Ba-rich celestine: new data and crystal structure refinement

M. F. BRIGATTI, E. GALLI AND L. MEDICI

Dipartimento di Scienze delia Terra, Universitii di Modena, Largo S. Eufemia 19,1-41100, Modena, lIaly

Abstract A Ba-rich celestine (SrOX7BaO13S04) filling cavities of volcanoclastic rocks from Montecchio Maggiore (Vicenza, lIaly) was studied. The role of the Ba content in the mineral was determined using X-ray powder data, single crystal X-ray refinement, thermal and chemical a!1alyses. The unit cell parameters (obtained by single crystal diffraction) are a = 8.408, b = 5.372, c = 6.897 A, and the refinement in the space group Pnma (Z =4) gives the final R value of 0.039. The average (Sr,Ba)-oxygen bond length is 2.842 A and agrees with an occupancy of Sr 87% and Ba 13%. Individual bond lengths (Sr,Ba)-O and bond strength calculations confirm that all twelve interactions are significant and define an irregular array around the cation.

KEYWORDS:celestine, barium, crystal structure refinement, Vicenza, Italy.

Introduction occurs as euhedral to subhedral crystals, as crystal fragments and as granular aggregates. It is white or TIlE strontium-barium sulphate system (celestine- light blue in colour and closely associated with baryte) has been extensively studied. II is known that analcite, natrolite, celadonite and apophyllite. a solid-solution series is formed over the entire Recently, a number of samples or celestine have compositional range (Bostrom et al., J968; Browner, been found in a slope-cutting carried out in basaltic 1973) and that the unit cell cdges vary non-linearly lava, near the town. It occurs rarely as single crystals with composition (Bostrom et al. 1968; Burkhard, (Fig. Ia), almost always as aggregates of subparallel 1973; Goldish, 1989). Furthermore, a statistical study lamellae from colourless to snow-white, which on naturally-occurring samples shows that the decussate in an irregular manner to fill the rock composition of the great majority or mixed cavities (Fig. Ib). celestine-baryte crystals is close to that of the end The crystal density, measured at room temperature members, and ranges between BaI.o-Sro I BaOYand by the Berman method (1939) is 4.046(3) g/cm' and between SrI.O-SrO.9BaO.1 (Hanor, 1968; Bouhlel, agrees with a composition of SrOX7BaO.13S04' 1985). Backscattered electron imaging on several crystal The end-member celestine structure type was first fragments (using a Philips SEM XL-40 equipped determined by James and Wood (1925) and later with energy dispersive electron microprobe EDS- refined by Garske and Peacor (1965), Hawthorne and EDAX 9900) did not reveal Sr or Ba zoning. The Ferguson (1975) and Miyake et al. (1978), but the quantitative chemical data gathered by a wavelength role of Ba in naturally occurring samples has not dispersive ARL-SEMQ electron microprobe been thoroughly investigated. Here we present the (operating voltage of IS kV, IS nA sample current, mineralogical and structural details of a natural Ba- 5 ~m electron beam diameter; (pZ) correction rich celestine (Ba = 0.13 atoms per formula unit). procedures; after Pouchou and Pichoir, 1985), on the same crystal used for the structure refinement, are reported in Table I. Occurrence, properties and composition Thermogravimetric analysis (SEIKO 5200 appa- The occurrence of celestine in basalts and in ratus; start temperature: 20°C; limit temperature: volcanoclastic sedimentary rocks outcropping near 1300°C; program rate: lOOC/min; Ar gas, flow rate: Montecchio Maggiore, Vicenza, north-eastern Italy, 30 ml/min) revealed only one weight loss, linked to a has already been reported (Billows, 1920; Boscardin broad derivative maximum at 1200°C which is and Sovilla, 1988). The mineral fills rock cavities and produced by SO, loss. Mineralogical Magazine, June 1997, Vo!. 61, pp. 447-451 (Q Copyright the Mineralogical Society 448 M. F. BRIGA TTI ET AL.

TABLE I. Representati ve electron microprobe anal ysis and chemical formula of Ba-rich celestine from Montecchio Maggiore (Yicenza, Italy)

Chemical data Atoms per formula unit (oxide wt.%)

