific surface area and pore volume. The mechanism might be attributed to the dehydration and dehydroxylation of goethite in the thermal treatment. It has been found that calcinating temperature was the predominant factor in the preparation of the porous hematite. Also, residence time in the calcinations greatly affected the pore characterization of the porous hematite.

Reflectance spectroscopy of soils post-heating—Assessing thermal alterations in soil minerals

We examined the potential of reflectance spectroscopy across the VIS-NIR–SWIR spectral region to identify, ex post facto, the thermal processes that occur in heated soils and minerals, and thus to monitor mineral changes in the soil. We systematically investigated five Israeli soils and six standard soil minerals using a controlled furnace from 150°C to 1000°C (at 50°C intervals). Samples which had been heated and cooled (soils and minerals) were measured for reflectance across the 350–2500nm spectrum using an ASD Field-Pro spectrometer. To confirm the findings extracted from the spectral information, traditional analytical methods (XRD, DTA-DTG, PSD, SEM–EDS) were used. The spectral information was found to be a good indicator for thermal processes occurring in the soil based on mineralogical changes. The spectral changes across different spectral regions provided information about dehydroxylation of goethite to form hematite (VIS-NIR); clay deformation (lattice collapse) due to dehydroxylation processes (VIS-NIR–SWIR); the condition of calcite (SWIR), and indirect processes, such as re adsorption of water and rehydration of the new minerals (e.g. pyrolysis of the calcite product) (SWIR). We propose the use reflectance spectroscopy for post-thermal analysis (RS-PTA), as a valid post-heating diagnostic method for thermal processes occurring in minerals and soils. The suggested method is simple and easy to perform, and can be used in all domains (laboratory and field, point and image) to monitor heat-induced changes in soil minerals.

Thermal behaviors and growth of reduced ferronickel particles in carbon-laterite composites

The thermal behaviors of single laterite ore and graphite-laterite mixtures were investigated by thermogravimetry (TG), derivative thermogravimetry (DTG), and differential thermal analysis (DTA). Four mass loss steps maximized at about 78, 272, 583, and 826°C are observed for the laterite ore, representing the vaporization of free water, the dehydroxylation of goethite, the decomposition of serpentines, and the second dehydroxylation of serpentines, respectively. The reduction reactions of the graphite-laterite mixtures start at around 700°C and can be divided into three major temperature regions. Coal-laterite composites with an addition of 10 wt.% CaO were roasted at 1100–1350°C for 30 min, and the reduced samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results indicate that the reduction reactions proceed more completely at higher temperatures. The growth of the reduced ferronickel particles is greatly influenced by the roasting temperature. Obvious growth of the reduced ferronickel particles appears with the formation of worm-like crystals for the sample reduced at 1250°C, and spheric particles are observed for the sample reduced at 1300°C. When the reduction temperature increases to 1350°C, the reduced ferronickel particles agglomerate to ferronickel granules of 3–8 mm in diameter. The main elements in the granules include iron, nickel, chromium, carbon, and sulfur, with the content of nickel and that of iron of 9.08 wt.% and 85.21 wt.%, respectively.

Adsorption of fluoride by waste iron oxide: The effects of solution pH, major coexisting anions, and adsorbent calcination temperature

In this study, a waste iron oxide material (BT3), which is a by-product of the fluidized-bed Fenton reaction (FBR-Fenton), was thermally treated between 200 and 900°C and was used as an efficient adsorbent for the removal of fluoride ions in an aqueous system. The highest fluoride adsorption capacity occurred at the termination of the BT3 goethite dehydroxylation phase at about 300°C calcination where both the volume of nanopores formed by dehydroxylation and the specific surface area reached their maximum values. Above 300°C, BT3 transformed to the hematite phase in which fluoride adsorption capacity decreased as calcination temperature increased. On the other hand, the effect of pH on the fluoride adsorption capacity of BT3 for various initial fluoride concentrations was examined. The optimum pH value was found to be about 4. After that efficiency decreased as pH became more alkaline. Finally, coexisting anions affected the fluoride adsorption capacity of BT3 at pH 3.9±0.2 in this order: PO43−>SO42−>Cl−>NO3−.

Effect of pre-roasting on leaching of laterite

The effect of pre-roasting on leaching of the gamierite laterite ore, obtained from Yunnan province, China, was investigated in this study. The phase transformation of laterite minerals roasted at different temperatures was investigated with X-ray powder diffraction (XRD). The roasting experiment results show that there are two phase transformation processes of dehydroxylation of goethite and lizardite at roasting temperatures of 277 °C and 610 °C, respectively, which accord with the result of DTA–TG analysis. Pre-roasting of the laterite not only alters its mineralogical composition but also increases its porosity and surface area, thus making it more amenable to leaching. Compared to the leaching result of raw ore and ores roasted at different temperatures, it indicates that increasing roasting temperature up to 300 °C appears to provide the optimum nickel recovery and further heating appears to be detrimental to the nickel recovery.

