Crystal structure, thermodynamic properties, and ...

133Structure and properties of bukovskýiteJournal of Mineralogical and Petrological Sciences, Volume 107, page 133─148, 2012

doi:10.2465/jmps.110930J. Majzlan, [email protected] Corresponding author

Crystal structure, thermodynamic properties, and paragenesis of bukovskýite,

Fe2(AsO4)(SO4)(OH)·9H2O

Juraj Majzlan*, Biljana Lazic**, Thomas Armbruster**, Michel B. Johnson***, Mary Anne White***, Robert A. Fisher†, Jakub Plášil§, Jan Loun§,

Radek Škoda§ and Milan Novák§

*Institute of Geosciences, Burgweg 11, Friedrich-Schiller University, D-07749 Jena, Germany **Mineralogical Crystallography, Institute of Geological Sciences, University of Bern,

Freiestrasse 3, CH-31012 Bern, Switzerland ***Department of Chemistry and Institute for Research in Materials, Dalhousie University,

Halifax, NS B3H 4R2, Canada †MSD, Lawrence Berkeley National Laboratory, University of California at Berkeley,

Berkeley, California 94720, USA §Department of Geological Sciences, Faculty of Science, Masaryk University,

Kotlářská 2, CZ-611 37, Brno, Czech Republic

Bukovskýite is a relatively rare secondary ferric arsenate-sulfate. At the type locality near the municipality of Kutná Hora (Czech Republic), it is the main secondary mineral in the medieval dumps, where it occurs in enor-mous amounts and forms nodules of prodigious dimensions. We investigated the mineral bukovskýite and the type locality in detail to understand the abundance of the mineral at this locality. The crystal structure of bu-kovskýite was solved for bukovskýite crystals from Großvoigtsberg (Germany) and found to be of the space group P1 with a final R factor of 5.08% from 2403 reflections. The lattice parameters at room temperature are a = 7.549(1) Å, b = 10.305(1) Å, c = 10.914(2) Å, α = 115.136(3)°, β = 99.798(3)°, and γ = 92.864(3)°. The struc-ture consists of octahedral-tetrahedral Fe-arsenate chains. Sulfate tetrahedra are bonded to the chains and free H2O molecules via a complicated network of hydrogen bonds. Calorimetric measurements (acid-solution calo-rimetry at T = 298.15 K and relaxation calorimetry yielded heat capacities from T = 0.4 K to 300 K) gave an en-thalpy of formation of −4742.4 ± 3.8 kJ·mol−1 and standard entropy of 615.2 ± 6.9 J·mol−1·K−1. A combination of these values gives a Gibbs free energy of formation of −3968.9 ± 4.3 kJ·mol−1 and aqueous solubility product (log K) of −30.627. Bukovskýite is metastable with respect to scorodite; if scorodite is not considered in the thermodynamic calculations, a stability field of bukovskýite appears at low pH and high sulfate and arsenate ac-tivity. Field observations showed that bukovskýite occurs in dumps where the space between the rock fragments is filled by clays. Bukovskýite crystallizes from Fe-As-S-rich gels that replace Si-Al gels. The exact mecha-nisms that control the entire process are not clear but will be the subject of further studies. We presume that the clays play an important role in creating microenvironments where the activity of the components needed for bu-kovskýite crystallization remains high for a long time. Bukovskýite is then an intermediate step in the conver-sion of the unstable gels to the stable assemblage of scorodite and iron sulfates.

Keywords: Bukovskýite, Crystal structure, Thermodynamics, Paragenesis

INTRODUCTION

Bukovskýite is a relatively rare secondary ferric arsenate-sulfate, described for the medieval dumps of the Kaňk de-

posit near the town of Kutná Hora, Czech Republic, by Novák et al. (1967). Although it was described for several other localities (see below), Kutná Hora remains the only site where bukovskýite occurs in massive quantities (Fig. 1), essentially dominating the secondary mineral assem-blage in some dumps. An association similar to that in

134 J. Majzlan, B. Lazic, T. Armbruster, M.B. Johnson, M.A. White, R.A. Fisher, J. Plášil, J. Loun, R. Škoda and M. Novák

Kutná Hora, with bukovskýite, scorodite, kaňkite, zýkaite, and pitticite was reported from Munzig in Saxony (Ger-many) (Hyršl and Kaden, 1992). As a product of the weathering of löllingite (FeAs2) and associated sulfides, bukovskýite was described in Přebuz (Czech Republic) in association with scorodite, arsenolite, coquimbite, sulfur, kaatialaite, and pharmacosiderite (Šrein et al., 1999), al-though the identification of the mineral was later ques-tioned by Filippi (2004). Acicular crystals of bukovskýite, which were also used in this study, were described for the Christbescherung mine near Großvoigtsberg in Saxony, Germany (Witzke and Hocker, 1993). Bukovskýite occurs in association with halotrichite-pickeringite, gypsum, scorodite, copiapite, melanterite, and jarosite in the lake pit of the former mine Getchell in Nevada (USA) (Bowell and Parshley, 2005). No other details about its paragenesis or formation were reported. Bukovskýite was reported in a property wall in SW England built from heavily weath-ered arsenopyrite-rich (>50% arsenopyrite) ore (Power et al., 2009). Bukovskýite was also found at the Golden Point mine in New Zealand, where it occurs together with scorodite and As-rich hydrous ferric oxide (Haffert et al., 2010). It forms cement and veinlets in the interior of the waste dump, where the secondary minerals at the surface of the dumps are dominated by scorodite. Those authors argue that bukovskýite and zýkaite originated as weather-

