Redetermination of ruizite, Ca2Mn3+ 2[Si4O11(OH)2](OH)2·2H2O

The crystal structure of ruizite, ideally Ca2Mn3+ 2[Si4O11(OH)2](OH)2·2H2O was redetermined based on single-crystal X-ray diffraction data of a natural sample from the Wessels mine, Northern Cape Province, South Africa, in space group C2. All non-H atoms were refined with anisotropic displacement parameters and all hydrogen atoms were located, improving upon previous results and yielding a significantly lower R factor.


Mineralogical and crystal-chemical context
Ruizite from the Christmas mine, Gila County, Arizona, USA, was originally described by Williams & Duggan (1977) with monoclinic symmetry in the space group P2 1 /c and unit-cell parameters a = 11.95, b = 6.17, c = 9.03 Å , = 91.38 based on rotation and Weissenberg photographs. Ideal chemistry was ISSN 2056-9890 Figure 1 Photograph of the ruizite specimen analyzed in this study.
proposed as CaMn(SiO 3 ) 2 (OH)Á2H 2 O. Wilson & Dunn (1978) reported a second occurrence of ruizite from the Wessels mine, Kalahari Manganese Field, Northern Cape Province, South Africa, with the same chemical formula as that given by Williams & Duggan (1977).

Figure 2
The crystal structure of ruizite as reported in this paper, viewed down b. Pink and gray ellipsoids represent O and Ca atoms, respectively. SiO 4 tetrahedra are shown in green and MnO 6 octahedra in yellow. Hydrogen atoms are represented by small dark-blue spheres.
locality may have occurred during the Cretaceous or sometime thereafter (Peterson & Swanson, 1956). The structure of ruizite was first determined by Hawthorne (1984) on the basis of space group A2, in the same setting as reported by Williams & Duggan (1977), using a crystal from the Wessels mine. The structure refinement yielded an R factor of 5.6% for an isotropic displacement model in which positions of three of the four hydrogen atoms were located. Refinement of anisotropic displacement parameters was not successful. The ideal chemical formula was revised to Ca 2 Mn III 2 [Si 4 O 11 (OH) 2 ](OH) 2 (H 2 O) 2 . Moore et al. (1985) reexamined the ruizite structure using a sample from the N'Chwaning mine and reported space group C2/m, a cell setting different from that adopted by Hawthorne (1984). The structure was refined with anisotropic displacement parameters for non-H atoms, yielding an R factor of 8.4%; no hydrogen atoms were located. However, most of the resulting displacement ellipsoids were unreasonable or non-positive definite. Moore et al. (1985) presented a disclaimer that 'the anisotropic thermal parameters for these crystals are more likely manifestations of intergrowths and domain disorder, rather than descriptions of true thermal motions'.
The current study reports a redetermination of the ruizite structure by means of single-crystal X-ray diffraction data of a natural sample from the Wessels mine, Kalahari Manganese Field, Northern Cape Province, South Africa ( Fig. 1).

