The Crystal Structure of Pararealgar, Asoso Plou Bonlzzro Srr,Vro Mnncnnrn, Grovlxrvr Pmrnsr Ansruct

Ameican Mineralogist, Volume 80, pages 400-403, 1995

The crystal structure of pararealgar, AsoSo

Plou BoNlzzro Srr,vro MnNcnnrn, Grovlxrvr Pmrnsr Dipartimento di Scienzedella Terra, Universiti di Firenze, via La Pira 4,l-50121 Florence, Italy

Ansruct Pararealgar,a polymorph of realgar(a-AsoSo), crystallizes in the monoclinic spacegroup p2,/c,witi a:9.9t09(2f,D:9.655(l), c:8.502(l) A, B: 97.29(l),z:806.8(2) A3. A single crystal of pararealgar was obtained by light exposure of realgar. The structure was determined by direct methods and refined by least-squarestechnique to a final R index of 3.39o/o.The structure consists of discrete covalently bonded AsoSomolecules, which are held together by van der Waals forces. In each molecule, one As atom links 2As * lS' another links 35. and the other two link lAs + 25. Therefore this molecule differs from that of a-AsoSobut is the same as that found in AsoSoGI).The difference between the structure of pararealgar and AsoSo(II) is due to different molecular packing.

Ivrnooucrrox On the basis of its cell parameters and X-ray powder pararealgarwas supposedto be relatedto p-AsoSo Many studies have been made concerning the crystal 93tt-"-: (fot9rt1 Douglass et al' (1992) suggested structures, phase transitions, and stability n.fOr liift" et al', 1980)' in samplesof realgarfrom Mina AsoSopolymorphs. A number of phasesut. t"o*"' t"- that the lack of alteration (Clark' 1970)' that had been exposed to algar (a-AsoS4)(Ito et ar.,1952;Pen'kov uno sunn, isii; Alacr6n' Chile be related to coexisting Hall, 1966; Forneris, 1969; Mullen u"o No*u.ia, iiii; sunlight for a long time could hypothesized that the struc- Bues et al., 1983; Bryndzia and Kleppa, 1988), plrarJ: B-Alq.' The same authors tural similarity of to pararealgarmay preclude its gar (Roberts et al., 1980); and the 1 phaset"f.ffl;; B-AsoSo p form occurs' bouglass et al. (1992)as the precursor to pararea[ar roi- formation where the mation. Two other phases,reported as high-temperature SOLUTTON forms,arealsodescribed:B-Asos"(Hall,1966;ctart, t9z0; E)pERTMENTALAND STRUCTTTRE Streetand. Munir, 1970; Porter and Sheldrick, 1972; Ro- A suitable singlg srys1al of pararealgar was obtained land, 1972; Yu and Zaltai, 1972) and AsoSo(II),synthe- from an unaltered fragment of realgar from Hunan, Chi- sized and studied by Kutoglu (1976). Furthermore, Hall na, through exposureto unfiltered visible light for 48 h. (1966) observed a thin orange-yellowfilm coating most The light sourcewas a l5-V and 150-W quartz-tungsten- museum specimensofrealgar, called 7-AsS, which he was halogen lamp manufuctured by Philips; optical fibers were able to produce by exposing both natural and synthetic employed to minimize overheating.After the treatment, realgar to infrared radiation. the crystal changed color from red to yellow and cracked Pararealgar from two localities in British Columbia into a number of minute fragrnents.One of these,because (Mount Washington on Vancouver Island and the Gray of its transparency,was chosen for the structural study. Rock property, Lillooet district), was first identified by Lattice parameters were determined by means of the Roberts et al. (1980), who described it as a new poly- least-squaresmethod using 25 high-O reflections mea- morph of AsS. According to theseauthors, the symmetry sured with a CAD4 single-crystaldiffractometer. Their of pararealgaris monoclinic, with spacegroup Pc or P2/c, values (Table l) compare favorably with those measured andcell parametersa:9.929(4), b:9.691(6),c: byRobertsetal.(1980)andareinbetteragreementwith 8.503(3) A, and P : 97.06(2). Pararealgaralways occurs the reported density. The systematicabsences of 0k0 (k asanalterationproductof realgar.Sinceitresemblesor- :2n * l) anidh)l reflections(l: 2n + l) led to the piment, pararealgarremained unidentified for a long time unique choice of spacegroup P2'/c. Ittensity data were (Roberts et al., 1980). Recently Douglass et al. (1992) collected and subsequentlycorrected for Lorentz and po- confirmed that the well-known friable degradationprod- larization factors and for absorption (North et al., 1968). uct of realgar that has been exposedto light consists of The crystal structure was solved using MULTAN-8O pararealgar.They emphasized that it is the stable AsS Main et al", 1980).AII the As and S atoms were located polymorph in the presenceof light, whereasthe forma- on the {-Fourier map, and isotropic full-matrix least- tion of AsrS, requires a much higher pressureof S than squaresrefinement led to R : 7.0,Vo.With the introduc- what is normally present in air. tion of anisotropic displacement parameters and w : Until now, the crystal structure of pararealgarwas un- l/o'zr",the structure refinement convergedto a final R" of known, and only a few hypotheseshad been put forward. 3.39o/o.The atomic scattering factors for neutral atoms 0003-004v95/0304-0400$02.00 400 BONAZZI ET AL.: CRYSTAL STRUCTURE OF PARAREALGAR 401