ing products where sulfate concentrations are elevated and mining waste remains moist most of the time. Other known sites at which bukovskýite occurs are the aban-doned mines near the Unterer Rotgüldensee lake near Sal-zburg (Austria) (Paar et al., 1993), dumps of the mines in Laurion (Greece) (Baumgärtl and Burow, 2002), the Kirki (Greece) high-sulfidation deposit (Triantafyllidis and Skarpelis, 2006), and Ashanti region in Ghana (Smedley, 1996). Bukovskýite can be formed during biologically in-duced mineralization, either during bacterial oxidation of gold ores (São Bento Mine, Brasil, Márquez et al., 2006) or in ferruginous bacterial accretions, morphologically

Figure 1. Czech mineralogist Petr Pauliš with a large bukovskýite nodule in Kutná Hora. Photograph by František Novák.

Figure 2. A secondary electron (SE) microphotograph of bu-kovskýite from Großvoigtsberg.

Table 1. Parameters for X-ray data collection and crystal-structure refinement

135Structure and properties of bukovskýite

similar to stromatolites (Carnoulès Pb-Zn mine, Gard, France, Leblanc et al., 1996).

The crystal structure of bukovskýite was unknown as yet. Novák et al. (1967) proposed that bukovskýite is the arsenate analogue of the mineral destinezite, Fe2(PO4)(SO4)(OH)·6H2O, but this claim was refuted by Johan (1986). The thermodynamic properties of this phase are also unknown, with the exception of an estimate of ΔfGo

of −3480 ± 20 kJ/mol by Gas’kova et al. (2008). This esti-mate was made using various other very uncertain esti-mates and an assumption of ideal mixing between two hypothetical compounds.

In this work, we have determined the crystal struc-ture of bukovskýite from Großvoitsberg (Germany) by single-crystal X-ray diffraction and compared this struc-ture to the structure of bukovskýite from the type locality, modeled from synchrotron powder X-ray diffraction data. The comparison was deemed necessary because the solved structure is that of a nonahydrate while the postu-lated composition of bukovskýite is heptahydrate (Novák et al., 1967). We also measured the enthalpy of formation and standard entropy by calorimetric techniques and cal-culated phase diagrams for the system Fe-As-S-C-O-H. The field observations and comparison with other sites polluted by arsenic supplied a few hints as to why bu-kovskýite is such a rare mineral, although it could be ex-pected to be quite commonly formed from arsenopyrite (FeAsS).

Kutná Hora − history of mining, geological and mineralogical settings

The first written report about the mining of silver ores at the Kaňk deposit near Kutná Hora dates back to the 13th century (Kořan, 1950). The exploitation peaked in the 14th century and started to decline slowly in the 15th and 16th centuries. The waste dumps explored in this study are 400-600 years old.

The danger of the ores and dumps as a source of ar-senic was recognized at an early stage. Secondary miner-als from the dumps were probably used to poison the army of Albrecht Habsburg in 1304 (Kořínek, 1675). Bu-kovskýite and associated minerals were used commercial-ly as mouse and rat poison under the name “the poisonous earth from Kaňk” (Katzer, 1887). The first chemical anal-ysis of the material was carried out by the high-school teacher Antonín Bukovský (Bukovský, 1915) in Kutná Hora. The mineral was further studied by Slavík (1925) and Ulrich (1930) and was known in the literature as arse-nian-destinezite (Winchell, 1961). The first modern de-scription using X-ray diffraction, chemical analysis, TEM, and IR spectroscopy was given by Novák et al. (1967) (Fig. 1), who also named the mineral after A. Bu-kovský.

The Kaňk deposit is located in the northern part of the Kutná Hora mining district and is hosted by Variscan biotite, biotite-muscovite, quartz gneisses, and migmatites with aplite and pegmatite veins (Holub et al., 1982). The deposit was traditionally referred to as pyrite-dominated to distinguish it from the deposits of the central and south-

Table 2. Atomic coordinates and isotropic displacement parameters (Å2) of bukovskýite

Uequ for Fe1 to OW3, fixed Uiso for H sites.* Uiso for H sites are fixed: at 0.08 Å2 for H atoms of H2O molecules

kept in position by soft hydrogen bonds only and at 0.05 Å2 for more rigid OH groups and water molecules directly bonded to cations.


ern parts of the district, which are silver-dominated. The principal ore minerals in Kaňk are pyrite, arsenopyrite, Ag-bearing galena, chalcopyrite, sphalerite, pyrrhotite, marcasite, freibergite/tetrahedrite, stannite, and Pb-Sb (-Ag) sulfosalts (Malec, 1997). The gangue contains only quartz. Carbonates are absent.

Materials and methods

The crystals of bukovskýite from Großvoigtsberg in Sax-ony, Germany, which were used for single-crystal X-ray diffraction and calorimetry, were kindly donated by T. Witzke. Details of their locality and other minerals found there were reported by Witzke and Hocker (1993). We collected a number of samples with bukovskýite and as-sociated minerals during several field campaigns at the dumps near the municipality of Kutná Hora, especially at the dump of the Kaňk deposit of the Šafary mine. Select-ed samples were prepared in the form of standard petro-logical thin sections with polished surfaces.

