FLORE Repository istituzionale dell'Università degli Studi di Firenze How many alacranites do exist? A structural study of non-stoichiometric As8S9-x crystals. Questa è la Versione finale referata (Post print/Accepted manuscript) della seguente pubblicazione: Original Citation: How many alacranites do exist? A structural study of non-stoichiometric As8S9-x crystals / P. BONAZZI; L. BINDI; F. OLMI; S. MENCHETTI. - In: EUROPEAN JOURNAL OF MINERALOGY. - ISSN 0935-1221. - STAMPA. - 15(2003), pp. 283-288. Availability: This version is available at: 2158/312415 since: Terms of use: Open Access La pubblicazione è resa disponibile sotto le norme e i termini della licenza di deposito, secondo quanto stabilito dalla Policy per l'accesso aperto dell'Università degli Studi di Firenze (https://www.sba.unifi.it/upload/policy-oa-2016-1.pdf) Publisher copyright claim: (Article begins on next page) 30 September 2021 Eur.J. Mineral. Dedicatedto thememory of 2003, 15, 283–288 LucianoUngaretti How many alacranitesdo exist? Astructuralstudy ofnon-stoichiome tricAs 8S9-x crystals PAOLA BONAZZI1, LUCA BINDI1, FILIPPO OLMI2 and SILVIO MENCHETTI1 1Dipartimento di Scienze della Terra,Universit` adegli Studi di Firenze, via LaPira4, I-50121 Firenze, Italy e-mail:[email protected] 2CNR –Istituto di Geoscienze eGeorisorse –sezione di Firenze, via LaPira 4, I-50121 Firenze, Italy Abstract: Crystals ofarsenic sulphide (reportedas alacranite),coming fromthe burningdump of Kate ÏrinaMine (Czech Republic), have been investigated by single crystal X-raydiffraction and chemical microanalysis. Both analytical data and unit-cellparameters stronglysuggest the existence ofa continuous series between the high-temperature polymorph ( -As4S4)and the mineralalacranite (As 8S9).As the Scontent increases inthe series, the unit-cellvolume increases accordingly.Thestructural model has been obtained fortwo crystals, exhibitingdifferent unit-cell volumes (ALA15: a = 9.940(2), b = 9.398(2), c = 9.033(2), =102.12(2), V = 825.0(3), Robs =6.12 %;ALA2: a = 9.936(2), b = 9.458(2), c = 9.106(2), =101.90(2), V = 837.3(3), Robs =6.41 %).W efoundthat the non-stoichiometriccompounds crystallize as adisordered mixtureof two kinds of cage-like molecules, packed togetheras inthe -As4S4 phase. Thefirst one is identical tothe As 4S4 molecule foundin the structures ofboth realgar ( -As4S4) and -As4S4.Thesecond molecule is chemically and structurallyidentical tothat foundin the As 4S5 compound. Thesimultaneous presence ofAs 4S4 (C2/c) and As8S9 (P2/c)microdomains could be areason forthe observed gradual change ofthe translation symmetry fromthe -phase toalacranite s.s.. Key-words: alacranite, crystal structure, arsenic sulphides, chemical composition, Kate Ïrina Mine. Introduction Table1. Unitcell parameters for alacranites from KateÏrinaMine to- getherwith data from literature. Threephases among the known crystalline modifications of 3 tetra-arsenictetrasulphide ( i.e. realgar,pararealgar, and - a (Å) b (Å) c (Å) (°) Vol. (Å ) phase1)occur as minerals. Realgar is the low-temperature ALA119.968(4) 9.317(2) 8.906(4) 102.45(4) 807.7(5) ALA129.963(9) 9.351(2) 8.984(5) 102.43(2) 817.4(9) form, -As4S4 (Hall,1966; Clark, 1970; Roland, 1972; Blachnik et al.,1980; Bryndzya &Kleppa, 1988), which ALA159.940(2) 9.398(2) 9.033(2) 102.12(2) 825.0(3) commonly occurs as asublimation product in active volca- ALA1 9.95(1)9.44(1) 9.07(1) 102.1(2) 833(2) nic areas,in low-temperaturehydrothermal deposits, and, to ALA2 9.936(2)9.458(2) 9.106(2) 101.90(2) 837.3(3) alesser extent, as aminor constituent of lead, silver, and ALA7 9.941(6)9.450(5) 9.110(2) 101.81(3) 837.7(7) ALA6 9.918(5)9.478(7) 9.145(6) 102.10(5) 840.6(9) gold ore veins. Pararealgaroccurs as ayellow filmcovering realgarcrystals which have been exposed to light (Roberts CL* 9.97(1)9.29(1) 8.88(1) 102.6(1) 803(2) PS 9.957(3)9.335(4) 8.889(5) 102.48(4) 806.7(6) et al.,1980; Bonazzi et al., 1995). The -As4S4 phase is the high-temperature form,stable inthe system As-S attemper- PPV 9.89(2)9.73(2) 9.13(1) 101.84(5) 860(3) atures higher than 256 5°C(Hall,1966). The occurrence BP9.943(1)9.366(1) 8.908(1) 102.007(2) 811.4(1) of anatural phase exhibiting adiffraction pattern quite simi- ZO 9.87(1)9.73(3) 9.16(2) 101.52(4) 858(4) larto that of the synthetic -As4S4 was firstreported by Note:CL =natural -As4S4 from Alacr`anMine,Chile (Clark, 1970); Clark (1970), who found both low- and high-temperature PS=synthetic -As4S4 (Porter& Sheldrick,1972); PPV =alacranite formsin the Ag-As-Sb vein deposit atAlacr an` (Chile). Ac- fromKamchatka (Popova et al.,1986);BP=natural -As4S4 (Burns &Percival,2001); ZO = alacranite from KateÏrina Mine (Z´Ï acekÏ & cording to Clark (1970), the natural -As4S4 phase formsir- regular masses and exhibits optical properties similarto Ondrus,Ï 1997).