American Mineralogist, Volume 80, pages 394-399, 1995 The crystal structure of gillulyite, TI2(As,Sb)sSm from the Mercur gold deposit, Tooele County, Utah, U.S.A. FRANKLIN F. FOIT, JR. Department of Geology, Washington State University, Pullman, Washington 99164, U.S.A. PAUL D. ROBINSON Department of Geology, Southern Illinois University, Carbondale, Illinois 62901, U.S.A. JAMES R. WILSON Department of Geology, Weber State University, Ogden, Utah 84408-2507, U.S.A. ABSTRACT Gillulyite, TI2(AsH ,SbH )8S13,is monoclinic with space group P2/n and a = 9.584(3), b = 5.679(2), c = 21.501(6) A, ~ = 100.07(2)°, V = 1152.2(7) A3, and Z = 2, The average structure was determined using direct methods and refined to a final residual of 0.046 using 930 reflections. The structure consists of TI-As-S- and As-S-bearing sheets, each present statistically 50% of the time. Four sheets, linked together by Tl-S, As-S, and S-S bonds, are stacked parallel to (00 I), yielding the 21-A c-axis repeat of the average unit cell. The Tll atom within the TI-As-S sheet is coordinated by six S nearest neighbors, which form a distorted trigonal prism. The partially occupied Tl2 position between the sheets is split symmetrically about the twofold axis with a separation of 1.28 A and equal but partial occupancies of25%. The disordering of the Tl2 position and extreme distortion of its coordination polyhedron is probably the result of the Tl+ 6s2lone pair effect. The mean bond distances for the AsS3pyramids range from 2.263 to 2.319 A. However, four of five pyramids have a fourth S neighbor at distances ranging from 2.943 to 3.001 A, which may be rationalized in terms of the AsH 4s210ne pair effect. Edge sharing among the AsS3pyramids results in an As-As distance of 2.562 A, comparable with that observed in elemental As. The presence of S-S and As-As bonding suggests that gillulyite might more appropriately be classified as a sulfide rather than a sulfosalt. INTRODUCTION tions were too weak to be measured and suggest minor The new mineral gillulyite, TI2(As,Sb)8SI3' one of sev- ordering in the b direction that was not investigated. eral unusual Tl minerals found in the Mercur gold de- Systematic absences indicate the space group to be Pn posit, Utah, was reported by Wilson et al. (1991) to be a or P2/n. Intensity statistics support the latter choice, which new thallium arsenic sulfosalt. The details of the occur- was confirmed by the successful solution and refinement rence, composition, optical and physical properties, and of the structure. The severe effects of absorption were powder difITaction and unit-cell data are presented in this corrected by the empirical 'II-scan method. Direct meth- earlier work. This paper presents the results of a struc- ods produced solutions that showed what appeared to be tural investigation of this unusual mineral. AsS3 groups in a linked pattern, but the expected large peaks representing the Tl atoms were absent; thus, these solutions were considered to be false. Only after many STRUCTIJRE DETERMINATION AND REFINEMENT attempts with various direct and heavy atom methods After considerable searching, an irregular fragment of were we able to decipher the highly disordered average gillulyite was found that produced relatively narrow and structure described below, with its partially occupied As, symmetrical diffraction