Hydrothermal precipitation of artificial violarite

Hydrometallurgy 115-116 (2012) 98–103

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Hydrothermal precipitation of artificial violarite W.H. Jørgensen a, H. Toftlund a, 1, T.E. Warner b,⁎ a b

Institute of Physics and Chemistry, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark Institute of Chemical Engineering, Biotechnology and Environmental Engineering, University of Southern Denmark, Niels Bohrs Allé 1, 5230 Odense M, Denmark

a r t i c l e

i n f o

Article history: Received 9 June 2011 Received in revised form 6 January 2012 Accepted 15 January 2012 Available online 21 January 2012 Keywords: Violarite Polydymite Thiospinel Hydrothermal synthesis Penicillamine

a b s t r a c t The nonstoichiometric nickel-ore mineral, violarite, (Ni,Fe)3S4 was prepared as a phase-pure fine powder by a comparatively quick hydrothermal method from an aqueous solution of iron(II) acetate, nickel(II) acetate and DL-penicillamine in an autoclave at 130 °C for 45 h. Powder-XRD showed that the violarite crystallised with the thiospinel structure, a = 9.478(3) Å, Z = 8; SEM revealed platy crystals 10 × 10 × 1 μm; EMPA gave a mean composition, Fe0.31Ni2.36S4. Violarite compositions with a higher Fe/Ni ratio than this were concluded to be metastable in this aqueous system. When DL-penicillamine was used in excess, nickeliferous pyrite formed in addition to violarite. This is the first successful method for preparing violarite directly from aqueous solution, and demonstrates that it is feasible for nickel-rich violarite to precipitate from aqueous media in a geochemical environment. The mechanism for the pseudomorphic replacement of pentlandite, (Fe,Ni)9S8 by violarite is discussed in the light of these new results. The Fe-free end-member, polydymite, Ni3S4 was also prepared by this method. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Violarite, (Ni,Fe)3S4 is a nonstoichiometric thiospinel that occurs in nature as an economically important nickel-ore mineral. It forms commonly through the supergene alteration (a weathering process) of the metal-rich species, pentlandite, (Fe,Ni)9S8; and to a lesser extent through the alteration of pyrrhotite, Fe7S8 and millerite, NiS (Nickel, 1973). The preparation of artificial violarite involved, until quite recently, a lengthy two-stage process in which the constituent elements were reacted together at high temperature. This method was far from optimal, yielding a product invariably contaminated with other phases. Over the past decade, various aqueous routes involving pH-control have been devised for the precipitation of the closely related thiospinels: polydymite, Ni3S4 and CoNi2S4 (Manthiram and Jeong, 1999; Shimizu and Yano, 2001). More recently, Xia et al. (2008) developed a hydrothermal method for converting artificial pentlandite to phase-pure violarite. Violarite was described originally by Lindgren and Davy (1924) as a violet-grey nickel sulfide mineral. Its composition was determined more precisely as, (Ni,Fe)3S4 by Short and Shannon (1930). 57FeMössbauer spectroscopy and powder neutron diffraction revealed that violarite crystallises with the inverse spinel structure, (Ni) tet [FeNi] octS4 (Tenailleau et al., 2006a; Townsend et al., 1977; Vaughan and Craig, 1985). The composition, FeNi2S4 was considered by Craig ⁎ Corresponding author. Tel.: + 45 6550 2575; fax: + 45 6615 8780. E-mail address: [email protected] (T.E. Warner). 1 In memory of the late Professor Hans Toftlund — deceased 27th November 2009. 0304-386X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2012.01.004

