American Mineralogist, Volume 97, pages 661–665, 2012 Structural trends for celestite (SrSO4), anglesite (PbSO4), and barite (BaSO4): Confirmation of expected variations within the SO4 groups SYTLE M. ANTAO* Department of Geoscience, University of Calgary, Calgary, Alberta T2N 1N4, Canada ABSTRACT The crystal structures of the isostructural orthorhombic sulfates celestite (SrSO4), anglesite (PbSO4), and barite (BaSO4) were refined by Rietveld methods using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. Their structural model was refined in space group Pbnm. The unit-cell 3 parameters are a = 6.87032(3), b = 8.36030(5), c = 5.34732(1) Å, and V = 307.139(3) Å for SrSO4; a 3 = 6.95802(1), b = 8.48024(3), c = 5.39754(1) Å, and V = 318.486(1) Å for PbSO4; and a = 7.15505(1), 3 b = 8.88101(3), c = 5.45447(1) Å, and V = 346.599(1) Å for BaSO4. The average <M-O> [12] dis- tances are 2.827(1), 2.865(1), and 2.953(1) Å for SrSO4, PbSO4, and BaSO4, respectively, and their corresponding average <S-O> [4] distances are 1.480(1), 1.477(3), and 1.471(1) Å. The geometrical features of the SO4 and MO12 polyhedra become more symmetrical from SrSO4 to BaSO4. Across the series, the a, b, and c parameters vary non-linearly with increasing V. The radii of the M2+ cations, rM, <M-O> [12], and <S-O> [4] distances vary linearly with V. These structural trends arise from the 2+ effective size of the M cation (rM: Sr < Pb < Ba) that is coordinated to 12 O atoms. Keywords: Celestite, SrSO4, anglesite, PbSO4, barite, BaSO4, Rietveld refinement, synchrotron high-resolution powder X-ray diffraction (HRPXRD), crystal structure INTRODUCTION using conventional X-ray diffraction because of strong absorption effects and the small relative contribution of the oxygen atoms Celestite (SrSO4) and barite (BaSO4) occur in hydrothermal veins and as secondary minerals in sedimentary environments. to the total scattering. Consequently, Jacobsen et al. (1998) used They are the main commercial sources of strontium and barium. single-crystal and high-intensity rotating anode Mo X-ray source data to refine the structure of these isostructural minerals. They Anglesite (PbSO4), a minor ore of lead, typically occurs in the oxidized portion of hydrothermal lead deposits as an alteration concluded that the average <M-O> [12] distance increases lin- product of galena and also as a primary mineral in some low- early with increasing cell volume, but the SO4 behaves as a rigid temperature oxidized hydrothermal deposits. group with an average <S-O> [4] distance of 1.476 Å, which is The crystal structures of isostructural celestite, anglesite, and constant across the series. Hawthorne and Ferguson (1975) and barite were determined by James and Wood (1925). Sahl (1963) Hill (1977) reported that the SO4 groups in all three minerals refined the structural model for anglesite and barite. The barite display identical geometries, and the M cation is [l2]-coordinated structure was also refined by Colville and Staudhammer (1967). to O atoms because of bond-strength sums. 2+ The structure of celestite was refined by Garske and Peacor Although previous studies indicate that the M cations (1965) and Hawthorne and Ferguson (1975). The structure of have no effect on the shape or size of the SO4 tetrahedron (e.g., 2+ a Ba-rich celestite is also available (Brigatti et al. 1997). The Jacobsen et al. 1998), the M cations have different sizes and structure of all three minerals was refined by Miyake et al. (1978) effective charge, so systematic variation in the geometry of the and Jacobsen et al. (1998) to determine structural trends that are SO4 group is expected. The purpose of this study is to examine expected across the series. The structure refinements were car- the structural trends for SrSO4, PbSO4, and BaSO4. Using high- ried out in space group Pbnm. The main structural features of resolution powder X-ray diffraction (HRPXRD), this study shows that the geometry of the SO4 group varies in a systematic the orthorhombic sulfates, SrSO4, PbSO4, and BaSO4, listed with 2+ and expected manner. increasing size of the M cation, are SO4 and MO12 polyhedra that share edges (Fig. 1). EXPERIMENTAL METHODS Miyake et al. (1978) indicated a possible systematic variation 2+ in the SO4 tetrahedron with field strength of the M cation across Sample characterization the series, but their SO4 geometry was statistically identical The celestite sample is from Saxony, Ngar Majunga, Madagascar (UC15824). (Jacobsen et al. 1998). Heavy M2+ cation makes it difficult to de- The barite sample is from Elk Creek, South Dakota. The synthetic PbSO4 sample termine the atom positions of light elements with high precision was obtained as high-purity (99.995%) reagent grade powder from Aldrich and no impurity phase was observed in the HRPXRD trace. The celestite and barite samples were analyzed using a JEOL JXA-8200 * E-mail: [email protected] electron microprobe and its standard operating program on a Solaris platform. 