maxima. A hemisphere of data S, and Tl positions. In hindsight, we realize that the direct was collected and averaged to yield a more accurate data methods program Mithril (Gilmore, 1984) had produced set. A scan speed of 6°/min (in w) was used. Weak reflec- correct locations for most of the atoms early in the struc- tions [/<IO.OO"[] were rescanned (maximum of two re- ture determination process, but we were initially unwill- scans) and the counts accumulated to improve accuracy. ing to accept them. The remaining atomic sites were found Experimental details are given in Table l. by difference-Fourier syntheses. Examination of a series of precession photographs con- Full-matrix, least-squares refinement was performed to )2. All computer programs used firmed the cell parameters and revealed a weak zone of minimize ~ w(IFaI - IFe I diffuse and split reflections in the b* direction, which ef- were from Texsan (Molecular Structure Corporation, fectively doubles the length of the b axis. These reflec- 1985). Atomic scattering factors and anomalous disper- 0003-004X/95/0304-0394$02.00 394 FOIT ET AL.: CRYSTAL STRUCTURE OF GlLLULYITE 395 sion corrections were taken from the International Tables TABLE1. Experimental details for gillulyite for X-ray Crystallography (Ibers and Hamilton, 1974). A Crystal data site occupancy refinement of the As positions indicated TI2(As7.78,5bo...)5'3 V = 1152.2(7) A3 M, that virtually all the Sb found in the microprobe analyses = 1435.20 Z=2 of gillulyite (Wilson et aI., 1991) resides at the As4 and Monoclinic 0, ~ 4.14 g/cm P2ln 4.02 g/cm As5 sites; the refined occupancy values are in good agree- Do = a = 9.584(3) A MoKa ment with the electron microprobe analytical values. Table b = 5.679(2) A A ~ 0.71069 A c ~ 21.501(6) A JL= 265.59 cm-1 2 lists the final refined positional, occupancy, and dis- (3 = 100.07(2f T~ 296 K placement parameters; bond distances and angles are pre- Cell parameters from irregular fragment sented in Table 3. The observed and calculated structure 20 refl. 0.22 x 0.14 x 0.10 mm 28 ~ 43.5-49.10 red factors are given in Table 4. I The presence of the weak and diffuse superstructure Data collection Rigaku AFC55 diffractometer 930 observed refl. [I> 4",] reflections, which double the length of b, abnormally large Absoprtion correction 28~ = 50" TI and S thermal parameters, a split Tl2 position, and w scan empirical h ~ -11-11 partial site occupancies reveal the existence of domains 4 refl. k ~ 0-6 ~ 0.48, Tm.. = Tm;" 1.0 I = - 25-25 and partial atom ordering in gillulyite. Thus, what was 4504 measured refl. 3 standard refl. determined and is described below is the average struc- 2031 independent refl. frequency: 150 refl. ture. intensity variation: 1.4% Refinement ~ 4F~/,,~ DISCUSSION Refinement on F w Final R = 0.046 127 parameters General structural details (~,,)~ Rw= 0.061 = 0.001 The structure of gillulyite is a composite of two types S = 1.57 (AP)m.. = 3.83 e/A3 930 refl. (AP)m.. -2.05 e/A3 of structurally similar sheets, TII-As-S-bearing and As- = S-bearing sheets situated parallel to (001). Each of the two sheet types is present statistically 50% of the time. est, and the Tl-S bonds to the bridging S are alternately The first type (Fig. I) consists of single chains of edge- long and short parallel to the chain length (Table 3, Fig. sharing Tll-S6 polyhedra extending