Tmu 1. Crystaldata and experimental details for pararealgar

Crystdsystem monoclinic Spacegroup P2.,lc Cellparameters a (A) 9.909(2) D(A) 9.65s(1) c (A) 8.502(1) pf) 97.29(1) v (41 806.8(2) Formulaunits per unitcell, Z 4 Diffractometer Enraf Nonius CAO4 Radiation MoKd (0.71069A) Crystalsize (pm) 25x30x60 Scanmode Scanwidth (") 1.60 Scanspeed ("/min) 0.78 Rangeof hk -9,9 -9,9 0,8 R",.. f/4 2.6 No.of independentreflections occ Nc 395 No. of refined parameters 7Q Fig. l. The AsoSnmolecule in pararealgar.Vibrational ellip- R*"(%) 3.39 soidsare scaled at 500/oprobability. Nore.' 4y.. : p {Iv > Iw(E F.)1}D [(N - 1) > (w4)])"; /v* : numberof reflectionswith F. > 4oF";R*: > (v*l F" - F.l\12(WrF"). different crystal chemical environments: Asl links three were taken from the International Tables for X-rav Crvs- S atoms (Sl, 53, S4);As2 and As3 eachlink two S atoms tallography, volume IV (Ibers and Hamilion, 197'4).ihe and one As atom (Sl, 52, As4 and 52, 53, As4, respec- refinement was perforrned using the program SHELX tively); As4 links one S atom and two As atoms (S4, As2, (Sheldrick,1976). As3). The pararealgar molecule therefore differs from both Experimental details are reported in Table l. The atomic coordinatesand anisotropic displacementparam- Tlar-e5. Selectedinteratomic distances (A; and angles(") for eters are given in Tables 2 and 3, respectively,and a list pararealgar of observed and calculated structure factors appears in Intramolecularbond distances Table 4.' As1-Sl 2.254(9) As2-As4 2.484(4) -s3 2.238(81 As3-S2 2.244(10) DrscussroN -s4 2.261(8) -S3 2.228(101 As2-S1 2.251(91 -As4 2.534(4) The pararealgarstructure consistsof discretecovalent- -s2 2.252(9) As4-S4 2.190(9) ly bonded AsoSomolecules, which are held together by Intramolecularbond angles van der Waals forces. In the unit cell there are four such S1-As1-S3 103.6(3) S3-As3-As4 100.0(3) '101.7(2) molecules. All the atoms lie in general positions (point S1-As1-S4 e6.2(3) S4-As4-As2 symmetry l); S3-As1-54 98.3(3) S4-As4-As3 101.0(3) however, on the