0003-004X/12/0004–661$05.00/DOI: http://dx.doi.org/10.2138/am.2012.3905 661 662 ANTAO: ORTHORHOMBIC STRUCTURE OF SrSO4, PbSO4, AND BaSO4 TABLE 1. Electron microprobe analyses (EMPA) for celestite and barite Celestite Barite Madagascar South Dakota wt% oxide wt% oxide CaO 0.01(1)† 0.03(4) MgO 0.00(1) 0.03(1) FeO 0.02(1) 0.00(2) SrO 54.83(7) 0.3(1) BaO 0.06(4) 64.9(5) F 0.02(3) 0.00(1) Cl 0.07(3) 0.00(1) SO3 45.06(5) 34.6(4) Total 100.07 99.76 apfu* Ca 0.000(1) 0.001(1) Mg 0.000(1) 0.002(1) Fe 0.001(1) 0.000(1) Sr 0.966(5) 0.007(3) Ba 0.001(1) 0.985(6) F 0.002(3) 0.000(2) Cl 0.004(1) 0.000(1) S 1.027(6) 1.005(7) Notes: Formula for celestite is [Sr0.966Fe0.001Ba0.001]∑0.97(SO4)1.03. Formula for barite is [Ba0.985Sr0.007Mg0.002Ca0.001]∑1.00(SO4)1.01. * The values of atoms per formula unit (apfu) are based on sum of cations and anions (except O) = 2. † The estimated standard deviations in brackets are based on analyses of 10 spots for each sample and are given to the last two decimal places for the oxides and three for the apfu. in the GSAS program. Full-matrix least-squares refinements were carried out by varying the parameters in the following sequence: a scale factor, cell parameters, atom coordinates, and isotropic displacement parameters. Toward the end of the refinement, all the parameters were allowed to vary simultaneously, and the refine- ment proceeded to convergence. The fitted HRPXRD traces are shown in Figure 2. IGURE F 1. Projection of the SrSO4 structure (based on data from The cell parameters and the Rietveld refinement statistics are listed in Table this study). The Sr atom is coordinated to 12 O atoms of six different 2. Atom coordinates and isotropic displacement parameters are given in Table 3. SO4 groups. The SO4 and SrO12 polyhedra share edges. (Color online.) Bond distances and angles are given in Table 4. The wavelength-dispersive operating conditions were 15 kV accelerating voltage, RESULTS AND DISCUSSION 10 nA beam current, a beam diameter of 5 µm, and using various standards [e.g., fluorapatite (CaKα, PKα, FKα), barite (BaKα, SKα), hornblende (FeKα, MgKα), Structure of SrSO4, PbSO4, and BaSO4 scapolite (ClKα), strontianite (SrKα), and rhodonite (MnKα)]. The crystals are homogeneous based on optical observations and electron microprobe analyses of The three isostructural minerals have SO4 and MO12 polyhe- 2+ 10 spots for each sample. Representative chemical compositions of the samples dra that share edges (Fig. 1). The M (=Sr, Pb, or Ba) cation is are given (Table 1). The MSO4 formula was used in the structure refinements as coordinated to 12 O atoms (Hawthorne and Ferguson 1975; Hill the samples are quite pure, as indicated by the chemical analyses. 1977; Jacobsen et al. 1998; Brigatti et al. 1997). However, there are ranges of various M-O distances, so the nearest O atoms can Synchrotron high-resolution powder X-ray diffraction be grouped into 8, 10, or 12 distances (Table 4). (HRPXRD) The samples were studied by HRPXRD experiments that were performed at Structural trends beamline 11-BM, Advanced Photon Source (APS), Argonne National Laboratory 2+ (ANL). Each sample was crushed to a fine powder using an agate mortar and The radii, rM, of the 12-coordinated M cations (Shannon pestle. The crushed samples were loaded into Kapton capillaries (0.8 mm internal 1976) and the a, b, c unit-cell parameters are plotted against the diameter) and rotated during the experiment at a rate of 90 rotations per second. The data were collected at 23 °C to a maximum 2θ of about 43° with a step size of 0.001° and a step time of 0.1 s per step. The HRPXRD traces were collected TABLE 2. Cell parameters and Rietveld refinement statistics for the with 12 silicon (111) crystal analyzers that increase detector efficiency, reduce orthorhombic sulfates the angular range to be scanned, and allow rapid acquisition of data. A silicon Celestite Anglesite Barite (NIST 640c) and alumina (NIST 676a) standard (ratio of ⅓ Si: ⅔ Al2O3) was used SrSO4 PbSO4 BaSO4 to calibrate the instrument and refine the wavelength used in the experiment (see Madagascar synthetic South Dakota Table 2). Additional details of the experimental setup are given elsewhere (Antao a (Å) 6.87032(3) 6.95802(1) 7.15505(1) et al.