(1971) to be the thermodynamically stable iron-rich end-member of the solid solution series, Fe1−xNi2 + xS4 (0 ≤ x ≤ 1), with the upper decomposition temperature just above 300 °C. However, there are many examples of both natural (Misra and Fleet, 1974; Nickel et al., 1974) and artificial (Rice et al., 1994; Townsend et al., 1977; Warner et al., 1992; Xia et al., 2008) violarite containing a higher Fe/Ni ratio than portrayed by the formula, FeNi2S4, although these are considered to be metastable with respect to the assemblage: FeNi2S4, (Fe,Ni)S2 and (Ni,Fe)1−xS (Vaughan and Craig, 1985; Warner, 2011; Warner et al., 1992). Likewise, the binary thiospinel, greigite, Fe3S4 is generally considered to be a metastable phase. One of the early methods for preparing violarite was that devised by Craig (1968, 1971). It was a lengthy three-month two-stage process that involved heating iron, nickel and sulfur powders together inside an evacuated quartz-glass ampoule at ~500 °C, followed by grinding and re-annealing to form the monosulfide solid solution, FeNi2S3.25. This intermediate phase was then ground and reacted with the necessary additional amount of sulfur inside yet another ampoule at ~ 300 °C. In spite of much laborious effort, the product materials were contaminated with (Fe,Ni)S2 and (Ni,Fe)1−xS. The preparation of artificial violarite by an aqueous route was first attempted by Michener and Yates (1944). These workers treated an aqueous solution containing equimolar amounts of nickel(II) sulfate and iron(II) sulfate with hydrogen sulfide. This resulted in an amorphous precipitate; and so the matter was not pursued. Lundqvist (1947) successfully prepared the nickel end-member, Ni3S4 via a reaction between an aqueous solution of sodium thiosulfate (Na2S2O3) and a boiling aqueous solution of nickel(II) sulfate. This route was modified by Manthiram and Jeong (1999), in which Ni3S4 was

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precipitated by adding an aqueous solution of sodium dithionite (Na2S2O4) to an aqueous solution of nickel(II) chloride, whilst maintaining a pH = 3. Shimizu and Yano (2001) prepared CoNi2S4 by adding an aqueous solution of cobalt(II) chloride and nickel(II) chloride to an aqueous solution of thioacetamide (CH3C(S)NH2) at 60−70 °C, whilst maintaining a pH = 9.9. Recently, Xia et al. (2008) reported a method for preparing violarite by a hydrothermal coupled dissolution−reprecipitation replacement reaction. These workers developed this synthesis route from the results of their investigations regarding the mechanism for the transformation of pentlandite to violarite; yet the essence of their studies was concerned with the processes of mineral replacement. Their procedure first requires the preparation of artificial pentlandite, (Fe,Ni)9S8 − by a conventional high temperature method − as a precursor compound for the subsequent preparation of violarite. The product was described as a phase-pure violarite (in either powder or bulk form) in which the Fe/Ni ratio could be controlled, to some degree, by varying the reaction conditions, yielding compositions spanning the range: Fe0.91Ni2.05S4 −Fe1.86Ni1.24S4. Although this method is clearly successful for obtaining iron-rich compositions, it beckons the development of a more simplified approach, and also a method for preparing violarite with nickelrich compositions. Hence, it became apparent to us, that the choice of sulfurising reagent for the precipitation of violarite from an aqueous solution of iron(II) and nickel(II) cations, is neither an arbitrary nor, trivial matter. The sulfurising reagent used in this present work for the synthesis of violarite is penicillamine which is a metabolite of penicillin. The D-enantiomer is manufactured as an immunosuppressant drug, Cuprimine™ and Depen™ for the treatment of rheumatoid arthritis (Furst, 1990). D-penicillamine is also used to treat Wilson's disease, where it functions as a chelating agent binding Cu2 + ions in the body, and gives the possibility of excretion via urine. The other enantiomer, Lpenicillamine is toxic to humans. A mixture of both enantiomers, DLpenicillamine, is available commercially at a comparatively lower price. Penicillamine was chosen as a sulfurising reagent since certain Lewis acids, such as, Cu2 + and Ni 2 + aqueous ions are capable of breaking the carbon−sulfur bond in molecular species similar to penicillamine at elevated temperature; cf. Becher et al. (1985). Furthermore, the mechanism for the transformation of pentlandite to violarite has become a topic of renewed debate. Pring et al. (2005), Tenailleau et al. (2006b) and Xia et al. (2008, 2009) have challenged the traditional interpretation that supergene violarite (after pentlandite) forms directly through the fractional de-intercalation of iron and nickel cations from pentlandite into the ground water (that is, essentially, a solid-state process), and have suggested an alternative mechanism that involves the complete dissolution of pentlandite (with the formation of aqueous metal and aqueous sulfur species) coupled to the subsequent in-situ epitaxial precipitation of violarite. Within this context, it is important to note that a method for the precipitation of violarite from an aqueous stock solution comprising soluble species of all the elemental components (Fe, Ni and S) has not been reported in the literature. The present paper describes our preliminary findings concerning the first successful method for preparing violarite by a direct hydrothermal route using iron(II) acetate, nickel(II) acetate and DL-penicillamine as the chemical reagents in aqueous solution. 2. Experimental methods The chemical reagents: iron(II) acetate (95%), nickel(II) acetate tetrahydrate (98%) and DL-penicillamine, (CH3)2C(SH)CH(NH2) COOH (97%) were purchased from Aldrich Chemical Company and used as received. 2 Based upon the above assays, attempts were 2 Preliminary attempts to prepare polydymite using thioacetamide (CH3C(S)NH2) as the sulfurising reagent (using a similar procedure) have been unsuccessful so far. In this case, hexagonal α-NiS was the only solid precipitated. Therefore, no attempt was made here to prepare violarite using thioacetamide.