A single crystal of bukovskýite from the sample from Großvoigtberg (Fig. 2) was measured with an Ox-fords Agilent SuperNova diffractometer equipped with an EoS detector. All data were integrated using CrysAlisPro (Oxford Diffraction, 2010). An empirical absorption cor-rection was applied using spherical harmonics imple-mented in the SCALE3 ABSPACK scaling algorithm. The structure was solved by direct methods using SHELXS (Sheldrick, 2008) and subsequent difference

Fourier analyses located the H sites. Structural refinement was performed using SHELXL-97 (Sheldrick, 2008) soft-ware in the space group P1, with Z = 2. The final aniso-tropic full-matrix least-squares refinement on F2 with 256 variables converged at R1 = 5.08% (I > 2σ(I)) and wR2 = 9.21% for all data. Scattering factors for neutral atoms were employed. Hydrogen positions were refined at a fixed value of Uiso = 0.05 Å2 for H2O molecules and OH groups bonded to cations. Uiso = 0.08 Å2 was used only for H2O fixed by soft hydrogen bonds. O-H distances were constrained to 0.90(5) Å and H-H distances in H2O molecules were constrained to 1.6(1) Å. Data collection and refinement details are summarized in Table 1. Tables 2 and 3 list the atomic positions and the anisotropic dis-placement parameters, respectively.

Powder XRD data were measured with a Bruker D8 DaVinci diffractometer with Cu Kα radiation (λ = 1.54056 Å). Powder XRD patterns were also collected at the bend-ing-magnet beamline DIFF at the synchrotron light source ANKA (Angströmquelle Karlsruhe, Germany). X-rays of wavelength of 0.82694(1) Å were selected by a double crystal Si(111) monochromator. The wavelength and the zero angle of the diffractometer were determined with sil-icon powder (NIST standard reference material 640). The sample was loaded into a 1.0-mm glass capillary, which was rotated about its axis during data collection. The in-tensity of the incoming beam was monitored during data collection by an ion chamber and the measured intensities of the diffracted beam were corrected for decay and fluc-

Table 3. Anisotropic displacement parameters of bukovskýite


tuations of the primary beam. The XRD patterns were collected at room temperature over the 2Θ angular range of 4° to 40° in steps of 0.005° and with a counting time of 1 s per point. Full-profile Rietveld refinement was applied to the diffraction data with the GSAS (General Structure Analysis System) program of Larson and von Dreele (1994).

For the electron microscopy work, selected frag-ments of the samples and the thin sections were carbon-

coated. The images and energy-dispersive analyses were acquired with a field-emission gun scanning electron mi-croscope (SEM) (Zeiss Ultra Plus, Friedrich-Schiller Uni-versity Jena, Germany) with an accelerating voltage of 15 kV. The chemical composition of selected samples was investigated using an electron microprobe Cameca SX100 (Masaryk University, Brno) with an accelerating voltage of 15 kV, current of 4 nA, beam diameter of 5 μm, and with the following standards: fluorapatite (P), sanidine (Al, Si, K), lammerite (Cu, As), vanadinite (Pb), barite (S), andradite (Ca, Fe), ZnO (Zn). F, Cl, Na, Mn, and Ba were sought but not detected. The measured data were pro-

cessed with PAP matrix corrections.For the solution calorimetric experiments at T = 25

°C, we used a commercial IMC-4400 isothermal micro-calorimeter (Calorimetry Sciences Corporation), which we modified for the purposes of acid-solution calorimetry. The liquid bath of the calorimeter was held at a constant temperature of 298.15 K with fluctuations smaller than 0.0005 K. The calorimetric solvent was 25 g of deionized water or 25 g of 5 N HCl contained in a polyetheretherke-

Figure 3. Crystal structure of bukovskýite. (a) A perspective (non-

parallel) view of a [100] chain. The Fe1, Fe2, and As sites are la-beled in the projection. (b) A parallel projection onto (100), or-thogonal to the chains. White hatched tetrahedra, sulfate; gray tetrahedra, arsenate; white hatched octahedra, Fe octahedra; black circles, oxygens; small white circles, hydrogen atoms. For clarity, the oxygen atoms (black circles) are displayed smaller if they are directly bonded to a cation (As5+, S6+, Fe3+).

Table 4. Selected interatomic distances (in Å) and the parameters for hydrogen bonds in the structure of bukovskýite


tone (PEEK) cup with a total volume of 60 mL. The cup was then closed with a PEEK screwable lid and was in-serted into the calorimeter well. The calorimeter stabilized after ~ 8 h. During stabilization and the experiment, the solvent was stirred with a SiO2 glass stirrer by a motor positioned about 40 cm from the active zone of the instru-ment. The samples were pressed into a pellet and weighed on a microbalance with a precision of 0.002 mg (as stated by the manufacturer). The pellets were then dropped through a SiO2 glass tube into the solvent and the heat produced or consumed during the dissolution was mea-sured. The heat flow between the reaction cup and the constant temperature reservoir was then integrated to cal-culate the caloric effect. A typical experiment lasted 50-

60 min, and the end of the experiment was judged by the return of the baseline to the pre-experiment position. The pellet mass for each measured phase was calculated ac-cording to the stoichiometry of the thermochemical cycle relative to 3.00 mg of MgO. The calculated mass of the bukovskyite pellets was 39.97 mg; the actual mass was 39.99 ± 0.48 mg. The calorimeter was calibrated by dis-solving ~ 20 mg pellets of KCl in 25 g of deionized water. Prior to each calibration measurement, the KCl was heat-ed overnight in the furnace at 800 K to remove the ad-sorbed water. The expected heat effect for the calibration runs was calculated from Parker (1965).

