Thermodynamics and Crystal Chemistry of the Hematite–Corundum Solid Solution and the Fealo3 Phase
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Phys Chem Minerals (2002) 29: 515–526 Ó Springer-Verlag 2002 DOI 10.1007/s00269-002-0261-7 ORIGINAL PAPER J. Majzlan Æ A. Navrotsky Æ B. J. Evans Thermodynamics and crystal chemistry of the hematite–corundum solid solution and the FeAlO3 phase Received: 11 February 2002 / Accepted: 5 June 2002 Abstract High-temperature oxide-melt calorimetry and mated standard entropy (includingconfigurational en- Rietveld refinement of powder X-ray diffraction patterns tropy due to disorder of Fe/Al) is 98.9 J mol)1 K)1, were used to investigate the energetics and structure of giving the standard free energy of formation at 298 K the hematite–corundum solid solution and ternary phase from oxides and elements as +19.1 ± 1.8 and )1 FeAlO3 (with FeGaO3 structure). The mixingenthalpies )1144.2 ± 2.0 kJ mol , respectively. The heat capa- in the solid solution can be described by a polynomial city of FeAlO3 is approximated as Cp(T in K) ¼ 175.8 ) )1 6 2 0.5 )6 DHmix ¼ WXhem(1)Xhem) with W ¼ 116 ± 10 kJ mol . 0.002472T ) (1.958 · 10 )/T ) 917.3/T +(7.546 · 10 ) The excess mixingenthalpies are too positive to repro- T 2 between 298 and 1550 K, based on differential duce the experimental phase diagram, and excess scanningcalorimetric measurements. No ferrous iron entropies in the solid solution should be considered. was detected in FeAlO3 by Mo¨ ssbauer spectroscopy. The hematite–corundum solvus can be approximately The ternary phase is entropy stabilized and is predicted reproduced by a symmetric, regular-like solution model to be stable above about 1730 ± 70 K, in good agree- ) with DGexcess ¼ (WH TWS)XhemXcor, where WH ¼ ment with the experiment. Static lattice calculations 116 ± 10 kJ mol)1 and W 32±4Jmol)1 K)1.In S ¼ show that the LiNbO3-, FeGaO3-, FeTiO3-, and disor- this model, short-range order (SRO) of Fe/Al is ne- dered corundum-like FeAlO3 structures are less stable glected because SRO probably becomes important only (in the order in which they are listed) than a mechanical at intermediate compositions close to Fe:Al ¼ 1:1 but mixture of corundum and hematite. At high tempera- these compositions cannot be synthesized. The volume tures, the FeGaO3-like structure is favored by its en- of mixingis positive for Al-hematite but almost ideal for tropy, and its stability field appears on the phase Fe-corundum. Moreover, the degree of deviation from diagram. Vegard’s law for Al-hematite depends on the history of the samples. Introduction of Al into the hematite Keywords Hematite Æ Corundum Æ Solid structure causes varyingdistortion of the hexagonal solution Æ FeAlO3 Æ Thermodynamics network of oxygen ions while the position of the metal ions remains intact. Distortion of the hexagonal network of oxygen ions attains a minimum at the composition Introduction (Fe0.95Al0.05)2O3. The enthalpy of formation of FeAlO3 )1 from oxides at 298 K is 27.9 ± 1.8 kJ mol . Its esti- It is important to understand the geochemical similari- ties and differences between ferric iron and aluminum. Both elements are commonly incorporated in minerals and dissolved in natural waters (Anovitz et al. 1991; & J. Majzlan ( ) Æ A. Navrotsky Cornell and Schwertmann 1996). Solid Fe3+ and Al3+ Thermochemistry Facility Department of Geology phases exhibit extensive polymorphism with a number of University of California at Davis isostructural compounds. On the other hand, there are Davis California 95616, USA also crystal structures of iron oxides (e.g., akaganeite) e-mail: [email protected] and structures of aluminum oxides (e.g., gibbsite) that Fax: +1-530-752-9307 Tel.: +1-530-754-2131 have no isostructural counterpart with the other metal, although some Al substitution in the akageneite struc- B. J. Evans Department of Chemistry ture has been reported (Chenget al. 2001). Some of the University of Michigan solid solutions between isostructural Fe and Al end Ann Arbor Michigan 48109, USA members appear to be continuous (jarosite–alunite, 516 Stoffregen et al. 2000); in others the solubility is limited ± 5 K of the preset value. Two days of annealingwere sufficient to (hematite–corundum, goethite–diaspore, Cornell and produce a coarse well-crystalline product. The samples were quenched in water and dried in vacuum at room temperature. Schwertmann 1996). These samples will be referred to as the high-temperature hematite– Hematite (a-Fe2O3) is abundant in soils and sedi- corundum samples. ments (Cornell and Schwertmann 1996), and aluminum Another set of Al-hematite samples was prepared by thermal is the most common substituent in its crystal structure. decomposition of Al-goethite at 975 K overnight. The goethite The hematite–corundum solid solution was studied by samples were synthesized by followingthe procedure described by Schwertmann and Cornell (2000). Characterization, thermody- Schwertmann et al. (1979), Barron et al. (1984), De namics, and crystal chemistry of