Rietveld refinement study of pyrite crystals

https://doi.org/10.1016/j.jallcom.2005.02.052Get rights and content

Abstract

Rietveld refinement was performed for two samples prepared from natural pyrite (a polymorph of iron disulphide or iron persulphide, FeS2) single crystals originating from Navajun (Spain) and from Ural Mts. (Russia). The present refinements provide the following crystallographic data: a = 5.41784(2) and 5.41819(2) Å, x = 0.3848(1) and 0.3840(1) for the above-mentioned crystals, respectively. They are comparable with those of several crystals from other geological deposits, reported in literature. The obtained diffraction results indicate an off-stoichiometry with a higher sulphur content in the sample from Ural. The last observation is qualitatively supported by the results of X-ray photoelectron spectroscopy. The crystallographic data are discussed on the basis of earlier literature information on composition and structure reported for synthetic bulk and thin-layered pyrite.

Introduction

The pyrite (a cubic polymorph of iron disulphide or iron persulphide, FeS2), is the most abundant sulphur-containing mineral in the earths crust [1]. Moreover, it is a second source of sulphur after that of native sulphur. Pyrite is also known as a natural component of sediments [1], [2]. The name of pyrite comes from a Greek expression pyrites lithos—“a stone which strikes the fire”. It crystallises in space group Pa3¯ with Fe atoms at (0, 0, 0) and S atoms at (u, u, u), with u = ∼0.385. [Bayliss has attributed a (pseudocubic) triclinic structure to some pyrite samples exhibiting a weak anisotropy [3]; for discussion of other pseudocubic structures in pyrite family see Ref. [4]]. The electronic structure of pyrite has been studied or discussed in Refs. [5], [6], [7], [8], [9], [10], [11], [12]. There are numerous indications that pyrite is or can be nonstoichiometric (see e.g. [5], [13], [14], [15], [16] and references therein). The deviation from stoichiometry is found to have a weak but well determined influence on the lattice parameter of bulk FeS2 crystals [5].

It is worth noting that the analysis of geochemical processes involving pyrite at high pressure–high temperature conditions is thought to be important for understanding of geochemical synthesis of organic molecules 4 gigayears ago [17]. Moreover, models exist connecting pyrite and iron sulphides with emergence of life on the earth (see [18], [19] and references therein). Just this last compound is present also in space, in protoplanetary disks and cometary dust [20], and detected in meteorites [21].

Photoelectrochemical energy conversion has been reported for ultrathin FeS2 organo-metallic chemical-vapour-deposited layer on TiO2 [22], so pyrite is a potentially photovoltaic material. The opportunity of its use in lithium batteries has been studied, e.g. in Refs. [23], [24]. Some pyrite ores include impurities (inclusions) of noble metals (gold, silver) [1], [25], [26], [27], [28] and can be used for gold recovery; pyrite itself is a frequent (harmful) impurity phase in coal [1]. One of studied applications is the soil improvement/amendment [29], [30], [31]. On the other hand, the degree of pyritisation can be treated as an indicator helping in studies of geological sediments [32]. Pyrite is a basic component used in production of the sulphuric acid [1]. It is known to interact with the natural environment: the sulphuric acid together with iron hydroxides may form on the surface of stored crystals in contact with water or humid air; (this reaction is highly exothermic) [1]; for discussion of mechanisms, including chemical or microbial oxidation, see Refs [9], [31], [33], [34], [35], [36], [37], [38]. On the other hand, pyrite is a largely ‘destructive’ mineral: as a major waste phase from mines it is a significant cause increasing the acidity of waters and thus a big menace for the environment (cf. remarks in Refs. [33], [39]).

Very recently, pyrite has been proposed as a fine-tuning component of a tunable source of highly spin-polarised electrons, (Co, Fe)S2 [40]. Moreover, (Co, Fe)S2 is considered as a system offering insights into the design of half-metallic ferromagnets [41].

Accurate structural data of the bulk pyrite can be helpful in understanding its physical properties and designing its applications. In particular, full understanding of structural features connected with nonstoichiometric composition and with cationic or anionic substitution in pyrite are, still, at their infancy. The reported experimental data concerning FeS2 include results of structural studies by single crystal diffraction and powder diffraction. Moreover, a number of papers are devoted to structure calculation from first principles [11], [39], [42], [43], [44]. The experimental structural data concern natural crystals [13], [23], [45], [46], synthetic bulk crystals [5], [23], [47], [48], [49], [50], [51], [52], and pure and doped polycrystalline [15], [16], [53] and monocrystalline [54] thin layers (see Table 1). Systematic, combined or comparative studies involve, e.g. XPS and theoretical investigation of pyrite surface [36], X-ray diffraction (XRD) and XPS study of high-quality thin FeS2 films [54], the variation of structural properties with Fe:S ratio [5], [15], [16] and a study of synthetic and natural crystals [23]. The (minor) discrepancies between the reported a and x values are due to both, the sample properties (nonstoichiometry, impurities in mineral samples) as well as to instrumental factors. Influence of some impurities on the lattice constant is known, e.g. a raising tendency in a is observed when substituting Fe by Ni (see Ref. [53]), while for Zn substitution at Fe sites no such influence has been detected [47].

In the present study, Rietveld refinements for FeS2 powdered natural single crystals were performed on sample A originating from Navajun (Spain) and sample B from Ural Mts. (Russia). The mentioned calculation method has already been found to be useful in studies of bulk FeS2 crystals [23], arsenopyrite formation [55], in crystallographic/mineralogical studies including the pyrite phase (Rietveld refinement study of pyrite in coals [14]) and in mixture with marcasite [48] as well as in investigation of synthetic pyrite thin films [15], [16].

Section snippets

Experimental

The purity was checked by X-ray photoelectron spectroscopy (XPS) (for experimental conditions see Ref. [56]). The amount of impurity elements in the studied samples is below the detectability level of this method. The X-ray diffraction data were collected in the 2θ range 5–160° using a Bragg–Brentano diffractometer. The instrument is equipped with a computerised GUR4 goniometer and a graphite monochromator cutting off the white radiation and fluorescence as well as Kβ spectral components. This

Results and discussion

Diffraction patterns of samples A and B exhibited the peak-to-background ratio (P/B), at the strongest 002 reflection, evaluated to be ∼95 and ∼115 for crystals A and B, respectively. Phase analysis indicated that there is no secondary phase in the studied specimens at the detectability limit of the applied measurement conditions (this limit is evaluated to be about 1% in the given case). Therefore, the initial structural model was built according to the single-phase pyrite structure (space

Conclusions

In the present work, the structural information on natural mineral pyrite crystals originating from Navajun (Spain) and Ural Mts. (Russia) was obtained from high-quality powder diffraction patterns. The obtained crystallographic data were determined by Rietveld analysis. The data are compared to few existing earlier results reported for single crystals and for bulk and thin-layered pyrite polycrystals. The lattice parameter a = 5.41784(2) and 5.41819(2) Å and the free positional parameter x = 

Acknowledgement

This study was supported in part by the European Commission Program G1MA-CT-2002-4017 (Centre of Excellence CEPHEUS). One of us (J.L.) is grateful to Markku Heinonen (University of Turku) for technical assistance.

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