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Table 1 Chemical composition for the various reaction solutions and the corresponding iron– nickel sulfide precipitates as investigated in this work. Specimen Elemental composition Precipitate Phases mass/g (XRD)a of the initial reaction solution/mmol 1

4

5Fe : 10Ni : 35S (75% excess S)c 2.5Fe : 12.5Ni : 25S (25% excess S)c 6Fe : 9Ni : 25S (25% excess S)c 5Fe : 10Ni : 20S

5

15Ni : 20S

6

2.5Fe : 12.5Ni : 20S

2 3

1.719 1.493 1.426 1.180 83% yieldd 1.459 96% yieldd 1.382 87% yieldd

Violarite Pyrite Violarite Pyrite Violarite Pyrite Violarite

Composition (EMPA)b

Fe0.2Ni2.7S4 Fe0.50Ni0.51S2 Fe0.31Ni2.36S4

Polydymite Ni3.06S4 Violarite

Fe0.02Ni3.22S4

a The mineral names listed in this column refer to the phases identified by powder XRD (cf. Figs. 1–6). b These compositions are the mean values of 6−9 points measurements by EMPA. c The ‘percentage excess S’ in the initial reaction solution is with respect to the sulfur content of the desired violarite, (Fe,Ni)3S4. d The yield percentages are expressed in terms of the fraction of the sulfur content within the initial reaction solution that became the thiospinel precipitate. The composition of the thiospinel precipitate was determined from the EMPA.

made to prepare 1.5 g samples of violarite, (Ni,Fe)3S4 with various Fe/Ni ratios, and in certain cases, deploying an excess of DLpenicillamine. The elemental composition of the initial reaction solution for these preparations is described in Table 1. The presence of the acetate ions in these solutions provided a convenient pH-buffer. The pH of the solution was measured using pH-indicator paper at the end of each reaction at ambient temperature. A pH = 4 was obtained in all cases. For example, in the attempt to prepare violarite with the ideal composition, FeNi2S4 (cf. specimen 4), 5 mmol of iron(II) acetate; 10 mmol of nickel(II) acetate tetrahydrate; and 20 mmol of DLpenicillamine were dissolved in 110 mL of distilled water at room temperature using magnetic stirring. This deep reddish purple solution was transferred into a Teflon™ container and inserted into a stainless steel autoclave (built in-house), and then placed in a preheated electric furnace (Aurora Classic Kiln P5923, Potterycrafts Ltd.) at 130 °C for 45 h. The autoclave was then removed from the furnace and left to cool to room temperature. The contents of the Teflon™ container were filtered using a medium speed filter paper and a glass funnel, and then washed with distilled water, followed by ethanol, in order to isolate the product precipitate. The precipitate, whilst still attached to the filter paper, was placed in a drying-oven at 60 °C for 4 h, before being weighed. The other specimens were prepared likewise. The precipitates were analysed by powder X-ray diffractometry using a Siemens D5000 powder diffractometer equipped with a Ge(111) incident beam monochromator and, Cu-Kα1 radiation (λ = 1.5405 Å), with a Bruker Diffrac-Plus data capture/treatment system. Quartz was used as the external standard in order to calibrate the instrument. The peak positions and intensities were determined using the Bruker Diffrac-Plus EVA programme. The powder patterns were indexed using the software: Program for the automatic indexing of powder diffraction patterns by the successive dichotomy method (DICVOL06) written by Boultif and Louer (2004). The dimensions of the unit cells were determined by least-squares fit of the data. Polished mounts were prepared by setting a dispersion of the precipitate in epoxy araldite DBF with the hardener HY951 (Vantico Ltd.). These were ground and then polished on a lapidary cloth to 1 μm using polycrystalline diamond spray. Finally, the polished mounts were coated with 15 nm of carbon using a Quorum Q150E carbon evaporator. The SEM micrographs were obtained using a FEI

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Intensity / arbitrary units

Intensity / arbitrary units

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2 theta / degree

2 theta / degree Fig. 1. Powder X-ray diffraction pattern (Cu-Kα1 radiation) for the precipitate in specimen 1. PDF 47-1740 for violarite (FeNi2S4) is shown in red. PDF 42-1340 for pyrite (FeS2) is shown in blue with heads.