whole, the molecule (Fig. S1-As2-S2 105.s(3) As2-As4-As3 83.4(1) l) shows a pseudomirror running through 52, As4, Asl, S1-As2-As4 100.0(3) As1-S1-As2 109.2(4) S2-As2-As4 87.0(3) As2-S2-As3 95.9(4) and 54. Deviations from mirror symmetry, in terms of 't03.2(4) S2-As3-S3 As1-S3-As3 11 0.5(3) interatomic distancesand angles,are <0.05 A and 2.5., S2-As3-As4 86.0(3) As1-S4-As4 104.4(4) respectively. Vibrational ellipsoids also display devia- Intramolecular nonbonded distances tions from mirror symmetry. The As atoms show three As1-As2 3.672(5) As4-S2 3.266(9) -As3 3.670(5) -S3 3.652(8) -As4 3.519(4) S1-S2 3.s86(12) ' As2-54 3.629(9) -S3 3.530(11) a *pV of fables 3 and4 may beordered as Document AM- -As3 3.338(4) -S4 3.36102) 95-581from theBusiness Office, Mineralogical Society of Amer- As3-S4 3.652(9) S2-S3 3.506(12) ica, ll30 SeventeenthStreet NW, Suite330, Washingron,DC As4-S1 3.630(8) S3-S4 3.404(11) 20036,U.S.A. Please remit $5.00in advancefor the microfiche. Intermolecular distances As1-S1. 3.473(6) As3-S3s 3.653(10) -s2" -S4" Tmu 2. Fractional 3.789(7) 3.618(9) atomiccoordinates and isotropicdisplace- -s3b 3.607(9) As4-S1r 3.6s2(9) mentparameters for pararealgar -As3 3.799(4) -S2, 3.592(9) As2-S2d 3.775(10) -S4h 3.720(9) uq -s4' 3.388(8) S1-S2. 3.478(10) -As2! 3.6s7(5) s2-s3t As1 0.3187(3) 3.345(11) 0.6355(3) 0.0432(4) 0.038(1) -As4' 3.687(4) s3-s4o 3.777('t1l As2 0.081e(3) 0.5427(31 0.3252(4) 0.03s(1) As3 0.36s8(3) 0.3607(4) 0.3431(4) 0.0s0(1) Note: the superscriptsdenote the symmetry and translationoperations As4 0.1455(3) 0.343s(3) 0.1643(4) 0.041(1) appfied:a: +x,%- y, - 1/z+ z;b: -x + 1, -y + 1, -zic: x + Sl , 0.164s(8) 0.7187(8) 0.1923(1 0) 0.040(3) 1, +y + y2,-z + lz;d: -x, y + 1, -z+ 1;g: -x, -y + 1, -z; S2 0.2s37(9) 0.4782(9) 0.5099(10) 0.046(4) t : x, +y - lz, -z + Vz;g: -x + 1, +y - Vz,-z + 1/2il.t : +x, S3 0.47O3(7) 0.5276(10) 0.2192(1 0) 0.052(4) -y + y2,+z + y2ii: +x, -y + y2,+z - Vz;i: -x + 1, -y +.1, S4 0.1964(8) 0.4483(8) -0.0492(10) 0.042(3) -z+1. 402 BONAZZI ET AL.: CRYSTAL STRUCTURE OF PARAREALGAR

