CHARACTERIZATION OF THE NATURAL MINERAL FORM FROM THE PbS – Sb 2 S 3 SYSTEM

This paper presents the results of experimental investigations of natural mineral form from the PbS – Sb2S3 system. Investigated sulfosalt appears in nature in the form of nanowires with diameters ranging from 200 to 500 nm and its length can reach up to 150 m. In most cases these nanowires are arranged like bundles, which were inspected with scanning electron microscopy (SEM). The chemical composition of this material was analyzed and its stoichiometry responded to “plumosite”. Crystal structure was analyzed using XRD analyses and compared to available literature data. Investigated samples were subjected to thermal analyses and resulting mechanism of oxidation was concluded. Also, kinetic parameters were calculated for the process of oxidation.


Introduction
It is well known that semiconductor nanowires have attracted great research interest in materials science for several years due to their potential use in microelectronic, electronic, optical, magnetic and sensor devices [1,2,3].For making semiconductor nanowire devices, the contacting is one of the most important procedures.The metallic contacts have to be stable and the contact resistance has to be low.Furthermore, for low-temperature measurements the contacts should remain of good quality at milli-

DOI:10.2298/JMMB1001075S
Kelvin temperatures.Thus, the design of nanostructures of the commonly-used semiconductor materials such as Si and ZnO could be appreciably improved using new materials [4].According to this, attempts were made to find alternative materials, which are promising for applications in the nanotechnology [5,6].It is known that the properties of the semiconductors are strongly influenced by the type of elements presented in the structure mainly because of the role that d electrons play in their chemical bonding [7].Recently, it was reported that it is possible to synthesize nanowires and nanotubes of binary sulfides like MoS 2 , [8] PbS [9,10] and Sb 2 S 3 [11] which are natural existing minerals as well.Synthesizing enables introduction of an element with unoccupied d orbitals that favors low energy electronic movements which influence the optical and physical properties of those materials.
As known from the literature the bulk materials of PbS-Sb 2 S 3 sulfosalts are semiconductors having band gaps between 1.2 and 1.7 eV including boulangerite with a band gap of 1.34 eV.[12] Sulfosalts are complex sulfides of similar structures, consisting of long layers or chains of M-Spolyhedra (M=Pb and Sb) on the basis of the archetype structures of SbS and PbS [13].Because of relatively strong bonds in chain direction, PbS-Sb 2 S 3 sulfosalts tend to grow as thin needles parallel to the chain direction.Furthermore, the formation of closed microrings was reported for PbS-Sb 2 S 3 natural minerals, [14] which have not been completely understood so far.
Based on the extremely rare occurrence of nanowires of natural boulangerite (Pb 5 Sb 4 S 11 ) and jamesonite (FePb 6 Sb 4 S 14 ), [14] synthesis of nanowires of boulangerite and other mineral forms in the PbS-Sb 2 S 3 system obtained scientific significance lately.Results of characterization of those sulfosalts are presented in literature references.[13,14,15] If having in mind growing interest for those compounds from the PbS-Sb 2 S 3 system, it should be noted that basic information concerning physicochemical and mineralogical characteristics of this system are not completely investigated yet.Fortunately, natural minerals from the PbS-Sb 2 S 3 system could be found in mineral deposits in our country, which gave us the opportunity to study this system using natural mineral forms instead of sensitized ones.Accordingly, in this paper we investigated characteristics of natural sulfosalt from the PbS-Sb 2 S 3 system obtained from Trepča mine locality (Serbia).According to the expert's evaluations this mine has reserves of 60 millions tons of ore containing 5 millions tons of lead and zinc metal.Sulfosalts from the PbS-Sb 2 S 3 system are largely contained at this locality.
Most common natural mineral form from the PbS-Sb 2 S 3 system is boulangerite.It is usually situated in ore deposits of antimony and lead ores and often contains Fe, Ni, Cu, Maybe the most interesting mineral form from the PbS-Sb 2 S 3 system is "plumosite" (Pb 2 Sb 2 S 5 ).This mineral form is occurring in Trepča mine, in many of ore mines in the vicinity of Baia Mare and in other Miocene vein type deposits elsewhere in northern Romania [16].There are lots of discrepancies concerning "plumosite" as mineral form.Many authors don't approve existence of "plumosite" as separate mineral form.It is often referred as iron free jamesonite [16,17] or even synonym of boulangerite [18].Although Mumme [18] noted the apparent structural identity of "plumosite" and suggested that it may in fact be a discrete phase within the meneghinite (Pb 13 CuSb 7 S 24 ) homologous series.In this manner, the term "plumosite" has been used to describe fibrous varieties of a number of Pb sulfosalts over the past 150 years.In the literature reference Cook and Damian, [19] the authors suggest usage of the term "plumosite" in the sense of Oudins [20] description: "as a descriptive term which may be applied to jamesonite, boulangerite, zinkenite, any other lead sulfosalt, or as mixture thereof, that has a plumosite texture or that occurs in a fibrous aggregate".On the other hand, on the basis of analysis of composition and structural investigations of falkmanite (Pb 5 Sb 4 S 11 ) and boulangerite, Mozgova [21] et al suggested that the name "plumosite" should be applied for Pb-poor boulangerite (Pb <4.7 Sb >4.2 S 11 ).
The aim of this paper is to present first hand results concerning the structure and physicochemical characteristics of natural PbS-Sb 2 S 3 sulfosalt from Trepča mine.Also, this paper presents the results of thermal analysis and kinetic parameters of the investigated sample oxidation process.

