Mineralogical Characteristics of Poorly Crystallized Precipitates

Chapter 14

Mineralogical Characteristics of Poorly Crystallized Precipitates Formed by Oxidation of Fe in Acid Sulfate Waters 2+

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E. Murad , U.Schwertmann ,Jerry M.Bigham ,and L. Carlson Downloaded by MICHIGAN STATE UNIV on July 17, 2013 | http://pubs.acs.org Publication Date: December 20, 1993 | doi: 10.1021/bk-1994-0550.ch014

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Lehrstuhl für Bodenkunde, Technische Universität München, D—85350 Freising-Weihenstephan, Germany Department of Agronomy, Ohio State University, Columbus, OH 43210-1086 Department of Geology, University of Helsinki, SF-00171 Helsinki, Finland 2

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Identification of the minerals produced by the oxidation of sulfides is commonly complicated by the poor crystallinity of the products. A combination of selective chemical extraction procedures and instrumental techniques such as X-ray diffraction and Mössbauer spectroscopy can enable an unequivocal identification of even the most poorly crystalline minerals to be made, and in favorable cases can also provide information on particle dimensions from magnetic blocking temperatures or hyperfine fields at 4.2 K. The products of sulfide oxidation are controlled by the elements released and the environmental conditions, which favor specific mineral assemblages by controlling the pathways of mineral formation and the kinetics of competing reactions. Well crystallized minerals formed as result of sulfide oxidation can usually be readily identified using standard mineralogical techniques such as X-ray diffraction (XRD) or electron microscopy. Normally, mineral formation takes place at very low pH values resulting from the formation of large amounts of H S0 . If, however, freshwater enters the system, higher pH values and lower S0 concentrations ensue. For Fe , this leads to a pronounced increase in the rate and degree of hydrolysis, causing rapid precipitation of Fe oxides and oxyhydroxides of extremely small particle size (< 10 nm). Such particles are difficult to characterize, especially if they are constituents of assemblages of complex mineralogy. Typical examples for such minerals are the poorly crystallized iron oxide ferrihydrite, which has often erroneously been referred to under various names such as "amorphous iron oxide" or "ferric hydroxide", and an iron oxyhydroxysulfate recently identified as a constituent of numerous precipitates of acid sulfate-rich waters all over the world (1). This phase is hereafter referred to as "mine drainage mineral", abbreviated MDM. Identification of these poorly crystalline products of sulfate oxidation may be possible using XRD carried out by slow step-scanning, where necessary combined with 2

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Current address: Bayerisches Geologisches Landesamt, Concordiastrasse 28, D—96049 Bamberg, Germany 0097-6156/94/0550-0190$06.00/0 © 1994 American Chemical Society In Environmental Geochemistry of Sulfide Oxidation; Alpers, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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selective dissolution procedures and diffraction line profile analysis. Fe Mossbauer spectroscopy has also proven useful for the characterization of iron-bearing minerals of small particle size, especially if these are only minor constituents of a sample. In this paper we show how data acquired by these techniques can be used to characterize selected minerals from acid sulfide oxidation environments, and thus lead to an improved delineation of the processes involved.

Downloaded by MICHIGAN STATE UNIV on July 17, 2013 | http://pubs.acs.org Publication Date: December 20, 1993 | doi: 10.1021/bk-1994-0550.ch014