parallel to [0 I 0] and 2). Despite having a lower coordination number (6), the isolated three-membered rings, Asl, As2, and As3, of mean Tll-S distance is larger than that observed for [SITI comer-sharing AsS, pyramids. The isolated As,S. rings (3.339 A) in bernardite, TIAssSs (Pasava et aI., 1989), PITI are sutured together by edge sharing with Tll polyhedra (3.294 A) in lorandite, TI2As2S. (fleet, 1973), and [7ITI in adjacent chains. Tli is coordinated by six S atoms (3.288 A) in fangite (Wilson et aI., 1993). Abnormally forming a distorted trigonal prism, with one side parallel long mean bond distances (e.g., [6ITI2-S = 3.445 A) and to (001) and the prism axis parallel to [100]. The Tll-S anomalously high isotropic displacement parameters (Bi", distances range from 3.300 to 3.438 A, with a mean of - 5) were also observed for the partially occupied TI 3.356 A (Table 3). The next nearest S neightbors are S4 positions in the averaged structure of imhofite (Divja- at distances of 3.915 and 3.983 A. Tl polyhedral edges kovic and Nowacki, 1976). shared with adjacent As and Tl polyhedra are the short- Two of the three pyramidal AsS, groups (Asl and As3) that comprise the three-membered rings have only As-S bridge bonds, and these range from 2.289 to 2.359 A (mean I To obtain a copy of Table 4, order Document AM-95-582 = 2.319 A) and 2.294 to 2.352 A (mean = 2.318 A), respectively. These means are almost identical to the ITom the Business Office, Mineralogical Society of America, 1130 Seventeenth Street NW, Suite 330, Washington, DC 20036, average bridge bond (2.31 A) observed in well-refined U.S.A. Please remit $5.00 in advance for the microfiche. sulfosalt structures (Takeuchi and Sadanaga, 1969). The TABLE 2. Atomic coordinates, anisotropic displacement parameters, and B equivalents for gillulyite y x z Occup. Vl1 V22 V33 V23 V'2 V'3 V'" TI1 0.8102(3) 0.9483(5) 0.1488(1 ) 0.5 0.035(1) 0.035(1) 0.066(2) -0.004(1) 0.024(1) -0.008(1) 0.044(1) TI2 0.1895(4) 0.8829(6) 0.2589(2) 0.5 0.130(4) 0.052(2) 0.125(3) -0.009(2) -0.077(4) -0.004(2) 0.114(2) As1 0.4732(2) 0.4472(4) 0.0883(1 ) 1.0 0.010(1) 0.013(1) 0.021(1) 0.001(1) 0.0023(8) 0.002(1) 0.015(1) As2 0.2587(2) 0.9009(5) 0.0205(1 ) 1.0 0.019(1) 0.025(2) 0.029(1) 0.003(1) 0.008(1) 0.014(1) 0.024(1) As3 0.0965(2) 0.4476(5) 0.0883(1 ) 1.0 0.011(1) 0.014(1) 0.023(1) -0.000(1) 0.0069(9) 0.001(1) 0.016(1) As4, 5b1 0.8077(5) 0.2107(8) 0.1446(2) 0.43, 0.07 0.015(3) 0.016(3) 0.016(2) 0.001(2) 0.008(2) 0.002(2) 0.015(1) As5, 5b2 0.8087(5) 0.6617(8) 0.1468(2) 0.46, 0.04 0.017(3) 0.014(2) 0.015(2) -0.002(2) 0.006(2) 0.002(2) 0.015(1) 51 0.3105(6) 0.388(1) 0.1543(3) 1.0 0.009(3) 0.025(4) 0.030(3) 0.002(3) 0.007(2) 0.009(3) 0.021(2) 52 0.6452(5) 0.437(1) 0.1813(2) 1.0 0.015(3) 0.024(3) 0.018(3) 0.001(3) 0.009(2) -0.002(3) 0.018(2) 53 0.4708(7) 0.850(1) 0.0876(3) 1.0 0.019(3) 0.014(4) 0.051(4) -0.004(3) 0.001(3) 0.000(3) 0.029(2) 54 0.2435(6) 0.543(1) -0.0191(3) 1.0 0.023(3) 0.052(4) 0.018(3) 0.005(4) 0.010(2) -0.004(3) 0.030(2) 55 0.0980(7) 0.852(1 ) 0.0879(3) 1.0 0.028(3) 0.023(4) 0.031(3) 0.001(3) 0.013(3) -0.002(3) 0.026(2) 56 0.9980(5) 0.434(1 ) 0.1811(2) 1.0 0.012(3) 0.027(3) 0.018(3) -0.004(3) 0.009(2) 0.000(3) 0.018(2) 57 0.844(1) 0.941(2) 0.2254(5) 0.5 0.028(6) 0.019(6) 0.013(5) 0.007(6) 0.005(4) 0.001(5) 0.020(3) 396 FOIT ET AL.: CRYSTAL STRUCTURE OF GILLULYITE TABLE3.