Heat capacity was measured by relaxation calorime-try using a commercial Physical Properties Measurement

System (PPMS, Quantum Design, San Diego). With due care, the accuracy can be within 1% for temperatures from 5 K to 300 K, and 5% for 0.7 K to 5 K (Kennedy et al., 2007). Owing to the hydrated nature of the mineral sample, it had to be isolated from the vacuum required for heat capacity measurements. To achieve this, the alumi-num DSC pan method by Marriott et al. (2006) was em-ployed. This method requires four heat capacity runs to obtain the sample heat capacity: (1) an addendum mea-surement, (2) the DSC pan with ~ 3 mg Apiezon® N grease within the pan, (3) an addendum measurement, and (4) the hermetically sealed DSC pan with grease and sam-ple enclosed. The heat capacity of run (2) is subtracted from run (4), giving the sample heat capacity. Measure-ments were conducted over the temperature interval 0.4 to 300 K. The sample was pressed into pellets using a stain-less steel die with a diameter of 7/64.” A Carver Model 3912 press provided an applied force of 375 kg, resulting in a pressure of 0.55 GPa on the pellet. The pellets were then weighed using a semi-microbalance, and the masses were 6.30 ± 0.02, 8.08 ± 0.02, and 6.57 ± 0.03 mg. These masses were chosen because, in a previous work, it was found that masses less than ~ 9 mg are optimal for accu-rate heat capacity measurements for mineral complexes (Majzlan et al., 2010).

Magnetic susceptibility measurements were recorded using the ACMS option of the PPMS. A polycarbonate capsule was filled with 115 ± 0.03 mg of bukovskýite

Table 5. The thermochemical cycle for bukovskýite


powder and the capsule was sealed with non-magnetic Kapton® tape. Data were taken under zero-field cooling in the temperature interval 300-2 K; the parameters used were a = 10 Oe, f = 1000 Hz, duration = 0.5 s, and H = 0 Oe. Magnetization curves were recorded at T = 2, 8, and 10 K for H ± 3 T.

RESULTS AND DISCUSSION

Crystal structure of bukovskýite

The dominant feature of the structure of bukovskýite are [100] chains composed of Fe(III) octahedra and arsenate tetrahedra (Fig. 3a). Within these chains, there are dimers of Fe(III) octahedra bridged by the OH groups. The two octahedra house two crystallographically independent Fe(III) cations, each coordinated by 2 O anions, 1 OH group, and 3 H2O molecules. Arsenate tetrahedra are linked via all of their O atoms to the Fe(III) dimers, thus creating a chain with the overall composition [Fe2(AsO4)(H2O)6(OH)]. Sulfate tetrahedra and three crystallographi-cally independent H2O molecules are located in the space between these chains and are bonded to them by an intri-cate network of hydrogen bonds (Fig. 3b). Selected inter-atomic distances are listed in Table 4 and correspond well to the average distances known for Fe3+, As5+, and S6+ in coordination with O atoms in the corresponding coordina-tion geometry. The structure of bukovskýite shows a very strict distribution of As and S over the available tetrahe-dral sites: essentially no substitution between the two tet-rahedral cations is observed, although this is known to oc-cur to a limited extent in some structures (e.g., in jarosite, Savage et al., 2005). The overall structural formula for bukovskýite is [Fe2(AsO4)(H2O)6(OH)](SO4){(H2O)3}.

To resolve the question of the hydration state of bu-kovskýite—nonahydrate versus heptahydrate—we select-ed a small nodule (~ 1.5 cm in diameter) from the materi-al from the Šafary dump (Kutná Hora), which seemed to contain bukovskýite of very good crystallinity. The nod-ule was broken apart with a scalpel under a binocular mi-croscope and bukovskýite was manually separated from the rest of the material (quartz, iron oxides). A small por-tion of the sample was used for the synchrotron powder XRD experiment and then refinement with the model ob-tained for the single crystals of bukovskýite from Großvoigtsberg was performed.

The refinement showed with no ambiguity that the structures of the two samples (Großvoigtsberg and Kutná Hora) are identical. The refined occupancies of all atoms, including the oxygen atoms of the free H2O molecules, were in agreement within the procedural uncertainty.

Discussion of the structural features

Bukovskýite belongs to a sparsely populated group of chain structures of arsenate minerals within the hetero-polyhedral classification. The only other arsenate chain structure in which the chains are cross-linked by hydro-gen bonds is kaatialaite, Fe[AsO2(OH)2]3·5H2O (Boudjada

Table 6. Measured, calculated, and tabulated enthalpy values for reactions 1−17*

* See Table 5 for the reactions.


and Guitel, 1981). These chains have the same topology as the chains found in the structure of aluminocoquimbite, FeAl(SO4)3·9H2O (Demartin et al., 2010). Quadruple chains of a topology different from that of bukovskýite were found in the recently described synthetic Fe2(SO4)3·3H2O (Xu and Parise, 2011). Strict partitioning of S6+ and As5+ over the available tetrahedral sites can also be seen in the structure of leogangite, Cu10(OH)6(H2O)4

(AsO4)4(SO4)·4H2O (Lengauer et al., 2004).

