Crystal structure of Cr-bearing Mg3BeAl8O16, a new polytype of magnesiotaaffeite-2N′2S

The crystal structure of a new polytype of magnesiotaaffeite-2N′2S is described. The S (Mg2Al4O8) and N′ (BeMgAl4O8) modules have the stacking sequence N′SSN′′.


Mineralogical and crystal-chemical context
The minerals of the taaffeite group form a polysomatic series, composed of spinel (S) and nolanite (N 0 ) modules (Armbruster, 2002). The nolanite modules in the taaffeites are modified with respect to the nolanite, (V,Fe) 5 O 7 (OH), crystal structure (Gatehouse et al., 1983), such that Be nominally substitutes for the hydrogen atoms of the nolanite OH group, while Mg and Al replace V and Fe, respectively. Variable numbers of the S-modules, Mg 2 Al 4 O 8 , and of the N 0 -modules, BeMgAl 4 O 8 , combine to yield different compositions of taaffeite minerals, i.e. different polysomes. Magnesiotaaffeite-2N 0 2S is composed of two modified nolanite modules N 0 and two spinel modules S, yielding an idealized composition of Mg 3 BeAl 8 O 16 . Be-doping of MgAl 2 O 4 has been shown to cause growth of twinned spinel crystals as a precursor to the formation of magnesiotaaffeite polytypes (Drev et al., 2013).
Here we report the crystal structure of a new polytype of magnesiotaaffeite, magnesiotaaffeite-2N 0 2S 2 which differs from the known magnesiotaaffeite-2N 0 2S (Nuber & Schmetzer, 1983) by the module stacking sequence. The resulting space group symmetry is P3m1, as opposed to the P6 3 mc symmetry of the previously known polytype.

Structural commentary
The crystal structure of the title compound is shown in Fig. 1. It can be described by the stacking of close-packed oxygen layers along [001], with layers of cations filling the interstices. Following the layer nomenclature of Nuber & Schmetzer (1983), the [6] Al1, [6] Al3 and [6] Al4 cations can be attributed to O-layers, the [6] Al5, [4] Mg1 and [4] Mg2 cations to T 2 -layers and ISSN 2056-9890 the [4] Be, [4] Al2 and [6] Mg3 cations to T 1 -layers. The cation stacking sequence is then T 1 -O-T 2 -O-T 2 -O-T 1 0 -Á Á Á while the anion stacking sequence is BACBACBCÁ Á Á. The orientation of T 1 0 is upside down with respect to T 1 . In the polytype described by Nuber & Schmetzer (1983), the stacking sequence is T 1 -O-T 2 -O-T 1 -O-T 2 -O-Á Á Á and BCABCBACÁ Á Á by comparison. In terms of polysomatism, the N 0 layer is composed of one T 1 and one O-layer. The second nolanite layer, N 00 , is also composed of these layer types, but its T 1 layer is inverted with respect to the stacking direction. The S-layer is composed of one O-layer and one T 2 -layer. Stacking these modules in the order N 0 -S-S-N 00 -N 0 -Á Á Á generates the new polytype structure (Fig. 1). The stacking sequence of the The composition obtained by structure refinement is in good agreement with the composition obtained by electron microprobe analysis (EMPA). The calculated bond-valence sums agree reasonably well with the formal charges (Table 1), and on average they support the assumption that Cr is trivalent. Significant amounts of Cr 3+ are found at the octahedrally coordinated Al3 and Al4-sites, where Cr 3+ is overbonded, as well as at the tetrahedrally coordinated Mg1 and Mg2 sites, where Cr 3+ is underbonded. Cr 3+ in tetrahedral coordination is unusual, but has recently been reported for the brownmillerite-type compound Ca 2 Cr 2 O 5 (Arevalo-Lopez & Attfield, 2015) and for Cr-doped BaAl 2 O 4 (Vrankić et al., 2015). However, without further confirmation by other methods, the appearance of tetrahedrally coordinated Cr 3+ in the title compound should be treated with caution. The tetrahedral Mg1 coordination, with one Mg1-O6 distance of 1.9537 (12) Å and three Mg1-O4 distances of 1.9296 (7) Å is more distorted than the Mg2 coordination environment, where the longer Mg2-O1 distance [1.9361 (13) Å ] hardly differs from the three 1.9300 (7) Å Mg2-O5 distances. The average bond lengths at the tetrahedral sites, nominally occupied by Mg (Mg1 1.936 Å , Mg2 1.932 Å ), and at the octahedral sites, nominally occupied by Al (Al1 1.909 Å , Al3 1.916 Å , Al4 1.913 Å , Al5 1.909 Å ), are similar to the T-O (1.936 Å ) and M-O (1.923 Å ) distances reported for a natural Cr and V-bearing spinel from Burma with a small inversion parameter (Widmer et al., 2015). This indicates that the degree of Mg, Al disorder is equally low in the title compound. The Al2 site is at the center of a nearly regular oxygen tetrahedron with an average Al-O distance of 1.785 Å . Al 3+ is slightly underbonded at this site (Table 1), which might indicate admixture of Mg atoms. The slightly overbonded Mg2 site might accommodate the resulting Al-excess. The Be 2+ cation forms one short bond with O7 [1.602 (2) Å ] and three longer bonds [1.6615 (13) Å ] with the O3-anions, while the tetrahedral angles are either 97.89 (9) (O3-Be1-O3) or 119.45 (7) (O7-Be1-O3). The Mg atom in the Mg3O 6octahedron exhibits a strong out-of-centre distortion, away from the Al3-cation, to which it has a distance of only 3.0580 (7)  Malcherek and Schlü ter Mg 3 BeAl 8 O 16 1061 Table 1 Bond-valence sums (BVS). Calculated using JANA2006 (Petřìček et al., 2014) with bond-valence parameters taken from Brese & O'Keeffe (1991). Angular brackets indicate site-occupancy weighted averages for the corresponding Mab sites.  form a strain free boundary after the first S-layer of the module sequence as shown in Fig. 1. At the twin boundary this results in a module sequence N 0 -S-N 0 -S, corresponding to the previously described polytype.