Experimental procedure
Starting sample for investigations presented in this paper was natural mineral form from the PbS-Sb 2 S 3 system obtained from Trepča mine, which appears in nature as wool of microscopic needles or nanowires with diameter range from 200 to 500 nm.Those wires are highly flexible.When crushed to powder, they become very adhesive.
Characterizations of investigated samples were done using scanning electron microscopy (SEM) with EDEX-9100 analyses and PAX software package, with resolution of 1 nm (30kV), excitation voltage of 0,2-30kV, maximum magnification of 500000 times and with the secondary electron detector.Energy Dispersion X-ray Fluorescence analysis was done on Canberra equipment with the radioisotopes: Cd-109 (22.1 keV) and Am-241 (59.5 keV).
The non-isothermal oxidation process was investigated using the results of DTA analysis.A thermal analysis Derivatograph 1500 (MOM Budapest) was used, operating under the following conditions: air atmosphere, heating rate of 20ºC/min and T max = 1273 K.

Results and discussions
Results of X-ray analysis of the starting sample were compared with results reported for the mineral forms of the PbS-Sb 2 S 3 system present in the literature (Table 1).This was the only way possible for discussing X-ray results, since mineralogical databases rarely contains enough data concerning this system.Diffraction study presented in this paper was compared to diffraction patterns from Herja (Romania) "plumosite" [19] and jamesonite; [22] jamesonite from Bolivia [23] and of benavidesite from Uchucchacua (Peru) [20], which had a Mn/(Fe+Mn) ratio of 0.61.
Results of X-Ray analysis for "plumosite" from Herja (Romania) [19] showed the phase to correspond to jamesonite with a slight peak shift toward the Mn-analogue of jamesonite, benavidesite, Pb 4 (Mn,Fe)-Sb 6 S 14 .It was concluded that the relatively high Mn content of the Herja "plumosite" caused this shift [20,24].Udubasa et al., [16] interpret "plumosite" to have been one of the latest minerals to have crystallized from hydrothermal solution and from this reason, collected the relatively incompatible elements such are Mn or Ti.
Presence of Mn is confirmed in investigated sample from Trepča, as well, using EDXRF analysis.Nevertheless, amount of Mn in sample was too small to have significant influence on RDA results.If compare peaks in table1, it is obvious that "plumosite" from Trepča has largest phase correspondence to jamesonite.This proved statement published in book of Tućan, [17] where "plumosite" from Trepča was described as iron free jamesonite.
Both natural (this work) and synthetic [14] PbS-Sb 2 S 3 sulfosalts with boulangerite as main mineral form were found to grow as microscopical needles consisting of parallel intergrown nanowires (Figure 1), where the diameter of each individual wire ranges between 200 and 500 nm and its length can reach up to 150 m.
Further EDXRF analysis upon fluorescence excitation with the 241Am radioisotope discovered small amount of Mn and Fe impurities, Figure 2.
The results of the differential thermal analysis (DTA), of the investigated sample are presented in Figure 3.The DTA results show the existence of a first exothermal peak in the temperature range 97.33 According to the DTA analysis, XRD characterization of the samples heated at 500 o and 700 o C, Gibbs free energies change versus temperature calculated for potential reactions (Figure 4) and previous investigations of Sb 2 S 3 and PbS oxidation processes, [24,25]    and consequently to PbO (peak at 602.7 o C), similar mechanism responded for pure PbS investigated earlier [26].PbSO 4 was determined using XRD both on 500 and 700 o C.