Tlale 6. X-ray powder diftraction pattern of pararealgar

Vancouver lsland. Present work*. ilt" d* (A) d* (A) ilt"

9.8289 I 100 6.8878 3 110 6.3515 2 011 91 5.56 5.5834 83 111 100 5.14 5 1164 100 111 29 4.901 4.9145 20 200 4.8275 16 o20 4.1896 o21 3.9448 3 121 3.8642 4 012 't7 3.7693 121 3.7477 48 112 3.7186 16 211 3.7044 8 102 3.4618 112 27 3.44 3.4439 4 220 3.4219 5 202 50 3.299 3.2924 o, 221 Fig. 2. The structureof pararealgarprojected along [00 I ]. As 3.2253 3 212 atomsare shown as solidcircles and S atomsas open circles. 3 3.184 3 1758 <3 o22 3.1025 3 310 33 3.105 3.0935 27 221 in both of which all four 3.0322 13 311 a-AsoSo(realgar) and B-AsoSo, 51 3.025 3.0166 28 202 As atoms are bonded to lAs + 25, but the molecule is 3.0068 the same as that found by Kutoglu (1976) for AsoS.(ID. 2.9408 15 031 2.9124 o 122 Interatomic distancesand anglesare reported in Table 5. 30 2.905 2.8793 26 131 As-S bond distancesare in the range2.228-2.261 4,, ex- 2.8793 11 212 As4-S4, which is only 2.190 A. As-As bond dis- 2.8396 3 131 cept for 2.8046 28 311 tancesrange from 2.484 to 2.534 A. These values agree 2.795 2.7917 52 222 closely with those reported for AsoSo[I) (Kutoglu, 1976)' 2.6990 9 013 and 2.6s60 I 312 a-AsoSo(Mullen and Nowacki,1972), 0-AsoSo@orter 2.6181 o 231 and Sheldrick,1972). 18 2.5251 2.5236 132 Figure 2 shows the projection ofthe pararealgarstruc- 2.4572 I 400 13 302 ture down [001]. In this view the pararealgar structure 28 2.445 2.4414 't32 2.4306 5 appearssimilar to the [100] projection ofAsoSoGI)shown 2.4209 3 123 in Figure 3, though the differencein molecular packing is 2.3977 b 322 2.3813 4 410 evident. In Table 5 intermolecular contacts shorter than 11 2.377 2.3669 312 3.85 A are reported. This limit correspondsto that adopt- 2.3441 3 140 (1972) for realgaron the basis 2.3206 041 ed by Mullen and Nowacki 2.278 2.2793 15 223 of the sum of the van der Waals radii of As and S given 2.2763 4 141 by Pauling (1960), though the radii given by Bondi (1964) 2.2528 2 213 2.2201 3 313 give a limiting value of 3.65 A. ttre shortest contacts 11 2.208 2.2009 3 232 found in this study are As2-S4": 3.388 A and S2-S3 : 2 420 2.1899 3.345 A: similar values are found in AsoSo(II)and in re- 2 2.106f 2.1117 o 133 2.0962 3 332 algar. b 2.069 2.0636 14 421 Table 6 comparesthe calculatedX-ray powder pattern 2.0333 6 204 pararealgar in this study with that observed 22 2 0301 2.0302 22 133 for the used 2.O157 3 233 for natural pararealgarfrom Vancouver Island (Roberts 2.O147 o 402 et al., 1980). The calculated and observed data match 2.0098 2 104 1.9724 4 242 closely. 1 976 1.9688 5 313 As a frnal observation, it may be pointed out that, be- 1.9676 4 114 have identical molecular weightsand Z values, 1.9530 5 430 causethey 1.9433 4 340 the densities of realgar and pararealgar should be very 11 1.862 1.8611 I 333 similar. Since the unit-cell volume of pararealgar(806.8 1.8407 e 512 1.7972 3 250 1.7441 1.7443 10 152 1.7309 5 224 1.7219 3 440 t 1.7159 2 413 X-ray powder data for pararealgarfrom Mount Washington Cu de- 1.7127 4 152 posit, Comox district, Vancouver lsland (Roberts et al.' 1980). .tI - *'X-ray 10 1.710 1.71 10 404 pattern calculated using XPOW software, version 2'0 (Downs 1.7055 c 333 et al., 1993). Only reflectionswith /1. > 2 are listed. 11 1.682f 1.6805 13 531 t Indexingdifferent from that of Roberts et al. (1980). BONAZZI ET.AL.: CRYSTAL STRUCTURE OF PARAREALGAR 403