Dehydroxylation and dissolution of nickeliferous goethite in New Caledonian lateritic Ni ore

Goethite is a major constituent in ferruginous soils in a lateritic nickel deposit on the Koniambo massif in New Caledonia, and dehydroxylation of nickeliferous goethite may occur naturally due to fire and during the processing of lateritic Ni ore. Goethite started to alter to hematite in 2 h of heating at 230–250 °C and had completely altered to hematite at 340 °C and higher temperatures. For most heated samples the rate of dissolution of Fe in 1 M HCl followed the cube root equation with a single straight line describing the data, however for samples where both goethite and hematite were present the data did not conform to this equation. There was an approximately 7 fold increase in the rate of dissolution for samples heated to 340 °C which can be accounted for by the increase in surface area (2-fold) due to the development of micropores and the increase of structural defects in poorly ordered hematite structures (protohematite and hydrohematite) (3.5-fold) formed by partial dehydroxylation of goethite. For heated samples Ni, Cr, Al, Co and Mn showed congruent dissolution with iron indicating uniform incorporation of these metals in the structures of the iron oxides and complete retention of these metals in crystals during the goethite to hematite transition. V and Cu showed only partial dissolution (i.e. noncongruent dissolution) indicating that some V and Cu may be associated with another less soluble mineral (e.g. ilmenite).

A combined FTIR and TPD study on the bulk and surface dehydroxylation and decarbonation of synthetic goethite

The thermal dehydroxylation of a goethite–carbonate solid solution was studied with combined Fourier-transform infrared (FTIR)–Temperature programmed desorption (TPD) experiments. The TPD data revealed dehydroxylation peaks involving the intrinsic dehydroxylation of goethite at 560 K and a low temperature peak at 485 K which was shown to be associated to the release of non-stoichiometric water from the goethite bulk and surface. The FTIR and the TPD data of goethite in the absence of adsorbed carbonate species revealed the presence of adventitious carbonate mostly sequestered in the goethite bulk. The release of carbonate was however not only related to the dehydration of goethite but also from the crystallization of hematite at temperatures exceeding 600 K. The relative abundance of surface hydroxyls was shown to change systematically upon goethite dehydroxylation with a preferential stripping of singly-coordinated OH sites followed by a dramatic change in the dominance of the different surface hydroxyls upon the formation of hematite.

Heating Fe Oxide-Rich Soils Increases the Dissolution Rate of Metals

AbstractEvidence for fire affecting the solubility of metals in Fe oxide-rich Oxisols of the Koniambo Massif of New Caledonia is presented. Acid-dissolution studies showed that Ni, Al and Cr are substituted for Fe in the structure of the Fe oxides. Thermal dehydroxylation of goethite under oxidizing conditions led to the formation of hematite and to the migration of some of these metals towards the surface of hematite crystals as indicated by their enhanced release during the early stage of dissolution. Dehydroxylation of goethite under reducing conditions led to the formation of hematite and maghemite. Nickel and Al were released preferentially during the early stages of dissolution whereas Cr was not released preferentially and may be uniformly incorporated within maghemite and hematite crystals. These results have significance to the mineral-processing industry, to geochemical exploration and to the availability of these metals to plants growing on burnt soils.

Thermal and Mineral Properties of Al-, Cr-, Mn-, Ni- and Ti-Substituted Goethite

AbstractMineralogical and thermal characteristics of synthetic Al-, Cr-, Mn-, Ni- and Ti-bearing goethites, synthesized via alkaline hydrolysis of metal-ferrihydrite gels, were investigated by powder X-ray diffraction and differential thermal analysis. Shifts in unit-cell dimensions were consistent with size of substituent metal ions and confirmed the incorporation of Al3+, Cr3+, Mn3+, Ni2+ and Ti4+ in the goethite structure. A weight loss of 6.2 wt.% for goethite containing 12.2 mol.% Ti, being significantly less than for stoichiometric goethite, is consistent with the replacement of Fe by Ti in the goethite structure coupled with the substitution of O2− ions for OH− (i.e. proton loss). These data provide the first confirmation of the direct replacement of Fe by Ti within goethite. Formation of multiple dehydroxylation endotherms for goethite containing 4.5 mol.% Al, 15.3 mol.% Mn and 12.2 mol.% Ti was not attributed to the decomposition of surface OH groups or related simply to the crystallinity of precursor goethite (‘high-a’ vs. ‘low-a’) as defined by the magnitude of a. Instead, endotherm doublet formation was associated with weight loss due to the dehydroxylation of goethite remaining after initial phase transformation to protohematite and to the evolution of OH− associated with the rapid increase in crystallite size of protohematite directed primarily along the a direction. Development of the first endotherm is due to initial dehydroxylation and transformation to protohematite. With continued heating of well ordered goethite or goethite containing moderate to high levels of substituent cations, domain growth along the a direction is delayed or inhibited to a critical point that provides enough thermal energy to enable goethite transformation to proceed to completion and for proto-hematite domain growth to occur. This results in the formation of a second endotherm. For less well ordered goethite and/or goethite containing only low levels of foreign metal cations, protohematite domain growth is not inhibited and proceeds continuously with heating to give only a single endotherm.

