NEW INSIGHTS INTO THE STABILIZATION OF TH E HULSITE STRUCTURE DURING CRYSTAL-STRUCTURE DETERMINATION OF MNII(2)MNIII(BO3)O2 AND MNIISRMNIII(BO3)O2

The compounds (Mn2MnIII)-Mn-II (BO3)O-2 (I) and (MnSrMnIII)-Sr-II(BO3) O-2 (II) were prepared by using a B2O3 flux technique. Single crystals were investigated by X-ray diffraction. I showed orthorhombic symmetr y, space group D-2h(9) -Pbam (No. 55), a=926.0; b=1241.5; c=304.96 pm; Z=4 and II monoclinic symmetry, space group C-2h(1)-P2/m (No. 10), a= 1107.5; b=311.04; c=542.93 pm, beta=95,10(2)degrees; Z=2. (Mn2MnIII)-M n-II(BO3)O-2 is isotypic with the mineral Ludwigite while (MnSrMnIII)- Sr-II(BO3)O-2 is isostructural with the mineral Hulsite. In the struct ure of (Mn2MnIII)-Mn-II (BO3)O-2 (I) all metal point positions showed octahedral oxygen coordination, one point position is occupied by Mn3. The structure of (Mn2Sr2Mn2III)-Sr-II(BO3)(2)O-4 (II) contains five octahedrally coordinated metal sites, three of them statistically occu pied by Mn2+ or Mn3+ and Sr2+. The incorperation of Sr2+ seems to be r esponsible for the Hulsite structure. Both structures contain isolated , trigonal planar BO3 units and oxygen atoms that are not coordinated to boron.