Sample details and EMPA
The studied natural sample of magnesiotaaffeite (m = 0.95 g) originates from Chaung-gyi, Mogok, Pyin-Oo-Lwin district, Burma (Myanmar). It has a red colour and a layered appearance (Fig. 2). A small fragment of the original sample was examined using single crystal X-ray diffraction. The same crystal fragment was subsequently prepared for electron microprobe analysis (EMPA) using a Cameca SX100 electron microprobe, operating in wavelength-dispersion mode at 15 kV and 20 nA.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The results of the EMPA indicate that the magnesiotaaffeite crystal contains significant amounts of Cr. In order to accurately refine small Cr-site populations against the major constituent elements Al and Mg, intensities at small scattering angles were systematically weighted down by a factor of 1Àexp[À5(sin /) 2 ] in order to emphasize core electron contributions to the X-ray scattering. For that purpose, Cr and Mg or Al were constrained to have the same coordinates and displacement parameters under consideration of full occupancy for the corresponding site. Scattering factors for neutral atoms were used and all atoms were refined with anisotropic displacement parameters. No evidence for mixed occupancy was found at the Be site; small Cr amounts were found for the Al3, Al4, Mg1 and Mg2 sites with occupation factors for Cr of 0.017 (3), 0.017 (5), 0.028 (5) and 0.048 (5), respectively. Two twin domains (twinning by merohedry) with volume fractions of 0.64 and 0.36 contribute to the total scattering intensity, related by reflection parallel to [110]    Data collection: COLLECT (Nonius, 1998); cell refinement: EVAL15/Peakref (Schreurs et al., 2010); data reduction: EVAL15 (Schreurs et al., 2010); program(s) used to solve structure: SUPERFLIP (Palatinus & Chapuis, 2007); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: VESTA (Momma & Izumi, 2011); software used to prepare material for publication: publCIF (Westrip, 2010).

Trimagnesium beryllium octaaluminium hexadecaoxide
where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.41 e Å −3 Δρ min = −1.01 e Å −3 Extinction correction: SHELXL2014 (Sheldrick, 2015), 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 twin