High-Temperature Dielectric Properties of Goethite From 400 to 3000 MHz

The dielectric properties of goethite and, in particular, the changes during the topotactic conversion of goethite to hematite were studied from room temperature to about 800 °C in the frequency range of 400 to 3000 MHz using the cavity perturbation technique. The complex permittivity, that is, both the real and the imaginary or absorptive parts (έ and ἕ), were measured under various heating regimens. In addition, thermogravimetric analysis (TGA) was performed to characterize the transformation of goethite to hematite. The Debye relaxation formalism was applied to the processes occurring both during and after the main dehydroxylation reaction to calculate the relaxation times. The Arrhenius equation for thermally activated relaxation times was used to determine the activation energies. Both the real and the absorptive parts of the permittivity exhibited a significant peak during the main part of the goethite to hematite decomposition reaction. Above the transformation, there was another, less dramatic, thermally activated increase in the permittivity values. The increase in the permittivities during the goethite to hematite transformation was attributed to the formation of quasi-free migrating radicals, for example, hydroxyl ions, oxygen ions, or hydrogen atoms, during the dehydroxylation of goethite. The derivative thermogravimetric analysis (DTGA) curve was found to be directly related to the transient values of the real and the imaginary permittivities. Higher heating rates resulted in an accelerated rate of dehydroxylation and therefore higher values of the transient permittivities. In the temperature range of 400 °C to 500 °C (i.e., just above the dehydroxylation peak), the real permittivity exhibited a varying frequency dependence, which suggested that changes were occurring in the newly formed, highly defected hematite structure, which is referred to as hydrohematite. During the reaction there were multiple relaxation processes and thus the Debye relationship could not be applied. However, at temperatures above about 500 °C, the structure stabilized, the Debye relationship was more closely followed, and the relaxation times could be determined as a function of temperature. The activation energy for the relaxation process above 500 °C was determined to be 0.47 kJ/mol.

Effect of precursor mineralogy on the thermal infrared emission spectra of hematite: Application to Martian hematite mineralization

Observations from the Thermal Emission Spectrometer (TES) instrument aboard the Mars Global Surveyor (MGS) spacecraft led to the discovery of two isolated deposits of gray, crystalline hematite located in Meridiani Planum and Aram Chaos and several smaller deposits in Valles Marineris. Several pathways for formation of these hematite deposits have been proposed, involving both aqueous and nonaqueous processes. This work uses the precise shape and position of spectral features in the Martian hematite thermal emission spectrum to constrain hematite formation pathways. Thermal infrared emission spectra, X‐ray powder diffraction patterns, Mössbauer spectra, and transmission electron microscope (TEM) photomicrographs were obtained for synthetic and natural hematite samples derived by (1) dehydroxylation of fine‐grained goethite and (2) oxidation of magnetite. Collectively, the instrumental analyses show that the mineralogical composition and crystal morphology of precursor samples and the time and temperature conditions under which decomposition to hematite occur determine the crystallinity and crystal morphology of the hematite product. Comparison of laboratory and MGS‐TES spectra shows that the Martian hematite spectra correspond closely with a synthetic hematite sample derived by pseudomorphic and topotactic dehydroxylation of goethite at 300°C. Spectra of goethite‐precursor samples dehydroxylated at higher temperatures provide increasingly poor fits. Spectra of hematite samples derived by high‐temperature thermal oxidation of magnetite are also poorer fits to the Martian hematite spectrum. Thermal emission spectra of goethites heated at lower temperatures are characterized by the spectral signatures of both hematite and goethite and are not consistent with the Martian spectra. The characteristic that distinguishes the synthetic hematite sample with the Mars‐like spectral signature from the other synthetic hematite samples is the high proportion of crystal surfaces that are crystallographic {001} faces (c faces) for the former but not the latter. The high proportion of {001} face area results because the largest surface of the lath‐shaped hematite particles is the (001) face, as determined by TEM. Thus a possible formation pathway for hematite in Meridiani Planum, Aram Chaos, and Valles Marineris is precipitation of goethite from aqueous solutions as lath‐shaped crystals, possibly as a stain, cement, and/or massive deposit, followed by pseudomorphic and topotactic dehydroxylation to hematite at temperatures below ∼300°C.