The crystal structure of a blatterite mineral, Mg_1.33 Mn_1.44Fe_0.05 Sb_0.17 O₂ BO₃, a combined single crystal X-ray and HREM study

The borate mineral Mg_1.33Mn_1.44Fe_0.05 · Sb_0.17O₂BO₃ from the Mossgruvan mine, Nordmark, Värmland, Sweden, has been studied by a combination of single crystal X-ray diffraction and electron microscopy (HREM, EELS and EDX) techniques. The HREM investigations showed that the structure belongs to the pinakiolite family, whose other members with increasing complexity are, e.g. pinakiolite, ludwigite, orthopinakiolite, takéuchiite and blatterite. The space group is orthorhombic, Pnnm (No. 58), a = 37.384(11) Å, b = 12.568(3) Å, c = 6.200(2) Å; Z = 32. The structural model, for which the initial coordinates could be deduced from the HREM images, has been refined versus the 2280 most significant X-ray reflection intensities with sin θ/λ ≤ 0.70 Å-1 to R-value of 0.056. As for many other members of the pinakiolite family some cation positions are disordered. The disorder is obviously related to the structural effects caused by the distorted oxygen coordination octahedra around the Mn3+ ions. As revealed by HREM, some blatterite crystals contain extended planar defects that can be explained as due to irregular repeat distances between the twin planes in the [100] direction.