Robertsite, Ca2MnIII 3O2(PO4)3·3H2O

Robertsite, ideally Ca2Mn3O2(PO4)3·3H2O [calcium manganese(III) tris(orthophosphate) trihydrate], can be associated with the arseniosiderite structural group characterized by the general formula Ca2 A 3O2(TO4)3·nH2O, with A = Fe, Mn; T = As, P; and n = 2 or 3. In this study, single-crystal X-ray diffraction data were used to determine the robertsite structure from a twinned crystal from the type locality, the Tip Top mine, Custer County, South Dakota, USA, and to refine anisotropic displacement parameters for all atoms. The general structural feature of robertsite resembles that of the other two members of the arseniosiderite group, the structures of which have previously been reported. It is characterized by sheets of [MnO6] octahedra in the form of nine-membered pseudo-trigonal rings. Located at the center of each nine-membered ring is a PO4 tetrahedron, and the other eight PO4 tetrahedra sandwich the Mn–oxide sheets. The six different Ca2+ ions are seven-coordinated in form of distorted pentagonal bipyramids, [CaO5(H2O)2], if Ca—O distances less than 2.85 Å are considered. Along with hydrogen bonding involving the water molecules, they hold the manganese–phosphate sheets together. All nine [MnO6] octahedra are distorted by the Jahn–Teller effect.


Crystal data
Robertsite and pararobertsite (Roberts et al., 1989;Kampf, 2000), are dimorphs with composition Ca 2 Mn 3+ 3 O 2 (PO 4 ) 3 .3H 2 O, and are the only phosphate minerals known to date with Ca 2+ and Mn 3+ cations (Fransolet, 2000). They occur in altered pegmatites and sedimentary phosphate ores as typical products of weathering (van Kauwenbergh et al., 1988), and are thus important to our understanding of the alteration processes of primary phosphate minerals. Crystalline manganese phosphates are also of particular interest for technological applications. For example, they have been studied as potential adsorbers of metal contaminants, such as Ag, Hg, and Pd, from industrial waste (Kulprathipanja et al., 2001). Robertsite was previously investigated by Moore & Ito (1974) using powder X-ray diffraction, but its crystal structure was not refined owing to the rarity of suitable single crystals. This study presents the first crystal structure determination of robertsite. The single-crystal data was obtained from a sample from the Tip Top mine.
The structure of robertsite is built from sheets of [MnO 6 ] octahedra sandwiched between layers of PO 4 tetrahedra. The [MnO 6 ] octahedra share edges to form nine-membered pseudo-trigonal rings that pack in monolayers (Fig. 1) ] dimers, which may be the reason for the symmetry reduction from trigonal to monoclinic for this mineral (Fig. 2). In addition, one-third of the water molecules are loosely bonded in cavities of the structure. Although H atoms were excluded from the refinement, it is obvious from O···O distances that medium-strong hydrogen bonds are present in the structure. In Table 1, a possible hydrogen-bonding scheme devised from O···O distances is presented.

Experimental
The robertsite specimen used in this study comes from the type locality, the Tip Top mine, Custer County, South Dakota and is in the collection of the RRUFF project (deposition No. R120040; http://rruff.info). The chemical composition, Ca The Raman spectra of robertsite and pararobertsite (R120119) were collected from a randomly oriented crystal at 100% power on a Thermo Almega microRaman system, using a solid-state laser with a wavenumber of 532 nm, and a thermoelectrically cooled CCD detector. The laser is partially polarized with 4 cm -1 resolution and a spot size of 1 µm.

Refinement
Due to the similar scattering power of Mn and Fe, any minor Fe was treated as Mn and therefore the ideal chemical formula was assumed during the refinement. The structure, in space group Aa, was refined on basis of data from a crystal twinned by inversion with a ratio of 0.676 (18):0.324 (18) for the twin components. The maximum residual electron density in the difference Fourier maps was located at (0.0711, 0.2978, 0.8376), 0.71 Å from Ca2 and the minimum at (0.1865, 0.4867, 0.0185) 0.68 Å from P7. H-atoms from water molecules could not be assigned reliably and were excluded from refinement. To keep consistent with the previous report on mitridatite (Moore & Araki, 1977), the nonstandard space group setting of space group No. 9 in Aa was adopted here, instead of the conventional Cc setting.    Raman spectra of robertsite and pararobertsite.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.