*indexed by thepresent authors. those of realgar,but the colour is slightly paler and moreyel- lowish than realgar.At the Alacr`an Mine the mineraloccurs closely associated to smithite, orpiment and arsenolamprite. 1 Muchconfusion exists in literature for the use of - and -descriptors.Fol- lowingDouglass et al.(1992)we willrefer tothe low-temperature form as Although the main properties of the mineral(XRD data, andto the high-temperature form as . chemicalcomposition, optical properties and Vickers’ 0935-1221/03/0015-0283 $2.70 DOI:10.1127/ 0935-1221/2003/0015-0283 2003 E.Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart 284 P.Bonazzi,L. Bindi,F. Olmi,S. Menchetti hardness) weredefined by Clark (1970), the proposal of a new mineralspecies corresponding to the natural -As4S4 polymorph was not approved by the NMMN– IMACom- mission (Popova et al.,1986). Later,a new arsenic sulphide was found atthe Uzon caldera(Kamchatka, Russian Feder- ation) by Popova et al. (1986). These authors assumed this mineralto be identical to the species previously described by Clark (1970) due to the sim- ilarity of their XRD powder patterns. For this reason, the mineralwas named alacranite(Popova et al., 1986). As shown in Table 1, the latticeparameters of alacraniteresem- ble fairlythose of the synthetic -As4S4 (Porter& Sheldrick, 1972) and those of the natural -phase fromAlacr an` (Clark, 1970). In keeping with the greaterunit-cell volume, alacra- nite exhibits adifferent chemicalcomposition (As 8S9). Ac- cording to Popova et al.(1986), alacraniteis monoclinic, P2/c,while the synthetic -As4S4 crystallizes in the C2/c space group. During arecent sampling of the seafloor around Lihir Island (Papua NewGuinea), aspecimen main- Fig.1. SEM micrographof a crystalof alacranite togetherwith ly consisting of pyrite, sphalerite, and galena, together with amorphousAs-S alloyfrom Kate ÏrinaMine, Czech Republic. red and orange arsenic sulphides, was recovered atthe top of Conical Seamount (Percival et al.,1999). According to KateÏrina Mine is located atRadvanice, near Trutnov and these authors, the XRD analysis of both the deep-red and or- belongs to the Lower-Silesian coal basin in the north-east- ange crystals revealed amixture of realgarand alacranite. ern part of Bohemia (Czech Republic). Disseminated sul- The subsequent detailed investigation (Burns &Percival, phide (pyrite, marcasite,pyrrothine, chalcopyrite, chalco- 2001) showed the mineralfrom Papua NewGuinea to be cite, bornite, galena and sphalerite) and uranium (earthy structurally and chemically identical to the synthetic - uranium oxides and rarecoffinite) mineralizations accom- As4S4.However, one maywonder whether this mineralis pany the coal measures of the Radvanice Group of Coals actually the samemineral species defined asalacraniteby (ZÏ a´cekÏ & Ondrus,Ï 1997). Asecondary mineralization origi- Popova et al. (1986). Indeed, the alacranite described by nated as aconsequence of morethan adecade lasting sub- Burns &Percival (2001) is quite similarto that fromAlac- surfacefire of alarge dump atthe Kate Ïrina colliery.Accord- ran` (Clark, 1970), but, as also noted by Jambor &Roberts ing to ZÏ a´cekÏ & OndrusÏ (1997), several “minerals”were (2002), itappears to differwith respect to chemicalformula, formedfrom escaping gasses and vapours, including native unit-cell volume and space group fromthe mineralap- elements (sulphur, selenium, Bi-antimony,bismuth and proved with the namealacranite by the NMMN– IMA lead), sulphides (galena, greenockite, antimonite, arsenic Commission (Hawthorne et al., 1988). sulfides, and monoclinic GeSnS 3 ),oxides (molybdite, arse- The complexity of the problem is further increased due to nolite, hexagonal GeO 2),halides (salammoniac, cryptoha- the alteration induced by light on the tetra-arsenictetrasul- lite, bararite),sulphates (anglesite, mascagnite, letovicite), phides. Itis long known that both polychromatic and mono- and organic compounds (kratochvilite, kladnoite). Inaddi- chromatic light alterrealgar and the -phase up to parareal- tion, avariety of sulphates formedby alteration. Among the gar through an intermediate product ( -phase) (Douglass et arsenic sulphides, As-S alloy,realgar,alacranite s.s., orpi- al.,1992; Bonazzi et al.,1996; Muniz-Miranda et al., 1996). ment and a“monoclinic As 4S4”close to alacranitehave The light-induced formation of the -phase obtained from been reported ( ZÏ a´cekÏ & Ondrus,Ï 1997). the -phase occurs with astrong anisotropic increase of the The sample examined consists of asiltstone partially unit-cell volume. Therefore,crystals of -phase partially al- covered by acicularcrystals of anhydrite. Scattered euhed- tered by exposure to light exhibit unit-cell volumes greater raldeep-red