Materials and Methods We collected ochreous precipitates resulting from the bacterial oxidation of Fe^q) in sulfate-rich waters draining abandoned mine works and acid sulfate soils from a variety of localities around the world. This collection of samples was complemented by material synthesized bacterially using Thiobacillus ferrooxidans and abiotically in the presence of sulfate, selenate and chromate in the laboratory. Data on complex samples and samples that have been selected on the basis of purity (i.e. as monomineralic as possible) is given in this paper. XRD was performed either on a Philips PW1050/70 instrument using C o i ^ radiation and a graphite monochromator or on a Huber System 600 Guinier diffractometer using C o K radiation. Random powder samples were counted in steps of 0.02° 2Θ for 10 to 50 s per step. Where necessary, silicon was added as an internal standard and peaks were computer-fitted with a combination of Gaussian and Lorentzian lines (Voigt profile) to obtain precise d values and half widths. Fourier-transform infrared absorption data were obtained from 5 mg of powdered sample mixed with 195 mg KBr. Diffuse reflectance (DRIFT) spectra were collected as the average of 100 sample scans at 1 cm" resolution using a Mattson-Polaris FTIR spectrometer equipped with a Hanick "Praying Mantis" cell. Fe Mossbauer spectra were taken at various temperatures between 295 and 4.2 Κ using a Co/Rh source. The samples were placed in plastic holders to give Fe concentrations between 6 and 12 mg/cm . Selected samples were also studied under applied magnetic fields up to 9 T. Spectra were collected in different velocity ranges between ± 2 and ± 13.6 mm/s until sufficiently good statistics had been attained. The spectra were fitted with Lorentzian lines or distributions of Lorentzians. Isomer shifts are given relative to the centroid of the room-temperature spectrum of metallic Fe. al

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Individual Minerals Ferrihydrite. The nominal formula of ferrihydrite is given as Fe H0 -4H 0 (2). Although the composition may vary somewhat as a function of particle size, recent work (3) has shown that the variation in composition is only in part related to the adsorption of water on particle surfaces. Ferrihydrite is reddish-brown in color (Munsell hue 5 YR -7.5 YR), and usually consists of minute, aggregated spheres 3 to 6 nm in size. It has a high specific surface area between 200 and 500 m /g, and is — in contrast to goethite and hematite — readily soluble in ammonium oxalate at pH 3 (4). Because of the small particle size, XRD diagrams of ferrihydrite consist of only six to two broad, weak bands. The characterization of ferrihydrite by XRD is complicated by the fact that the broad 5

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In Environmental Geochemistry of Sulfide Oxidation; Alpers, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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ENVIRONMENTAL GEOCHEMISTRY OF SULFIDE OXIDATION

bands go hand in hand with very low count rates. A typical ferrihydrite from an acid drainage environment has an XRD diagram (shown in Figure 1) that consists of four such bands located at 0.25, 0.22, 0.17 and 0.15 nm. When counted for 20 s per step, this ferrihydrite appeared to be to be monomineralic. A prolonged counting time of 50 s, however, revealed additional peaks at 0.42 and 0.34 nm (Figure 1), showing up the presence of minor amounts of goethite and MDM. These admixtures imply a different mineral assemblage, and thus could indicate the crossing of a stability field boundary; they will naturally also contribute to other physical (e.g. magnetic) properties of this sample. Because of small particle size, natural ferrihydrite is superparamagnetic at room temperature and may remain superparamagnetic down to temperatures as low as 23 Κ (5). In the superparamagnetic state, Mossbauer spectra of ferrihydrite consist of a broad Fe doublet of non-Lorentzian shape. The high line widths indicate a variability of atomic environments, and the spectra consequently have to be fitted with distributions of quadrupole-split doublets rather than with single Lorentzian doublets. The maxima of the distributions, which are higher than those of almost all other (superparamagnetic iron oxides, shift from 0.62 to 0.80 mm/s with decreasing particle size, and the distribution half-widths simultaneously increase from about 0.70 to 0.86 mm/s. Since the quadrupole splitting of high-spin Fe in octahedral coordination is directly proportional to the site distortion, this shows that the site distortion and the variation of nuclear environments increase with decreasing particle size. For ferrihydrites of different particle size, magnetic blocking temperatures (50 % magnetic order) between 28 and 115 Κ have been described (5), corresponding to average particle sizes of 3 and 5 nm, respectively. Superparamagnetic particles possess uncompensated surface magnetic moments, and will therefore order magnetically above the magnetic blocking temperature under an applied magnetic field (Figure 2). The magnetic hyperfine fields observed at 4.2 Κ vary between about 50 Τ for ferrihydrites with 6 XRD bands and 47 Τ for such with 2 XRD bands. Because of an increasing randomization of the angle between the magnetic hyperfine field and the electric field gradient, the quadrupole interaction of magnetically ordered ferrihydrites decreases with decreasing particle size, and approaches zero for 2-XRD-band ferrihydrites. 3+