srO 47.41 Sr 0.87 BaO 10.49 Ba 0.13 SO] 42.11 S 1.00 Sum 100.01

Collection of Xray single-crystal data and structure refinement. For intensity data collection a plate crystal measuring 0.17 [] 00] x 0.04 [0] 0] x 0.1] [00 I] mm was mounted on a Siemens P4 rotating anode automated four-circle diffractometer (Mo-Kex graphite monochromatized radiation operat- ing at 45 kY, ]40 mA), equipped with XSCANS software (Siemens, 1993). Cell dimensions were refined using the 28 values for 30 centred retlections with 25 ~ 28 ~ 45. Intensities for retlections :t h+k+l were collected to a maximum value of 70° using the (J)scan mode, with a scan width of 1.6° and corrected for background, Lorentz, polarization and absorption effects. Intensity data of symmetrically equivalent retlections were averaged. Three standard FIG. I. Scanning electron micrographs showing: a) Ba- reflections measured every I h showed less than rich celestine crystals and b) the crystal nets. The J.0% variation in intensity. photographs were taken using a Philips SEM XL-40 Structure refinement was carried out using 596 scanning electron microscope. unique retlections (I ~ 30'(1), Rim = 0.025) with the SHELX76 and SHELX-S86 programs (Sheldrick, 1976, 1986), starting from the atomic parameters given by Pasero and Davoli (1987) for isostructural X-ray crystallography hashemite in the centrosymmetric space group Pnma. The final agreement factors were R = 0.039 and wR = X-ray powder diffraction. X-ray powder diffrac- 0.037. The structure refinement in the non-centro- tion data sets were collected using an automated symmetric space group Pn2Ja produced neither an Philips diffractometer, Cu-Kex Ni-filtered radiation improvement in the R agreement value nor a with PbNO] as an internal standard. significant departure of the atoms from the position The unit cell parameters based on orthorhombic on the mirror plane. The e.s.d.' s on bond distances indexing (space group Pl1;,ma) are: a = 8.4] 4(3), b = and angles were computed using the program ORFFE 5.369(2), c = 6.909(3) A, and were derived from (Busing et ai, 1962). The atomic scattering factors for d-spacing by a least squares refinement program neutral atoms were taken from the International (Alberti, 1976). Goldish (1989) emphasized that Tables for X-ray Crystallography (Ibers and synthetic solid solution unit cell parameters show Hamilton, 1974). Fractional atomic coordinates, deviations from Yegard's Law, concerning mostly isotropic and anisotropic thermal factors are reported the c parameter, and he therefore suggests that the in Table 2, selected bond distances and angles are SrS04 molar fraction can be derived from the reported in Table 3. The observed and calculated equation d(z! I) = 3.1023 - 0.1317 x S, (where S is structure factor list is available on request. the SrS04 mole fraction). Our data agree with the Thermal factors are generally lower than those composition Srl!.8XBall.lzS04' which in turn agrees previously reported for the two end-members SrS04 very well with the one determined by chemical and BaS04. This feature could be ascribed to analysis. experimental conditions. Furthermore, they confirm Ba-RICH CELESTINE 449

TABLE2. Single-crystal data; lattice constants, atom coordinates and displacement factors (A2) of Ba-rich celestine from Montecchio Maggiore (Vicenza, Italy)

Cell parameters (A) a 8.408(2) b 5.372(1 ) c 6.897(2)

Atomic coordinates and isotropic thermal factors. Atom x/a y/b z/c Beq M 0.1836(1) 0.25 01585(1 ) 0.66(1 ) S 0.4377(2) 0.75 0.1851(2) 0.32(3) 01 0.5939(5) 0.75 0.0945(8) 1.5(1) 02 0.3083(5) 0.75 0.0437(7) 1.1(I) 03 0.4220(3) 0.9760(5) 0.3106(5) 0.80(9)

Anisotropic thermal factors.

Atom ~II ~22 ~33 ~12 ~1J ~23 M 0.39( I) 0.87(2) 0.72(1 ) 0 -0.03(2) 0 S 0.23(4) 0.24(4) 0.47(4) 0 -0.05(3) 0 01 0.9(1 ) 1.9(2) 1.6(2) 0 0.5(]) 0 02 0.8(1) 1.6(2) 0.9(1 ) 0 -0.4(1) () 03 I.O(I) 0.43(8) 0.9(1) 0.2(1 ) -0.16(9) -0.29(9)

The form of the anisotrof.ic thermal parameter is: exp[-I /4(~1 ,h2a *2+~22k b *2+~3312c2 +2~12hka*b*+2~13hla*c*+2~23klb*c*)1.