Hydrogen bonding

The crystal structure of bukovskýite shows an intricate network of hydrogen bonds. There is one hydroxyl group and nine H2O molecules in the structure. Of these nine H2O molecules, six are coordinated to the Fe3+ cations (O4, O8, O10, O12, O13, O15), and three are not directly bonded to any cation (Ow1, Ow2, Ow3). All of these oxy-gen atoms are donors in hydrogen bonds, whose parame-ters are summarized in Table 4. A detailed inspection of Table 4 reveals that most hydrogen bonds are accepted by the sulfate tetrahedron. There is only one crystallographi-cally independent SO4 unit in the structure of bukovskýite. The four oxygen atoms in this SO4 unit (O6, O7, O11, O14) accept a total of 10 hydrogen bonds. The atoms O11 and O14 each accept three hydrogen bonds. There are two hydrogen bonds (O12-H12A···O3, O13-H13···O9) whose donor and acceptor oxygens are located in the het-eropolyhedral chains. Additional hydrogen bonds (via H15, H15A, H2, H8, Hw12, Hw32) link the chains with the free H2O molecules. There is only one hydrogen bond (via Hw21) whose donor and acceptor are oxygen atoms of the free H2O molecules.

Thermodynamic properties of bukovskýite

The enthalpies of dissolution of bukovskýite and the ref-erence compounds were measured sequentially in 5 N HCl (Table 5). The reference compounds were all syn-thetic: α-MgSO4, γ-FeOOH, H2O, KCl, MgO, and KH2A-sO4. With the exception of KH2AsO4, they were all used in our previous thermodynamic studies (see Majzlan, 2010 for a review). The validity of KH2AsO4 as a refer-ence compound was extensively tested in a separate study on the thermodynamics of ferric arsenates, especially scorodite (FeAsO4·2H2O), and led to satisfactory results on this well researched phase (Majzlan et al., 2012).

The dissolution and formation reactions for the refer-

Figure 4. (a) Molar heat capacity (Cp) data over the entire mea-sured range. Circles show the measured data and the solid line is the fit with orthogonal polynomials used to calculate the entropy. Note the absence of any sign of magnetic transition in the data. (b) A Cp/T 2 versus T plot shows an anomalous break in the mea-sured data at 8 K. Compare with the magnetic susceptibility data in the same temperature range (Fig. 5). (c) A Cp/T versus T plot with measured data (circles). The open circles were included in a fit to the function Cp = B3T 3 + γT, with resulting parameters B3 = 6.22 × 10−3 J·mol−1·K−4 and γ = 0.098 J·mol−1·K−2. The fit is shown as a solid line in the figure. Gray circles were not included in the fit. The anomaly centered at T = 0.5 K is likely caused by the magnetic ordering in chains.

100 20 30 40

18

20

22

24

26

T (K)

χ‘ (e

mu·

g)

-1

Figure 5. Zero-field-cooled magnetic susceptibility (χ’) for bu-kovskýite.


ence compounds and bukovskýite can be assembled into a thermochemical cycle (Table 5). The algebraic expression for this cycle is a linear equation (equation for ΔH18 in Ta-ble 5) with one unknown, namely the enthalpy of forma-tion (ΔfHo) for bukovskýite at T = 298.15 K. All measured values and values used in the calculations of ΔfHo for bu-kovskýite are listed in Table 6. The uncertainties of the measured data are reported as two standard deviations of the mean and are propagated by a standard procedure

(Taylor, 1982, p. 73).The dissolution enthalpies of two natural bu-

kovskýite samples (Großvoigtsberg and Kutná Hora) were measured, although the sample from Kutná Hora has physical and chemical impurities (see below). As a result of the presence of these two impurities, the two measured values differ (Table 6). The final ΔfHo value for bu-kovskýite was calculated from the dissolution enthalpy of bukovskýite from Großvoigtsberg and is −4742.4 ± 3.8 kJ/mol.

The heat capacity of bukovskýite was measured from T = 300 K down to 0.4 K (Fig. 4a). The most peculiar feature of the dataset is the absence of a prominent phase transition between 0.7 and 300 K. For compounds with Fe3+ as a major ion, a magnetic ordering transition would be expected at low temperatures. Fe3+ is a d

5 ion and unpaired electrons are present in both high-

spin and low-spin states. In the ligand field of the oxygen anions, the high-spin state with five unpaired electrons is more likely (Burns, 2005). Consequently, we measured the magnetic susceptibility (χ’) of bukovskýite from T = 300 to 2 K in the absence of an applied field (Fig. 5). The plot of χ’ versus T data set shows a peak at T = 8 K, and this peak disappears in the presence of an external magnetic field of 1000 Oe (data not shown). This peak suggests that at 8 K bukovskýite undergoes magnetic spin ordering. Note that the slope of Cp/T2, shown in Figure 4b, changes near 10 K. Although the exact nature of this process is not clear, its existence is further substantiated by an extremum in the Cp/T2 plot (Fig. 4b) and by the B(H) curves at low temperature. At 10 K, the B(H) curve is linear, typical of a paramagnetic system. At T = 8 K, the system shows some hysteresis indicative of magnetic ordering of the iron atoms within the structure. At 2 K, the B(H) curve shows an increased deviation from linearity and increased hysteresis.