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M D M . The major component of all precipitates formed between pH 2.5 and 4.0 is a poorly crystallized, yellowish-brown (Munsell 9-10 YR) iron oxyhydroxysulfate with an ideal chemical formula of Fe O (OH) S0 (1). This component has a fibrous morphology, a high specific surface area between 175 and 225 m /g, and is readily soluble in ammonium oxalate at pH 3. The XRD diagram of MDM consists of 8 broad bands for d > 0.15 nm (Figure 1), and indicates a tunnel structure akin to that of akaganéite, p-FeCKOH)^^ (nominally β-FeOOH). The sulfate is believed to be located in the mentioned tunnels, taking over the role of chloride in akaganéite, but sharing (two) oxygen atoms with neighboring iron atoms. The MDM scan shown in Figure 1 has an additional peak at 0.31, which probably originates from a minor admixture of jarosite. Mossbauer spectra of MDM taken at room temperature consist, in contrast to those of ferrihydrite, of a broad asymmetric doublet, the low-velocity line of which has a higher dip and is narrower than the high-velocity line (6). To account for the g

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Figure 1. X-ray diffraction diagrams of goethite, ferrihydrite and MDM from aciddrainage environments. Marked foreign mineral admixtures are jarosite (Js), goethite (Gt) and MDM; scale bars correspond to 400 counts per second.



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asymmetry, the spectra were fitted with two alternative quadrupole-splitting distribution models that imply either a linear correlation between the quadrupole splitting and the isomer shift, or the existence of two discrete quadrupole-splitting distributions with different isomer shifts. Both fitting models yield average quadrupole splittings of maximum probability of about 0.65 mm/s. The distribution half-widths of MDM (0.46 to 0.52 mm/s for one-distribution fits) were considerably narrower than those of even the best crystallized ferrihydrite (~ 0.70 mm/s), indicating a smaller variation of nuclear environments in MDM than in any ferrihydrite. Mossbauer spectra show that both natural and synthetic MDM have an average magnetic ordering temperature of 75 K, although the onset of magnetic order is smeared out over about 10 K. The ordering temperature of MDM thus is lower than the Néel temperature of any iron oxide. The application of an external magnetic field of 6 Τ between 75 and 90 K, however, did not induce magnetic order (Figure 3). This shows the absence of superparamagnetic relaxation, and indicates that the ordering temperature of 75 Κ is a genuine Néel temperature, and thus that a distribution of Néel temperatures causes the magnetic ordering temperature to be smeared out. The spectra of magnetically ordered MDM are broad and asymmetric, the first (low-velocity) line having a higher dip and being narrower than the sixth line. Therefore fits of hyperfine field distributions that imply a variation of quadrupole splitting as a function of the hyperfine field, or two discrete hyperfine field distributions with different isomer shifts had to be used. Independently of the fit model, the hyperfine fields of maximum probability averaged 45.4 Τ at 4.2 K. This is lower by about 1.5 Τ than those of even the most poorly crystallized ferrihydrite. The asymmetry of the Mossbauer spectra of both paramagnetic and magnetically ordered MDM and the range of magnetic ordering temperatures indicate a diversity of iron sites. The most obvious explanation is that the sulfate incorporated in the crystal structure is responsible for this diversity. Different locations of iron atoms relative to sulfate groups in the structure would cause the development of inequivalent iron sites. The sulfate probably also inhibits magnetic exchange interactions between neighboring iron atoms and thus may be responsible for the low magnetic ordering temperature and hyperfine fields. MDM can also be synthesized in the presence of selenate and chromate, whereas arsenate and phosphate inhibit the formation of MDM. This is probably due to the fact that arsenate and phosphate have a higher affinity to iron than sulfate and selenate, which prevents the formation of a phase with iron in an ordered, octahedrally coordinated structure (to be published elsewhere). Room-temperature Mossbauer spectra of MDM synthesized in the presence of chromate no longer show the characteristic asymmetry of sulfate MDM. An exchange of arsenate for sulfate can be effected by treating MDM synthesized with sulfate with 0.04 M Na^AsC^ for 24 hours. The effects of this treatment are particularly evident in infrared spectra, in which the SO " bands have almost completely vanished (Figure 4). This is accompanied by a marked attenuation of the (200, 111) XRD peak at 0.48 nm, indicating that the anion exchange has also affected the structure. 2

Goethite. In contrast to ferrihydrite, which is a common mineral in environments where Fe is rapidly oxidized, and MDM, which is specific to acid mine drainage 2+



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Figure 2. Mossbauer spectra of a ferrihydrite from an acid drainage environment taken at 78 Κ with and without an externally applied magnetic field.