the trend previously reported for celestine and baryte: coordi nated cation M, from baryte «M -0> == isotropic thermal factors of oxygen atoms lying on 2.952 A, Hill, 1977: ==2.952 A, Miyake et al., 1978) to celestine «M-O> the mirror planc (01 and 02) are significantly higher == 2.827 A, than that of 03 in general position (Hill, 1977). Hawthorne and Ferguson, ]975; == 2.83] The structure of Ba-rich celestine consists of A, Miyake el al., 1978). In Ba-rich celestine the isolated (S04) tetrahedra linked by Sr (or Ba) bond length agrees with a site occupancy of twelve-coordinated atoms. This implies that every Sr 87% and Ba 13% (ionic radii by Shannon, 1976). o atom is fourfold coordinated and linked to three Sr The site occupancy was also confirmed by the (or Ba) atoms and to one S atom. evaluation of the ionic radii and cell volume, as The tetrahedral mean bond length (1.477 A) is suggested by Hawthorne and Ferguson (1975) (V 1/3 ] == close to the mean bond distances reported both for 6.783 A, rM == .462 A). celestine (1.473 A, Hawthorne and Ferguson, 1975; The bond strengths for Ba-rich celestine, calcu- 1.474 A, Miyake et al., ] 978) and baryte (1.476 A, lated using the curves and parameters given by Hill, 1977: 1.478 A, Miyake et al., 1978) and to the Brown and Shannon (1973) and Brown and Wu mean sulphur-oxygen distance of 1.473 A after Baur (1976), compared with those of the end-member (1970) for sulphate structures. Furthermore, Ba-rich celestine (Hawthorne and Ferguson. 1975), confirm celestine shows a small increase in tetrahedral that all twelve M-O interactions are significant: the 2.58, BLDT 1.34%) with distortions (TAV == == two longest M-O interactions also have to be taken respect to those observed both for celestine and for into account, as indicated by the bond strength sums baryte. According to Hawthorne and Ferguson around 01 and M (Table 4). (1975), this indicates that tetrahedral distortions are a function of the geometry of the structure rather than Acknowledgements the chemistry of the twelve-coordinated site. The metal-oxygen mean bond length decreases The authors are indebted to Mr A. Zordan for making according to the ionic radius of the twelve specimens available for investigation and to Dr P.L. 450 M. F. BRIGA TTI ET AL.

TABLE 3. Selected bond-lengths and angles from structure refinement of Ba-rich celestine from Montecchio Maggiore (Vicenza, Italy)

SO~- group Metal ion-oxygen polyhedra M = Sr, Ba .~._~,-, ~- S - 00) (~) 1.454(5) M-O(l)(A) 2.558(5) S - 0(2) (A) 1.461 (5) M - 0(1) (x2) (A) 3.269(3) S - 0(3) (x 2) (A) 1.497(3) M - 0(2) (A) 2.658(5) (A) 1.477 M - 0(2) ( x 2) (A) 2.990(2) M - 0(3) ( x 2) (A) 2.655(3 ) 00) - 0(2) (A) 2.427(6) M - 0(3) ( x 2) (A) 2.699(3) 0(1) - 0(3) (x 2) (A) 2.405(5) M - 0(3) (x 2) (A) 2.832(3) 0(2) - 0(3) (x 2) (A) 2.403(5) (A) 2.842 0(3)' - 0(3)" (A) 2.428(6) <0 - 0> (A) 2.412 Selected distorsion parameters. 0(1)-S-0(2)(") 112.7(3) 0(1) - S - 0(3) (x2) n 109.2(2) BLDTt (%) 1.34 0(2) - S - 0(3) (x 2) (0) 108.7(2) ELDTtt (%) 0.44 0(3)' - S - 0(3)" n 108.4(3) TAyt (0) 2.58 <0 - S - 0> n 109.5 BLDI121t (%) 6.74

g lOo"n I(X-O),-(X-O)ml% t BLD (bond len th distorsions) = n L.,,=l (X-O)m tt ( g ) 100 I(O-O),-(O-O)ml ELD ed<>eo len th distorsions = n L.,,="n 1 (O-O)m (Renner and Lehmann, 1986; Kunz et ai., 1991). :j: T A V (tetrahedral angle variance) = L:~=l (8; - 109.47)2/5 (Robinson et ai., 1971).

Fabbri for obtaining scanning electron microprobe (CNR) of Italy. CNR is also acknowledged for the images. Financial support was provided by the use of the ARL-SEMQ electron microprobe at the Ministero dell' Universita e delia Ricerca Scientifica Dipartimento di Scienze delia Terra of Modena (MURST) and Consiglio Nazionale delle Ricerche University.

TABLE 4. Bond strengths(]) for Ba-rich celestine compared with those reported for celestine by Hawthorne and Ferguson (1975)

This study Hawthorne and Ferguson (1975) S M L* S M L* ------,,----'.-----.-. --'-'------_. ----- 0.309 0.325 01 1.599 2.020 ( I. 908)* 1.512 1.945 (1.837)* 0.056( x 2) 0.054( x 2) 0.236 0.244 02 1.566 2.010 1.589 0.I04( x 2) 0.098( x 2) 2.029 0.238( x 2) 0.206( x 2) 03 1.411( x 2) 0.2l2(x2) 2.013 1.469( x 2) O.226( x 2) 2.049 O.152( x 2) O.148( x 2) L* 5.987 2.069 (1.957)* 6.039 2.033 (1.925)*

(I) Calculated from the curves and parameters of Brown and Shannon (1973) and Brown and Wu (1976), considering the linear relation between metal-oxygen bond lengths and SrS04 molar fraction. * Yalues in parentheses are for a M coordination of [10]. Ba-RICH CELESTINE 451

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