Structural commentary
The structure of ruizite is characterized by chains of edgesharing MnO 6 octahedra extending along [010], which are linked by corner-sharing with SiO 4 tetrahedra that form short [Si 4 O 11 (OH) 2 ] chains, giving rise to a three-dimensional network (Fig. 2). The finite [Si 4 O 11 (OH) 2 ] chain in ruizite is the only reported silicate chain of this type. The relatively large Ca 2+ cations occupy the interstitial cavities and exhibit a coordination number of seven [Ca-O bond-length range 2.348 (4)-2.606 (3) Å ]. The Mn 3+ cations are bonded to four O atoms (O3, O4, and two O1 atoms) and two OH groups (O8-H2), resulting in a distorted MnO 6 octahedron, with an octahedral angle variance of 27.35 and a quadratic elongation index of 1.015 (Robinson et al., 1971). The Si1O 4 tetrahedron is more distorted than Si2O 4 , as indicated by angular variances (26.37 vs 10.18) and quadratic elongation indices (1.007 vs 1.003). Both average Si1-O nbr and Si2-O nbr bond lengths (nbr = non-bridging) are 1.618 Å . The Si2-O7 separation (1.642 Å ) is longer than the average Si2-O nbr length because O7 is the hydroxyl group that is also bonded to a Ca 2+ cation. The O5 atom is located on a twofold rotation axis and bridges the two Si2 atoms with a Si-O bond length of 1.6031 (13) Å , which is most likely a result of the considerably large Si2-O5-Si2 angle [162.9 (3) ] when compared to the Si1-O2-Si2 angle [128.27 (18) ] (Gibbs et al., 1994).
The hydrogen-bonding scheme in ruizite is presented in Table 2. The O9 atom is bonded to atoms H3 and H4, forming a water molecule whereas the O7 and O8 atoms are bonded to H1 and H2, respectively, to form two distinct OH groups. The bond-valence calculations (Brown, 2002) confirm the model ( Table 3). The O6 atom is markedly underbonded because it is an acceptor for both H1 and H3, and consequently it is associated with the shortest Si-O and Ca-O bond lengths. It is interesting to note that all hydrogen bonds in ruizite are shorter than 2.85 Å (Table 2). Nevertheless, the Raman spectrum shows a relatively sharp band at 3570 cm À1 (see below). According to Libowitzky (1999), this band would correspond to a hydrogen bond length (OÁ Á ÁO) of 3.1-3.3 Å . Perhaps O7-H1 forms a bifurcated hydrogen bond, where H1 is bonded to both O6 and O5. The O7Á Á ÁO5 distance is 3.354 Å , which could explain the band at 3570 cm À1 (Libowitzky, 1999). Broad-scan Raman spectrum of an unoriented ruizite specimen (R130787). Table 2 Hydrogen-bond geometry (Å , ).  (4) 167 (9) Symmetry code: (i) Àx þ 3 2 ; y À 1 2 ; Àz þ 1.  Fig. 3 is a plot of the Raman spectrum of ruizite. A tentative assignment of the major Raman bands is as follows: The bands between 2800 and 3600 cm À1 are due to the O-H stretching vibrations. The short H2Á Á ÁH4 distance (1.58 Å ) may be a result of disordering of one of the hydrogen atoms, which may also explain the considerably broad O-H stretching band in the Raman spectrum around 2940 cm À1 . The bands in the 1050-800 cm À1 region can be attributed to the Si-O stretching vibrations within the SiO 4 groups and those in the range of 670-520 cm À1 to the O-Si-O bending vibrations within the SiO 4 tetrahedra. The bands below 500 cm À1 are mainly associated with the rotational and translational modes of SiO 4 tetrahedra, and the MnO 6 and CaO 7 polyhedral interactions.

Synthesis and crystallization
The ruizite crystal used in this study is from the Wessels mine, Kalahari Manganese Field, Northern Cape Province, South Africa (Fig. 1) and is in the collection of the RRUFF project (http://rruff.info/R130787). Its chemical composition was measured using a CAMECA SX 100 electron microprobe at the conditions of 15 keV, 20 nA and a beam size <1 mm. The composition, calculated on the basis of 17 oxygen atoms and an estimation of H 2 O by difference is (Ca 1.90 Sr 0.06 Mg 0.04 )-(Mn 3+ 1.88 Fe 3+ 0.07 Al 0.05 )Si 4.01 O 11 (OH) 4 Á2H 2 O. The Raman spectrum of ruizite was collected from a randomly oriented crystal at 100% power of 150 mW on a Thermo Almega microRaman system, using a solid-state laser with a wavelength of 532 nm, and a thermoelectrically cooled CCD detector. The laser was partially polarized with 4 cm À1 resolution and a spot size of 1 mm.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The crystal structure was solved and refined based on space group C2 because it yielded better refinement statistics than in C2/m in terms of bond lengths and angles, atomic displacement parameters, and R factors. The crystal under investigation was twinned by inversion (Table 4). Electron microprobe analysis revealed that the ruizite sample studied contains small amounts of Sr, Mg, Fe, and Al. However, the overall effects of minor and trace amounts of these elements are negligible; therefore, the ideal chemical formula Ca 2 Mn 3+ 2 [Si 4 O 11 (OH) 2 ](OH) 2 Á2H 2 O was assumed during refinement. The H atoms were located from difference Fourier syntheses and confirmed by bond valence sum calculations. Their positions were refined with a fixed isotropic displacement parameter (U iso = 0.04). The maximum residual electron density in the difference Fourier map, 0.60 e Å À3 , was located at 0.85 Å from O8 and the minimum density at 0.17Å from H4. Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT (Bruker, 2004 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refined as a 2-component inversion twin.