The absence of any sign of prominent magnetic tran-sition can be traced back to the principal structural fea-tures of bukovskýite. There are two possibilities for the magnetic ordering in bukovskýite. The ordering could take place within the chains and then between the chains. It can be shown that in a linear chain, there can be no long-range ordering (de Jongh and Miedema, 1974). The chain entropy is lost through the short-range ordering that produces a Schottky-like anomaly. In most cases, three-dimensional magnetic ordering of the chains takes place at lower temperatures and is associated with a second-or-der anomaly—only a small entropy change is involved. This type of magnetic behavior is well known (e.g., Fisher et al., 1975). The long interchain distances complicate the ordering between the chains. The chain-to-chain (chain axis to chain axis) distance is 10.3 Å in the b direction. In

Table 7. Thermodynamic functions for bukovskýite

Calculated per mole of Fe2(SO4)(AsO4)(OH)·9H2O (Mr = 525.8608 g·mol−1).


the c direction, the chains are separated not only by H2O molecules but also by SO4 tetrahedra, and the chain-to-

chain distance is 10.9 Å. In Fig. 4c, the value of γ = 98 mJ·mol−1·K−2 from the fit is probably too large to have its origin in vacancies and/or disorder, thus lending support to linear-chain ordering. The small anomaly located be-low ~ 0.7 K could be associated with three-dimensional ordering of the chains. As would be expected, the anoma-ly has only a small amount of entropy associated with it. Of course, the anomaly could possibly be associated with an Fe-containing impurity ordering.

For fitting purposes, the heat capacity data were ex-trapolated from 0.4 K down to 0 K with a simple polyno-mial Cp = ΣanTn, where an are adjustable parameters and n are the integers 1-6. This polynomial was fitted to the measured data up to 13 K. The data above ~10 K were fit-ted with two orthogonal polynomials. The smoothed val-ues of thermodynamic functions obtained from the best-fit polynomials are reported in Table 7. The measured data are presented in the supplementary information (Tables S1 and S2).

The measured formation enthalpy and standard en-tropy were combined to calculate the Gibbs free energy of formation of bukovskýite (Table 8). Using auxiliary data listed in Table 8, we calculated the equilibrium constant for a dissolution reaction (reaction 19 in Table 9). The equilibrium constants for the reaction (19) were calculat-ed at temperatures from 273.15 to 373.15 K (Table 9), with the assumption that the reaction heat capacity, ΔrCp, is a constant in this temperature range. The log K value for T = 298.15 K is not subject to this assumption as it was directly calculated from the measured data. Although the assumption that ΔrCp is a constant in this temperature range may not be highly accurate, log K values calculated with the usual assumption of ΔrCp = 0 are subject to much larger errors when aqueous species are involved.

Thermodynamic stability of bukovskýite

The thermodynamic stability of bukovskýite was assessed by the construction of phase diagrams (Figs. 6 and 7). The first and most obvious conclusion is that bukovskýite is metastable with respect to scorodite. The reaction relating scorodite and bukovskýite is

Fe2(AsO4)(SO4)(OH)·9H2O + H+ = FeAsO4·2H2O + Fe3+(aq) + (SO4)2−(aq) + 8H2O (20).

This reaction shows that bukovskýite should be fa-vored at higher pH or higher sulfate and Fe(III) activities. Bukovskýite remains, however, metastable with respect to scorodite, even at unrealistically high pH values or ex-tremely high activities of sulfate or ferric species.

Stability relations can be explored under conditions that could be expected for an acid-mine drainage system polluted by arsenic (Figs. 6a-6c). We have chosen the pH value of 3.5 and the activities of aqueous ions marked in Fig. 6. The stable phases in the system are goethite and scorodite (Fig. 6a). If goethite is not considered, the poor-

Table 8. Thermodynamic properties of bukovskýite

Formation reaction listed as reaction 18 in Table 5.Thermodynamic properties used to calculate the thermodynamic properties of a dissolution reaction (reaction 19, see Table 9).Ref.: (1), this work; (2), Lemire et al. (2011); (3), Nordstrom and Archer (2003); (4), estimated; see Helgeson et al. (1981).

Table 9. Equilibrium constants of the dissolution reaction

Fe2(SO4)(AsO4)(OH)·9H2O (cr) + H+ (aq) =2Fe3+ (aq) + (SO4)2− (aq) + (AsO4)3− (aq) + 10H2O (l) (19)

Properties at T = 298.15 K. ΔrHo and ΔrSo calculated from the data in Table 8. ΔrGo and log K calculated from the ΔrHo and ΔrSo values.


age systems polluted by arsenic. Figure 7c shows a pε-pH diagram for the system Fe-S-As-C-O-H. As noted above, bukovskýite shows no stability field in this diagram. If scorodite is suppressed from the calculations, a narrow stability field for bukovskýite appears at low pH values (Fig. 7d).

Field observations in Kutná Hora

Field work and subsequent laboratory studies, principally powder XRD, distinguished two types of associations and dumps in the region of Kutná Hora. The first type is the rocky dumps, which are composed of large rock frag-ments with open space between the fragments. These dumps contain the mineral association of scorodite (FeAsO4·2H2O), kaňkite (FeAsO4·3.5H2O), and zýkaite [Fe4(AsO4)3(SO4)(OH)·15H2O]. Non-arsenate secondary minerals here include gypsum and jarosite.