Kinetics
According to the nature of the investigated oxidation process in the PbS-Sb 2 S 3 system, which results in the corresponding shape of DTA curve, Figure 3, it is possible, using non-isothermal model of Borchardt and Daniels [27], to determine the activation energy of oxidation, in the temperature ranges: 97, 33    these results, the corresponding Arrhenius diagrams were constructed and presented in Figure 5.
Based on the diagrams in Fig. 5, the corresponding values for activation energy have been calculated for the process.Activation energies for characteristic segments on DTA curves, are found to be 69, 154, 15; 175 and 90 kJ/mol, respectively, which means that oxidation process starts in the kinetic domain, according to classical kinetic theory [28,29,30].Endothermal dissociation is happening under diffusion domain and followed with exothermal oxidation in the kinetic domain, again.

Conclusions
Natural mineral form from the PbS-Sb 2 S 3 system has been experimentally investigated by DTA, RDA, SEM and EDX analysis.According to the chemical analysis it was found that its composition is close to the "plumosite" (Pb 2 Sb 2 S 5 ) stechiometry, with small content of Mn and Fe discovered using EDX.The RDA analysis positioned this mineral form in the range of "iron free" jamesonite.According to the SEM analysis it was concluded that this natural mineral form is composed of numerous nanowires with diameters ranging from 200 to 500 nm and the length up to 150 m.
The results including kinetic parameters and mechanism of the investigated process have been determined and presented in the paper.The mechanism of oxidation was determined using DTA and RDA analysis coupled with calculation of Gibbs free energies for potential reactions.It was found that first two exotherms, on DTA curve, respond to two stadiums in Sb 2 S 3 oxidation process: Sb 2 S 3 ® Sb 2 (SO 4 ) 3 and Sb 2 S 3 ® Sb 2 O 3 .Second peak is also representing oxidation of PbS to PbO 2 .Endothermal peak is representing decomposition of PbO 2 to PbO.Next two exotherms cover oxidation of remaining PbS to PbSO 4 and PbO.This mechanism was in partial accordance with mechanism of separate Sb 2 S 3 and PbS samples oxidation [25,26].Kinetic analysis, carried out according to Borchardt and Daniels [27], showed that the values of activation energy are consistent with oxidation taking place in kinetic region for all exotherms, which implies that reaction surface and temperature have the limiting influence on reaction rate.It is also determined that endothermal dissociation is taking place in diffusion region.This means that diffusion of gaseous reactants, through formed oxide layer covering each particle of the starting sample, have the limiting influence on overall reaction rate.
As the next step in further investigation of PbS-Sb 2 S 3 system, research of synthesized samples with different stoichiometry could be done, since there is a lack of references dealing with the mentioned system.Also, this kind of new materials investigation is in line with recent trends in extractive metallurgy [31], which could be interpreted as matching strategies to situations [32,33,34].

Fig. 4 .Fig. 5 .
Fig.4.Graphic presentation of Gibbs free energy change vs. temperature for the potential reactions in the PbS-Sb2S3 system (HSC software)