Fig. 3. Powder X-ray diffraction pattern (Cu-Kα1 radiation) for the precipitate in specimen 3. PDF 47-1740 for violarite (FeNi2S4) is shown in red. PDF 42-1340 for pyrite (FeS2) is shown in blue with heads.

Quanta 650 FEG-ESEM scanning electron microscope. The polished cross-sections were imaged in backscattered electron Z contrast mode. The accelerating potentials were adjusted depending on the sample to either 5 or 20 kV in order to optimise textural features and atomic number contrast (see Figure captions for details). The electron microprobe analyses (EMPA) were performed using a Jeol JXA-8230 Electron Probe Microanalyser. The operating parameters were: beam energy 20 kV, beam current 20 nA with 20 s peak time per element. Each specimen was sampled at 6–9 points. The standards used were: pyrite for Fe, S; nickel metal for Ni. Apparent concentrations were corrected for atomic number, absorption and fluorescence using Jeol phi-rho-z software. Element intensity distribution maps for Fe, S and Ni were obtained on the microprobe by stage scanning a 61 × 61 μm area at a pixel resolution of 1024 × 1024 with step interval of 0.06 μm and a pixel dwell time of 10 ms.

The powder X-ray diffraction patterns for the product precipitates are shown in Figs. 1–6. The phases that were identified in these patterns are described in Table 1. Regarding the single phase materials in specimens 4 and 5, the X-ray reflections in the corresponding Figs. 4 and 5 have been indexed to the spinel cubic cell, as listed in Tables 2 and 3 respectively. The cell constants for the violarite precipitate (a = 9.478(3) Å) and the polydymite precipitate (a = 9.483(1) Å), as calculated from these data, are in reasonable agreement with the values reported in PDF 47-1740 for violarite (a = 9.458 Å) and PDF 47-1739 for polydymite (a = 9.476 Å) respectively. These values are in sharp contrast to greigite, Fe3S4, in which the cell constant is much larger, a =9.876 Å (Skinner et al., 1964). Furthermore, the presence of strong X-ray fluorescence (as manifested by the raised background level) in the powder pattern of violarite using Cu-Kα1 radiation (Fig. 4) when compared with that in the powder pattern of polydymite (Fig. 5) is consistent with the incorporation of iron into the thiospinel structure; although a similar effect would arise, of course, if the product material was contaminated by an amorphous iron-containing phase. The results of the electron microprobe analyses on these precipitates are shown in Table 1. It is to be emphasised that the very small grain size and the poor polish of the mounted specimens reduced the precision of these analyses. The compositions listed in Table 1 are the mean values of 6−9 points per measurement. Nevertheless, these results confirm that the polydymite, Ni3S4 precipitate in specimen 5 is free from iron. And these show, most importantly, that

3. Results and discussion

Intensity / arbitrary units

Intensity / arbitrary units

The dried precipitates produced in this work were dark-grey finegrained powders. None of these particles were attracted towards a NEOMAX™ magnet, indicating that ferrimagnetic (or ferromagnetic) phases, such as, greigite, Fe3S4 and pyrrhotite, Fe7S8 were unlikely to be present as impurities since these would have been attracted to this magnet. (N.B. Pauli-paramagnetic phases, such as, violarite and polydymite should not be attracted to this magnet.)

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2 theta / degree Fig. 2. Powder X-ray diffraction pattern (Cu-Kα1 radiation) for the precipitate in specimen 2. PDF 47-1740 for violarite (FeNi2S4) is shown in red. PDF 42-1340 for pyrite (FeS2) is shown in blue with heads.

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2 theta / degree Fig. 4. Powder X-ray diffraction pattern (Cu-Kα1 radiation) for the precipitate in specimen 4 (Fe0.31Ni2.36S4). The peaks marked with green lines correspond to a thiospinel and are indexed in Table 2. The cell constant, a = 9.478(3) Å.

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2 theta / degree Fig. 5. Powder X-ray diffraction pattern (Cu-Kα1 radiation) for the precipitate in specimen 5 (Ni3.06S4). The peaks marked with green lines correspond to a thiospinel and are indexed in Table 3. The cell constant, a = 9.483(1) Å.

