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Crystal Structure of Bultfonteinite, Ca4[Sio3(OH)]2F2·2H2O, from N’Chwaning II Mine (Kalahari Manganese Field, Republic of South Africa)

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Crystal Structure of Bultfonteinite, Ca4[Sio3(OH)]2F2·2H2O, from N’Chwaning II Mine (Kalahari Manganese Field, Republic of South Africa) atti Soc. tosc. Sci. nat., Mem., Serie a, 115 (2010) pagg. 9-15, figg. 3, tabb. 5; doi: 10.2424/ASTSN.M.2010.02 C. Biagioni (*), E. Bonaccorsi (*), S. Merlino (*) CRySTaL structure OF bultfonteiNiTE, Ca4[SiO3(OH)]2F2·2H2O, from N’CHWaNiNG ii MiNE (KaLaHaRi MaNGaNESE FiELD, REPUBLiC OF SOUTH aFRiCa) Abstract - The finding of well-crystallized specimens of with tobermorite and afwillite. Bultfonteinite was also bultfonteinite from the N’Chwaning II mine (Kalahari identified in the Wessels and N’Chwaning II mines Manganese Field, Republic of South africa) in the 1980s (Kalahari Manganese Field, Republic of South africa; allowed the collection of new data about the crystal structure Von Bezing et al., 1991); its origin is related to an of this calcium silicate hydrate. The crystal structure study is in agreement with the general features outlined by Mciver important hydrothermal process, known as the Wes- (1963) and allows the description of a reasonable hydrogen- sels alteration event, occurred about 1.0-1.25 Ga ago bond scheme. (Gutzmer & Beukers, 1996). The widespread twinning shown by bultfonteinite Key words - Bultfonteinite, C-S-H, crystal structure, made the crystal structure determination difficult; the N’Chwaning ii mine, Republic of South africa. cell parameters were determined by Murdoch (1955), whereas Megaw & Kelsey (1955) identified the twin Riassunto - Struttura cristallina della bultfonteinite, laws: according to them, twinning takes place accord- Ca4[SiO3(OH)]2F2·2H2O, dalla N’Chwaning II mine (Kala- ing to axes normal to the (100) and/or (010) planes, with hari Manganese Field, Repubblica Sudafricana). il ritrova- an angle between the two twin axes of about 90°. The mento, negli anni Ottanta, di campioni ben cristallizzati di bultfonteinite nella miniera N’Chwaning ii (Kalahari Man- determination of the crystal structure was performed by ganese Field, Repubblica Sudafricana) ha permesso di rac- Mciver (1963), using two-dimensional Fourier maps, cogliere nuovi dati sulla struttura di questo silicato idrato achieving R indexes, for the [100], [010], and [001] di calcio. Lo studio strutturale ha consentito di confermare projections, of 0.087, 0.095, and 0.114 respectively. l’assetto generale descritto da Mciver (1963) e di ipotizzare The well-crystallized specimens from the Kalahari Man- un ragionevole schema di legami a idrogeno. ganese Field, and in particular from the N’Chwaning ii mine, allowed us to perform a new structural study, in Parole chiave - Bultfonteinite, C-S-H, struttura cristallina, order to fully characterize this phase. The aim of this N’Chwaning ii mine, Repubblica Sudafricana. paper is to illustrate the results of this crystal structure refinement and the description of the complex hydro- gen bond scheme shown by bultfonteinite. introduction Bultfonteinite is quite a rare fluorine-bearing hydrated Experimental calcium nesosilicate. it was described by Parry et al. (1932) from the Bultfontein diamond mine (Republic acicular colorless crystals of bultfonteinite were used of South africa) where it was found at the beginning th throughout this study. These crystals are associated of the 20 century in a xenolith embedded in a kimber- with a member of the series poldervaartite-olmiite. in lite, associated with «apophyllite», calcite, and natro- the studied specimens, bultfonteinite can be associated lite. initially it was confused with this latter zeolite, also with oyelite; because of the morphological simi- but new findings in the Dutoispan and Jagersfontein larity between these two silicates, bultfonteinite was diamond mines allowed Parry et al. (1932) to describe identified by X-ray powder diffraction using a 114.6 the new mineral species. as an alteration product of mm Gandolfi camera with Ni-filtered CuKα radiation. basaltic xenoliths embedded in kimberlitic rocks, bult- Preliminary X-ray investigations by single-crystal oscil- fonteinite was also described from Lac de Gras (Cana- lation and Weissenberg photographs showed that bult- da; Chakhmouradian & Mitchell, 2001) and Damtshaa fonteinite is pseudo-orthorhombic, with cell parameters (Botswana; Buse et al., 2010). a 11.03, b 8.26, c 5.68 Å; the elongation direction is Murdoch (1955) described the occurrence of bult- [001]. The hk0 layer showed reflections elongated paral- fonteinite at Crestmore (Riverside County, Califor- lel to the rotation axis, indicating that the studied crystal nia, USa), in saccharoidal aggregates together with was formed by a parallel association on [001], with the afwillite and scawtite. Successively, bultfonteinite was single individuals