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Figure 3. Mossbauer spectra of MDM taken at 75 Κ with and without an externally applied magnetic field.




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environments rich in sulfate, goethite, α-FeOOH, is a very common mineral in a variety of different terrestrial and aquatic environments, for example in soils or sediments. XRD lines of a goethite from acid mine drainage, shown in Figure 1, have widths at half height of about 1° 2Θ for CoK^, corresponding to a particle size of about 15 nm. In contrast to other goethites, which usually have only a very low solubility in ammonium oxalate at pH 3, goethite-rich samples from mine drainage environments have higher ratios of oxalate-soluble to total iron of about 0.3. This oxalate solubility may result in part from the admixture of ferrihydrite and/or MDM, minor amounts of which are difficult to detect by XRD, but which show up clearly in Mossbauer spectra taken at 4.2 Κ (Figure 5). Although well crystallized, pure goethite has a Néel temperature of 400 K, room-temperature Mossbauer spectra of goethites from acid drainage environments consist of a doublet. This is probably a result of small particle size, leading to superparamagnetic relaxation. At 4.2 Κ all studied goethites from acid drainage environments were magnetically ordered, with magnetic hyperfine fields that were, however, noticeably lower than that of pure, bulk goethite (50.6 T). In the absence of other effects that could have an influence on the magnetic properties (for example aluminum-for-iron substitution and/or excess water content), this may be attributed to small particle size. A hyperfine field of 49.88 T, for example, was derived from the spectrum shown in Figure 5. This would correspond to a mean crystallite diameter of 11 nm in the [lll]-direction (7), and compares favorably with the crystal dimensions observed by XRD. Complex Natural and Synthetic Assemblages Ochreous precipitates of complex mineralogy, collected from streams affected by acid mine drainage in the U.S.Α., Australia, and Bosnia (Yugoslavia), were studied by XRD and by Mossbauer spectroscopy at temperatures between 295 and 4.2 Κ before and after treatment with acid ammonium oxalate. Figure 5 shows Mossbauer spectra taken at 4.2 Κ of a precipitate from a stream in southeastern Ohio. Published work indicated this precipitate to consist of goethite and an oxalate-soluble ferrihydrite-like material (8). The spectrum of the untreated sample consists of two sextets, which can be assigned to goethite and MDM in a proportion of about 1:2 (assuming equal recoil-freefractions),and a central doublet with a relative area of about 3%. After a short oxalate treatment of 15 minutes duration (spectrum not shown), the MDM sextet had disappeared. The spectrum now comprised only a goethite sextet and the paramagnetic doublet with a relative area of 4 %. After an oxalate treatment of 2 hours the spectrum still consisted mainly of a goethite sextet, but the relative area of the paramagnetic component had increased to 12 %. This increase in the doublet intensity indicates that the extended oxalate treatment has resulted in a partial removal of goethite (8). The magnetic hyperfine fields of goethite in the untreated sample (49.96 T) and that remaining after the 15-minute (49.99 T) and the 2-hour oxalate treatments (49.69 T) were nevertheless identical within experimental error. Thus the oxalate treatment did not preferentially dissolve goethite of smaller particle size, which should have lower magnetic hyperfine fields. The quadrupole interaction (-0.24 ± 0.01 mm/s) was the same throughout for the component identified



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Figure 5. Mossbauer spectra taken at 4.2 Κ of material from an acid mine drainage containing MDM and goethite before (top) and after (bottom) a 2-hour treatment with ammonium oxalate at pH 3. Subspectra indicated in the untreated sample are goethite (broken line) and MDM (dotted line).