Intensity / arbitrary units

Fig. 7. Backscattered electron micrograph of the precipitate in specimen 4. This image reveals a mass of platy crystals of violarite (Fe0.31Ni2.37S4) with typical dimensions 10 × 10 × 1 μm.

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2 theta / degree Fig. 6. Powder X-ray diffraction pattern (Cu-Kα1 radiation) for the precipitate in specimen 6 (Fe0.02Ni3.22S4). PDF 47-1740 for violarite (FeNi2S4) is shown in red.

a significant amount of iron has been incorporated, together with nickel, into the thiospinel phase in specimens: 3, 4 and 6. But in all cases, to a lesser extent than anticipated by the Fe/Ni ratio within the initial reaction solution (see Table 1). A backscattered electron micrograph of the precipitate in specimen 4 is shown in Fig. 7. This precipitate was prepared with the chemical reagents in stoichiometric amounts, in an attempt to yield the ideal composition, FeNi2S4. This micrograph reveals a porous agglomerate of platy crystals of violarite (Fe0.31Ni2.36S4) with typical dimensions 10 × 10 × 1 μm. This image can be contrasted with the backscattered electron micrograph of the precipitate in specimen 3 (see Fig. 8) that was prepared with 25 mol% excess sulfur. In this case, the micrograph also reveals an agglomerate of platy crystals of violarite (Fe0.2Ni2.7S4) with typical dimensions 10 × 10 × 1 μm, but these are embedded within a nickeliferous pyrite (Fe0.50Ni0.51S2) matrix or infill. This feature is also revealed clearly in the element

Table 2 Powder X-ray diffraction data for artificial violarite (Fe0.31Ni2.37S4) indexed to a spinel cubic cell (cf. Specimen 4 and Fig. 4). Cell constant a = 9.478(3) Å, Z = 8. 2θ / °

dobs / Å

I/I1 obs

hkl

16.24 26.61 31.28 37.96 46.99 49.95 54.69

5.453 3.348 2.858 2.369 1.932 1.824 1.677

35 51 100 56 20 33 65

111 220 311 400 422 333 440

Table 3 Powder X-ray diffraction data for artificial polydymite (Ni3.06S4) indexed to a spinel cubic cell (cf. Specimen 5 and Fig. 5). Cell constant a = 9.483(1) Å, Z = 8. 2θ / °

dobs / Å

I/I1 obs

hkl

16.19 26.56 31.27 37.91 46.93 49.96 54.69 64.34 68.52

5.470 3.353 2.859 2.371 1.935 1.824 1.677 1.447 1.368

24 41 100 49 13 32 61 7 6

111 220 311 400 422 333 440 533 444

Fig. 8. Backscattered electron micrograph of the precipitate in specimen 3. This image shows platy crystals of violarite (similar to those in Fig. 7) embedded within a nickeliferous pyrite matrix or infill.

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intensity distribution maps for this specimen as shown in Fig. 9. Incidentally, this texture is rather similar to that described by Ramdohr (1980) for a petrological specimen from Horbach, Schwarzwald, Germany, in which bravoite (nickeliferous pyrite) and violarite occur together as secondary minerals in association with the supergene alteration of pentlandite. Ramdohr reported that the violarite seems to form mainly a network along the (111) cleavage planes in the pentlandite that are infilled with bravoite. It is very likely that the platy crystals of violarite (Fe0.2Ni2.7S4) produced in this present work formed initially and were then infilled with nickeliferous pyrite (Fe0.50Ni0.51S2). With regard to the recent work by Xia et al. (2008), it was anticipated that artificial violarite with the ideal composition, FeNi2S4 might form as the product in specimen 4. The results of the EMPA indicate, however, that the nickel-rich violarite, Fe0.31Ni2.36S4 has formed preferentially. The results from specimen 3 show that an increase in the Fe/Ni ratio within the initial reaction solution (albeit with a greater sulfur content) does not increase the iron content of the violarite precipitate. It is important to note that these results are consistent with the supergene nickel-enrichment that is observed in related petrological material occurring just beneath the water table. In this geochemical environment, the violarite is known to become