The second type is the clayey dumps with abundant bukovskýite and very rare parascorodite (FeAsO4·2H2O). As the description suggests, the space between the rock fragments is completely filled by earthy to clayey materi-al. Pieces of anthropogenic material such as charcoal, burnt clay, or remnants of timbering, which are all related to the medieval mining technologies, are relatively com-mon.

As mentioned above, most of the material used in this study was collected at the dump of the Šafary mine. This dump is of the clayey type, and at present, it is well exposed because of determined erosion by mineral collectors.

Bukovskýite grows here in the form of nodules that may achieve prodigious dimensions (Fig. 1). The growth process itself is an intriguing mystery. It is certain that bu-kovskýite grows in the dumps and was not a mineral from the natural oxidation zone of the deposit. A simple growth model suggests that the nodules began their growth at a nucleus within the dump and bukovskýite simply grew over any larger fragments. However, the nodules, when cut, do not contain any rock or mineral fragments that are otherwise common in the dumps (Fig. 8). This observa-tion could be explained if the growing nodule pushed and expelled the fragments beyond its surface, perhaps similar to the formation of polygonal soils by the action of ice on rock fragments (e.g., Easterbrook, 1999). This is not the case, as we never observed accumulation of fragments, large or small, at or near the surface of the nodules. It ap-pears that the bukovskýite nodules “absorb” and eliminate the rock fragments as they grow, even if these fragments are constituted from minerals not subject to fast weather-ing, such as quartz, feldspars, or amphiboles.

Microscopic studies revealed that bukovskýite is as-

ly crystalline Fe oxides (ferrihydrite, schwertmannite, As-rich hydrous ferric oxide) emerge in the diagram (Fig. 6b). Only once scorodite is suppressed from the calcula-tions, a small stability field of bukovskýite opens at si-multaneously high sulfate and arsenate activities (Fig. 6c).

Figures 7a and 7b present pε-pH diagrams for the systems Fe-S-C-O-H and Fe-As-C-O-H, respectively. Note that hematite and goethite were not considered in these calculations, as they would displace all poorly crys-talline Fe oxides, which are the hallmarks of mine drain-

goethite

scorodite

-14

-12

-10

-8

-6

-4

-2

0

2

log a(SO )2-4

log

a(H

AsO

)2

4-

Fe(S

O) 42

(aq)

-

a

-14 -12 -10 -8 -6 -4 -2 0 2

Fe(S

O) 42

(aq)

-As-HFO

ferrihydrite

schwertmannite

scorodite

-14 -12 -10 -8 -6 -4 -2 0 2

-14

-12

-10

-8

-6

-4

-2

0

2

log a(SO )2-4

log

a(H

AsO

)2

4-

b

-14 -12 -10 -8 -6 -4 -2 0 2log a(SO )2-

4

bukovsk iteý

As-HFO

ferrihydrite

schwertmannite

Fe(S

O) 42

(aq)

-

-14

-12

-10

-8

-6

-4

-2

0

2

log

a(H

AsO

)2

4-

c

Figure 6

Figure 6. A series of phase diagrams for the system Fe2O3-As2O5-

SO3-H2O. The diagrams were constructed for pH = 3.5 and T = 298.15 K. (a) Stable phases in the system. (b) Goethite sup-pressed to show the fields of poorly crystalline iron oxides. (c) Goethite and scorodite suppressed to show the small stability field of bukovskýite.


sociated with amorphous materials, hereafter referred to as gels, of variable composition (Figs. 9-11). We distin-guish the dark and light gels based on the apparent shade of gray seen in the back-scattered electron (BSE) images. The BSE images show dissolution of the minerals present in the dump, the subsequent formation of the light and dark gel, and finally crystallization of bukovskýite from the light gel (Fig. 11). In addition to dissolving the prima-ry silicates (quartz, feldspars, amphiboles), the gels also

replace older bukovskýite, parascorodite, or charcoal. Chemically, the gels are dominated by Si, Fe, As, and S (Table 10). The dark gel, which seems to be preceding the light gel, is richer in Si, Al, and Ca. The light gel is de-pleted in these three elements and enriched in Fe, As, and S. It is interesting that the gels maintain a Fe:(As+S)

0 2 4 6 8 10 12 14

-15

-10

-5

0

5

10

15

20

pH

pε

d

pyrite

schwertm.

symplesite

magnetite

ferrihydriteAs-HFO

bukovsk iteý

pyrrhotite

Fe2+

Fe3+

0 2 4 6 8 10 12 14

-15

-10

-5

0

5

10

15

20

pH

pε

c

pyrite

pyrrhotite

symplesite

scorodite

magnetite

ferrihydriteFe2+

Fe3+

As-

HFO

0 2 4 6 8 10 12 14

-15

-10

-5

0

5

10

15

20

pH

pε

b

ferrihydrite

magnetite

Fe3+

Fe2+ symplesite

scorodite

As-

HFO

siderite

ferrihydrite

magnetitepyrite

schwertmannite

Fe3+

Fe2+

0 2 4 6 8 10 12 14

-15

-10

-5

0

5

10

15

20

pH

pε

a

pyrrhotite

siderite

Figure 7. pε-pH diagrams constructed at T = 298.15 K, assuming an activity of 10−3 for dissolved Fe, 10−4 for dissolved As, 10−5 for dissolved S, and 10−2 for dissolved C species. Goethite and hematite are suppressed in all diagrams as they would displace all poorly crystalline iron ox-ides. (a) Fe-S-C-O-H system. (b) Fe-As-C-O-H system. (c) Fe-As-S-C-O-H system. (d) Fe-As-S-C-O-H system with scorodite sup-pressed. Note the small stability field of bukovskýite in the last diagram. As-HFO is arsenate-enriched hydrous ferric oxide [see Majzlan (2011) for the thermodynamic data].