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as goethite. This shows that ferrihydrite, which may have a similar hyperfine field but a different quadrupole interaction (between -0.02 and -0.10 mm/s), was absentfromall samples. The fact that the relative area of the paramagnetic resonance did not increase significantly following the first oxalate treatment, but showed a strong increase following the second treatment, indicates that this is due to at least two different components. The first of these is as readily soluble in oxalate as MDM, whereas the second is not (or less) soluble in oxalate, and could be due to Fe in clay minerals. XRD showed the complex precipitates from the other mentioned mine drainage environments, New South Wales (Australia) and Bosnia (Yugoslavia), to be made up mainly of MDM, with subordinate admixtures of jarosite in the former and of jarosite and poorly-crystalline goethite in the latter sample. Mossbauer spectra taken at 4.2 Κ confirmed the presence of MDM in both samples, but because jarosite and poorlycrystalline goethite have similar parameters, a distinction of these in the untreated Yugoslavian sample was not possible. A Mossbauer spectrum of the Australian sample taken at 4.2 Κ after oxalate treatment showed that MDM had been quantitatively removed, and that only jarosite was left over. To complement our work on natural precipitates of acid mine drainage and to simulate the conditions under which MDM and goethite coexist in nature, we undertook a long-term laboratory experiment. Synthetic MDM, prepared as described in (1), was dialyzed to remove excess sulfate from the solution and stored in distilled water for up to 1000 days. Aliquots of the suspension were collected at intervals and studied by chemical analysis and XRD. Preliminary results showed that traces of goethite could be detected after 72 days, and that conversion of MDM to goethite was complete after about 200 days. During this process the pH and Fe concentration decreased, whereas the sulfate concentration increased, supporting the reaction


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This reaction indicates that MDM is metastable with respect to goethite; it will transform to goethite within a period of several months even in the presence of 2 m molA S0 . Further experiments to determine the conditions under which the conversion of MDM to goethite is inhibited, with the purpose of arriving at a (conditional) solubility product of MDM, are presently under way. 4

Conclusions Poorly crystallized minerals formed in acid mine drainage environments and the pathways of their transformations can be characterized by combinations of modern mineralogical techniques such as X-ray diffraction using step-scanned and computerfitted data, Fourier-transform infrared spectroscopy, and Mossbauer spectroscopy. In this paper we give examples for the characterization of individual minerals by these methods. Further examples are given to show how a combination of these techniques with selective dissolution procedures, for example acid oxalate extraction, also enables the characterization of these minerals to be made in natural and synthetic assemblages of complex mineralogy from acid sulfate oxidation environments.


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Addendum. After submission of this article, the phase designated herein as MDM was approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association as a new mineral with the name "schwertmannite". Acknowledgments


We are indebted to L.H. Bowen (North Carolina State University, Raleigh, N.C.) for taking some of the low-temperature Mossbauer spectra of MDM and to J.D. Cashion (Monash University, Melbourne) and F.E. Wagner (Technische Universitat Munchen, Garching) for providing access to liquid-helium Mossbauer facilities. This study has been supported by the Deutsche Forschungsgemeinschaft and the Alexander von Humboldt Stiftung. Literature Cited 1. Bigham, J.M.; Schwertmann, U.; Carlson, L.; Murad, E. Geochim. Cosmochim. Acta 1990, 54, 2743-2758. 2. Towe, K.M.; Bradley, W.F. J. Colloid Interf. Sci. 1967, 24, 384-392. 3. Stanjek, H.; Weidler, P.G. Clay Minerals 1992, 27, 397-412 4. Schwertmann, U.; Fischer, W.R. Geoderma 1973, 10, 237-247. 5. Murad, E.; Bowen, L.H.; Long, G.J.; Quin, T.G. Clay Minerals 1988, 23, 161-173. 6. Murad, E.; Bigham, J.M.; Bowen, L.H.; Schwertmann, U. Hyperfine Interact. 1990, 58, 2373-2376. 7. Murad, E.; Schwertmann, U. Clay Minerals 1983, 18, 301-312. 8. Brady, K.S.; Bigham, J.M.; Jaynes, W.F.; Logan, T.J. Clays Clay Minerals 1986, 34, 266-274. RECEIVED March 16,

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Mineralogical Characteristics of Poorly Crystallized Precipitates

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