increasingly nickel-rich towards the surface. It has been suggested by Nickel (1973) and Thornber (1975) that the violarite undergoes cation exchange, taking up nickel from these relatively nickel-rich solutions and releasing iron into solution. Moreover, these results are also consistent with the equilibrium EH−pH diagram for the Fe−Ni −S−H2O system at 25 °C as constructed by Warner et al. (1996); in that nickel-rich violarite, Fe1−xNi2 + xS4 predominates over violarite with the ideal composition, FeNi2S4, within the stability domain of H2O, whereby the extent of the nickel enrichment would appear to be strongly influenced by the ratio of the Ni 2 +/Fe 2 + aqueous activities in the solution. At a first glance, the successful precipitation of violarite as reported in this present article, would appear to support the solution mediated mechanism of the pseudomorphic replacement of pentlandite by violarite as proposed by Pring et al. (2005), Tenailleau et al. (2006b) and Xia et al. (2008, 2009), in that violarite has now been demonstrated to precipitate directly from aqueous solution. However, the rather limited incorporation of iron into the violarite structure (Fe/Ni ≲ 1/8) can be interpreted to indicate that violarite with a Fe/Ni ratio more akin to that in the ideal composition, FeNi2S4 (Fe/Ni ~ 1/2) is metastable at 130 °C in an aqueous system. Consequently, this implies that violarite with a composition

Fe

Ni

S

BSE

Fig. 9. Element intensity distribution maps for Fe, Ni and S for the precipitate in specimen 3. The BSE micrograph (bottom right) shows platy crystals of light grey violarite (Fe0.2Ni2.7S4) embedded in a medium grey nickeliferous pyrite (Fe0.50Ni0.51S2) matrix or infill (cf. corresponding BSE micrograph in Fig. 8). Colour key: the spectral colours ranging from red to blue represent graduations from high to low element concentration. Field of view 61 × 61 μm.

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comparable to or, more iron-rich than FeNi2S4, is most probably formed under metastable conditions; for instance, through the irreversible de-intercalation of metal ions from pentlandite in aqueous media. Within this context, it is important to realise that violarite is a nonstoichiometric phase. That implies that its chemical potential, μ(Ni,Fe)3S4 is a function of composition in addition to temperature and pressure. Therefore, at a given temperature and pressure, the chemical potential of violarite is also dependent on the ratio of its Fe/Ni content. So, it is quite plausible for violarite with the ideal composition, FeNi2S4 to be thermodynamically stable within the Fe−Ni −S ternary system at 300 °C under an equilibrium sulfur vapour pressure; as indeed appears to be the case (Craig, 1971). But, the ideal composition, FeNi2S4 may not necessarily be thermodynamically stable within an aqueous Fe−Ni−S−H2O quaternary system at 130 °C; although it can form as a metastable product from an irreversible process. This present work has shown that nickel-rich violarite precipitates preferentially to iron-rich violarite from an aqueous solution. Conversely, the work by Xia et al. (2008) showed that iron-rich violarite formed preferentially to nickel-rich violarite during the alteration of pentlandite in their experiments. It should be interesting to account for this significant difference in the Fe/Ni ratio of the phases produced by these two different preparative routes. Therefore, it is important to find a sulfurising reagent (preferably one that is soluble in water) that is able to produce violarite with a Fe/Ni ratio more comparable with that in the ideal composition, FeNi2S4. Hence, it is crucial to identify the nature of the sulfur species formed during the dissolution of the pentlandite as deployed in the preparation of violarite by Xia et al. (2008). If this proved to be successful, it should give more credibility to the mechanism suggested by Pring et al. (2005), Tenailleau et al. (2006b) and Xia et al. (2008, 2009). And possibly extend the range of violarite compositions that can be precipitated directly from aqueous solution. 4. Conclusions This work has demonstrated for the first time that nickel-rich violarite (Fe0.31Ni2.36S4) can precipitate directly from an aqueous solution of iron, nickel and sulfur species. Indicating it is feasible for nickel-rich violarite to precipitate hydrothermally in a geochemical environment. As a preparative method, this process has the advantage of yielding a nickel-rich violarite as a phase-pure fine powder within a shorter period of time (~ 48 h) than many of the other methods reported in the literature and, is complimentary to the method developed by Xia et al. (2008) that produces iron-rich violarite using artificial pentlandite as a precursor compound. Violarite with a composition in which Fe/Ni≳ 1/8 is most probably metastable in the Fe−Ni−S−H2O quaternary system at 130 °C. The hydrothermal method reported here should, in principle, be applicable for the preparation of other thiospinels. Furthermore, the use of possible selenium and tellurium analogues to DL-penicillamine, for the preparation of other chalcogenide spinels, is a worthy area to investigate. Acknowledgements Acknowledgement is given to Dr Eric Condliffe, University of Leeds, for performing the electron microprobe analyses and for recording the backscattered electron micrographs. We thank Professor

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