these goethite samples will be the Grave et al. (1988), and others (see Cornell and Schw- subject of a separate contribution. In this study, only the Al-hema- ertmann 1996 for more references). The experimental tite samples derived from these Al-goethite preparations have been techniques employed in these studies included X-ray studied, and are referred to as low-temperature Al-hematite samples. diffraction, infrared spectroscopy, transmission electron microscopy, thermal analysis, Mo¨ ssbauer spectroscopy, Chemical analysis and EXAFS. Mongelli and Acquafredda (1999) ana- lyzed ferruginous concretions from Italian karst bauxites The high-temperature hematite–corundum samples were mounted and found that they consist of Al-hematite (average for optical and electron microprobe examination. We found that optical microscopy in reflected polarized light is more sensitive to composition Fe0.88Al0.12O1.5) and boehmite. Grubb impurities than X-ray diffraction. The samples were polished only (1971) and Wells et al. (1989) addressed the occurrence with diamond pastes to avoid contamination of the surface layer and origin of corundum and Al-hematite in bauxitic with Al from Al2O3-based abrasives. Electron microprobe analyses rocks. were performed with a Cameca SX-50 electron microprobe, with an Mixtures of hematite and corundum are also en- acceleratingvoltageof 15 kV, a current of 10 nA, a beam size of 3+ 5 lm, and countingtime of 10 s for each analyte, with synthetic countered in industrial applications. Addition of Fe corundum and hematite as the standards. to Al2O3 precursors may enhance nucleation kinetics Low-temperature Al-hematite samples were analyzed by elec- and preserve nanocrystalline features of corundum ce- tron microprobe although it was much more difficult to obtain ramics (Polli et al. 1996). Fe O /Al O assemblies find good analyses owing to poor polish of the fine powder. The pre- 2 3 2 3 cursor Al-goethite samples were analyzed for Fe and Al by ICP- their use in catalysis for the reduction of NOx in flue AAS, and the results of the two analytical techniques were in good gases (Willey et al. 1991) and oxidation of CO and C2H4 agreement. in automotive exhaust gases (Walker et al. 1988). Al2O3- Water content of low-T Al-hematite was determined by heating rich scales (with corundum structure) are a common the samples at 1180 K in corundum crucibles overnight because Gualtieri and Venturelli (1999) have shown that hematite loses all corrosion product of Fe–Al–Cr alloys (e.g., Stott et al. excess (adsorbed and structural) water by firingat >1080 K. Re- 1998). peated firingof empty corundum crucibles showed that an error of In this study, we investigated the thermochemistry ± 0.03 mgis associated with the procedure. Approximately 20 mg and crystal chemistry of the hematite–corundum solid of sample was used in each experiment, and the samples lost at least 0.07 mgupon firing,well outside the random error quoted above. solution series and the ternary phase FeAlO3. Measured enthalpies of formation and heat capacity were com- bined with the phase diagram of Muan and Gee (1956) X-ray diffraction (XRD) to present a full thermodynamic description of the solid solution and the ternary phase. The experimental cal- XRD patterns were collected with a ScintagPAD V diffractometer orimetric data are supported by static lattice calcula- (Cu Ka radiation) with a diffracted beam graphite monochroma- tor. The high-T hematite samples were ground in a McCrone tions. In addition, the calculations allow assessment of micronizingmill to particle size 1–10 lm. The millingtime was the energetics of ordered FeAlO3 phases which cannot 2 min, as determined by particle-size measurement of a milled he- be synthesized and were not found in nature. The crys- matite sample usinga laser-scatteringparticle-size distribution tallographic data are used to explain previous observa- analyzer (Horiba LA-910). Only one corundum sample was milled to minimize abrasion load on the plastic grinding jar and agate tions of Schwertmann et al. (1979) and De Grave et al. cylinders. The low-T Al-hematite samples were not milled as they (1988). are sufficiently fine-grained. Milling significantly improves the fit of the peak shape and intensities of the X-ray lines, but the position of the XRD peaks does not shift. The lattice parameters derived from the XRD patterns of the ground and unground sample are equal. Experimental procedures The samples were backloaded on a frosted glass slide to minimize preferred orientation. GSAS software (Larson and von Dreele Sample synthesis 1994) was used to calculate the lattice parameters (and atomic positions and site occupancies for selected samples) by Rietveld The hematite–corundum solid solution samples and FeAlO3 (AL- refinement. Startingmodels for the Rietveld refinement were taken 1.6 through AL-1.18) were prepared by mixing appropriate pro- from Finger and Hazen (1978) for corundum, Blake et al. (1966) portions of reagent grade ferric and aluminum nitrate nonahydrate.