Bues,W., Somer,M., and Brockner, W. (1983) Schwingungsspektrenvon AsoSnund AsoSeo.Zeitschrift fiir anorganische und allgemeine Chemie, 499,7-14. Clark, A.H. (1970) Alpha-arsenic sulfide, from Mina Alacr6n, Pampa Larga, Chile. American Mineralogist, 55, 1338-1344. Douglass,D.L., Shing, C., and Wang, G. (1992) The light-hduced alteration of realgar to pararealgar. American Mineralogist, TT, 1266-127 4. Downs, R.T., Bartelmehs, K.L., Gibbs, G.V., and Boisen, M.8., Jr. (1993) Interactive software for calculating and displaying X-ray or neu- tron powder diftactometer patterns of crystalline materials. American Mineralogist,78, I lOzt-1107. Forneris, R. (1969) The infrared and Raman spectraofrealgar and orpiment. American Mineralogist, 54, 1062-1074. Hall, H.T. (1966) The systemAg-Sb-S, Ag-As-S, and Ag-Bi-S: Phasere- lations and mineralogicalsigrrificance. Ph.D. thesis,Brown University, Providence,Rhode Island. Ibers,J.A., and Hamilton, W.C., Eds.(1974) International tables for X-ray crystallo$aphy, vol. IV, 366 p. Kynoch, Birmingham, U.K. Ito, T., Morimoto, N., and Sadanaga,R. (1952) The crystal structure of realgar.Acta Crystallographica,5, 77 5-782. Kutoglu, A. (1976) Darstellungund Kristallstruktur einer neuenisomeren Form von AsnSo.Zeitschrift fiir anorganische und allgemeine Chemie, 4r9,176-184. Main, P., Fiske, S.J., Hull, S.E., Lessinger,L., Germain, G., Declercq, J.P., and Woolfson, M.M. (1980) MULTAN80: A systemof computer programs for automatic solution of crystal structures from X-ray dif- fraction data. University of York, York, U.K., and University of Lou- Fig. 3. The structure ofAsnSoflI) projected along [100]. As vain, Belgium. atoms are shown as solid circles and S atoms as ooen circles. Mullen, D.J.E., and Nowacki, W. (1972) Refinement of the crystal stmc- tures ofrealgar, AsS and orpiment, AstS,. Zeitschrift fiir Kristallogra- phie,136,48-65. A') is somewhal grcaterthan that of realgar (799.7 Lr, North, A.C.T., Phillips, D.C., and Mathews, F.S. (1968) A semiempirical in the light-induced transformation of realgar to para- method of absorption correction. Acta Crystallographica,A24, 351- realgar the density decreases(realgar: D-"." : 3.56; D^" 359. : : Pauling, L. (1960) The nature ofthe chemical bond and the structure of 3.553;pararealgar: D-*" 3.52;D^": 3.521).Fur- molecules and crystals: An introduction to modern structural chemistry thermore, it should be noted that, becauseofthe different (3rd edition), 644 p. Cornell University Press,Ithaca, New York. configuration of the AsoSomolecule in the two structures, Pen'kov, L.N., and Sa6n, L.A. (1963) Nuclear quadrupole resonancern some As-As and As-S bonds have to break during this realgar.Doklady Akademii Nauk SSSRNovaia Seriia, 153, 144-146. process. Porter, E.J., and Sheldrick, G.M. (1972) Crystal structure of a new crystalline rnodification of tetra-arsenic tetrasulphide (2,4,6,8-tettathia- 1,3,5,7-tetra-arsatricyclo[3,3,0,0]-octane).Journal of the American AcxNowr,nocnnnNrs Chemical SocietyDalton, 13, 1347-1349. M. (1980) Pararealgar,a new The authors wish to thank the staff of the Mineralogical Museum of Roberts, A.C., Ansell, H.G., and Bonardi, polymorph Mineralogist, 18, the University ofFlorence for providing the sampleofrealgar (no. 46768). of AsS, from British Columbia. Canadian The authors are also grateful to J.D. Grice and G. Robinson for their 525-527. (1972) realgar inversion. Canadian helpful reviews. Thanks are due to Johanna Krueger for reading the orig- Roland, G.W. Concerningthe a-AsS inal manuscript. This study was funded by the C.N.R. and by M.U.R.S.T. Mineralogist, ll, 520-525. (grant 60%). Sheldrick, G.M. (1976) SHELX-Program for crystal structure determi- nation. University of Cambridge, Cambridge, U.K. RrrnnBNcns crrED Street,B.G., and Munir, Z.A. (1970)The structureand thermal properties of synthetic realgar (AsnSo).Journal of Inorganic Nuclear Chemistry, Bondi, A. (1964) Van der Waals volumes and radii. Journal ofPhysical 32.3769-3774. Chemistry,68,441. Yu, S.C., and Tnltai, T. (1972) Crystallography of a high-temperature Bryndzia, L.T., and Kleppa, O.J. (1988) Standard molar enthalpies of phaseof realgar.American Mineralogist, 57, 1873-187 6. formation of realgar (a-AsS) and orpiment (AsrSr) by high-temperature direct-synthesis calorimetry. Joumal of Chernical Thermodynamics, 20, M,lxuscrrsr RrcErvEDJr.rr-v 18, 1994 755-764. Meruscnn'r AccEprEDNovetvtBen 7. 1994