Figure 8. Cut of a bukovskýite nodule from Kutná Hora.Figure 9. A secondary electron (SE) image of bukovskýite crystals

from Kutná Hora. The crystals lie on the surface of a small quartz fragment and are covered by minute flakes of another mineral, most likely a phyllosilicate. The anhedral fragments, prominently seen in the center of the microphotograph, are probably the gels from which bukovskýite crystallizes.


As+S Fe

Si

at.%

Figure 10. A triangular plot of As+S−Fe−Si concentrations (at%) in light and dark gels and bukovskýite from Kutná Hora. Dark gels − diamonds, light gels−squares, bukovskýite−gray circles.

Figure 11. A back-scattered electron (BSE) image of a bukovskýite nodule from Kutná Hora. The microphotographs show dissolu-tion of the primary gangue minerals (quartz, Q; amphibole, A; K-feldspar, K-f) by a dark gel (DG) and a light gel (LG). Note that the fragments of quartz and K-feldspar are rimmed by dark gel. The aggregate of acicular bukovskýite (B) crystals is the fi-nal product of crystallization of the gels. Image taken from Loun (2010).

Table 10. Selected electron microprobe data

All data in weight%.DL, detection limit.


atomic ratio of approximately 1 (Fig. 10), as required by the stoichiometry of bukovskýite. Yet, the Fe:As:S ratio scatters fairly widely and most of the gels are depleted in S relative to the ideal 2:1:1 stoichiometry of bukovskýite. The Fe:As:S ratio of 2:1:1 cannot originate by simple weathering and decomposition of pyrite (FeS2) or arseno-pyrite (FeAsS). Within the gels, there must be an addi-tional control over their chemical composition and the fi-nal crystalline product. This control will be the subject of further studies here.

There are only small differences between the chemi-cal compositions of the bukovskýite from Kutná Hora and Großvoitsberg (Table 10). The only impurity present in both bukovskýite samples is P2O5; the other analyzed ele-ments are essentially not present. The bukovskýite crys-tals from the locality in Germany are of slightly higher purity.

SEM observations (Fig. 9) show that bukovskýite from the type locality is associated with tiny flakes down to 50 nm. The diminutive size of the flakes and their close spatial association with bukovskýite precluded a confident analysis with the EDS system. The identity of these flakes will be the subject of further studies. We can only specu-late now that these flakes represent phyllosilicates that crystallize from the dark, Si- and Al-rich gels.

CONCLUSIONS

The crystallographic, thermodynamic, and field observa-tions gathered during this study provide clues about the formation of the mineral bukovskýite. This information will serve as the working hypotheses for our further re-search, especially at the field site in Kutná Hora.

The crystal structure comprises two “modules,” namely, the iron-arsenate chains and the inter-chain space with H2O and (SO4)2− groups. We are not aware of any isostructural or structurally related phases.

Bukovskýite is metastable with respect to scorodite. If the equilibrium assemblage of scorodite + goethite were to form, sulfate would be released into the aqueous fluids and carried away. The absence of signs of a mag-netic transition in the heat capacity data can be explained by the chain nature of the structure of bukovskýite.

An undeniable field observation is that bukovskýite is concentrated in clayey dumps, and the role of clays in the formation process of this mineral must be considered. We suspect that bukovskýite forms in microenvironments sealed by the clay minerals where high concentrations of aqueous H+, Fe(III), SO4, and AsO4 are maintained for a long time. Bukovskýite forms from the amorphous gels via a series of intermediate steps. The mechanisms that control the formation, chemical composition, and crystal-

lization of the gels are unknown.

ACKNOWLEDGMENTS

We thank two anonymous reviewers for constructive criti-cism. We are grateful to T. Witzke for the bukovskýite sample for studying the structure and for thermodynamic measurements, and to P. Pauliš for providing very useful information about the locality of Kutná Hora. We ac-knowledge the ANKA Angströmquelle Karlsruhe for per-mitting beamtime at the PDIFF beamline and thank S. Doyle for help during the measurements. We also ac-knowledge the Canada Foundation for Innovation, the At-lantic Innovation Fund, and other partners that fund the Facilities for Materials Characterization, managed by the Institute for Research in Materials at Dalhousie Universi-ty, for time on the PPMS. The thermodynamic study con-ducted in this work was financially supported by the Deutsche Forschungsgemeinschaft, grant no. MA 3927/16-1. This work was supported by the research proj-ect MSM0021622412 (INCHEMBIOL) to M.N.

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Table S1. Measured heat capacity values for bukovskýite (run 1)

Reported per mole of Fe2(SO4)(AsO4)(OH)·9H2O (Mr = 525.8608 g·mol−1)

Table S2. Measured heat capacity values for bukovskýite (run 2)

Reported per mole of Fe2(SO4)(AsO4)(OH)·9H2O (Mr = 525.8608 g·mol−1)

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Manuscript received September 30, 2011Manuscript accepted April 16, 2012

Manuscript handled by Kazumasa Sugiyama

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