slightly misaligned in the ab plane. identified in the skarns of Mihara (Miyake, 1965) and a small acicular crystal was selected and used for the Fuka (Kusachi et al., 1997), both in Japan, and in the intensity data collection performed at the CIADS (Centro Hatrurim Formation (israel; Gross, 1977), associated Interdipartimentale di Analisi e Determinazione Strut- (*) Dipartimento di Scienze della Terra, Università di Pisa, via Santa Maria 53, 56126 Pisa. 10 C. BiaGiONi, E. BONaCCORSi, S. MERLiNO turale) of Siena University, using an Oxford Xcalibur S in agreement with those reported by Mciver (1963). diffractomer with a CCD detector. The software Crysal- The crystal structure of bultfonteinite presents four isPro allowed the selection of reflections between two sets independent Ca sites and two independent Si sites. The clearly belonging to twinned crystals. The refined cell Ca sites have a seven-fold coordination, with an aver- parameters are a 10.999(1), b 8.176(1), c 5.6678(5) Å, α age bond distance around 2.40 Å; Ca-O distances range 94.24(1), β 89.00(1), γ 90.18(1)°, V 508.2(1) Å3, space from 2.304(6) Å (Ca3-O7 bond) to 2.560(7) Å (Ca3- group P1ˉ. a total of 2289 reflections was collected and O8 distance). These bond lengths are in agreement with corrected for Lorentz-polarization and absorption factors. those given by Mciver (1963), ranging from 2.31(2) to The structure was refined using the SHELX software 2.59(2) Å. The coordination polyhedra of Ca sites can be (Sheldrick, 2008) starting from the atomic coordinates described as a monocapped trigonal prism; the capping given by Mciver (1963), by accounting for a twinning ligands are O9, O10, O5, and O6 for the four indepen- through the TWiN instruction, adding a twin axis nor- dent Ca sites, respectively. The average Si-O distance is mal to (100). The isotropic refinement converged to 1.632 and 1.631 Å for Si1 and Si2 sites, respectively; the R = 0.11; after the introduction of the anisotropy of bond lengths range from 1.596(6) to 1.654(7) Å. thermal parameters, the R factor dropped to 0.087 for The crystal structure of bultfonteinite can be described 1630 observed reflections. The refined ratio between as formed by two kinds of layers, stacked along [010] the two individuals of the twinned crystals is 84:16. (Fig. 1). Table 1 reports the details of the data collection and The first layer (layer a; Fig. 2a) is composed by Ca1, the crystal structure refinement. Ca2, Si1, and Si2 polyhedra. Ca1 and Ca2 share edges, forming ribbons running along [001], with the alternation of Ca1 and Ca2 sites. Every ribbon has the anionic sites Structure description O9 and O10 (the capping ligands of the Ca polyhedra) pointing in the same direction; adjacent ribbons have General features these sites pointing in the opposite direction. The layer Table 2 shows atomic coordinates, equivalent displace- assumes a wavy character. Every ribbon is connected to ment parameters and site occupancies, whereas Table 3 the adjacent ones by Si tetrahedra and hydrogen bonds. reports the bond distances. The atomic coordinates are The second layer (layer B; Fig. 2b) is formed by Ca3 and Tab. 1 - Crystal data and details of intensity data collection and structure refinement. Dati cristallografici Chemical formula Ca4[SiO3(OH)]2F2·2H2O Crystal size (mm3) 0.5 x 0.05 x 0.05 mm3 Symmetry, space group Triclinic, P1ˉ a, b, c (Å); α, β, γ (°) 10.999(1), 8.176(1), 5.6678(5); 94.24(1), 89.00(1), 90.18(1) V (Å3) 508.2(1) Z 2 Raccolta e raffinamento strutturale Radiation, wavelenght (Å) MoKα, λ = 0.71073 Temperature (K) 293 Sample-detector distance (mm) 45 Maximum observed 2θ 58.06 Measured reflections 3796 Unique reflections 2289 Observed reflections (Fo > 4σ Fo) 1630 Rint 0.0429 h, k, l range -14 ≤ h ≤ 14, -5 ≤ k ≤ 11, -7 ≤ l ≤ 6 R [Fo>4 σ Fo] 0.0868 R (all reflections) 0.1056 2 wR2 (on Fo ) 0.2286 Goof 1.016 Number of refined parameters 164 CRySTaL structure OF bultfonteiNiTE, Ca4[SiO3(OH)]2F2·2H2O, from N’CHWaNiNG ii MiNE 11 Tab. 2 - atomic coordinates and equivalent displacement parameters. Site x y z Ueq Site occupancy Ca1 0.1326(2) 0.7934(2) 0.7841(3) 0.0108(4) Ca Ca2 0.3686(2) 0.7940(2) 0.2849(3) 0.0107(4) Ca Ca3 0.1195(2) 0.4778(2) 0.2492(3) 0.0105(4) Ca Ca4 0.3820(2) 0.4804(2) 0.7467(3) 0.0105(4) Ca Si1 0.4280(2) 0.2392(3) 0.2142(4) 0.0091(6) Si Si2 0.0701(2) 0.2393(3) 0.7278(4) 0.0089(6) Si F1 0.2513(5) 0.6226(6) 0.0130(9) 0.0122(11) F F2 0.2490(5) 0.6230(6) 0.5118(9) 0.0113(11) F O3 0.0438(6) 0.0413(8) 0.6971(10) 0.0132(13) O O4 0.4563(6) 0.0400(8) 0.1965(11) 0.0152(14) O O5 0.2810(6) 0.2685(8) 0.1897(10) 0.0133(14) O O6 0.2172(6) 0.2688(8) 0.7540(10) 0.0119(13) O O7 0.0101(6) 0.3214(7) 0.5005(11) 0.0120(13) O O8 0.0069(4) 0.3184(3) 0.9647(10) 0.0112(13) O O9 0.1908(6) 0.9587(8) 0.1603(11) 0.0196(15) O O10 0.3102(6) 0.9564(9) 0.6566(12) 0.0205(15) O O11 0.5066(6) 0.6809(7) 0.0034(10) 0.0094(12) O O12 0.4867(6) 0.3208(7) 0.4580(10) 0.0112(13) O Ca4 polyhedra.
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