Crystal chemistry of two natural magmatic norsethites, BaMg(CO3)(2), from an Mg-carbonatite of the alkaline carbonatitic complex of Tapira (SE Brazil)

The crystal chemistry of two natural magmatic norsethites, BaMg(CO3)(2), wa s investigated by single-crystal X-ray diffraction and microprobe analyses. R-3m seems to be the most probable space group with respect to R32 and R-3 space groups. The main structural features, including the aplanarity of th e carbonate group, do not differ from those of synthetic norsethite. Slight chemical variations occurring in both A and B polyhedra of the two crystal s cause slight changes in the cation-to-oxygen and oxygen-to-oxygen distanc es between different layers along the z axis; as a consequence of the contr ibution of these slight polyhedral variations, the two crystals are charact erized by significantly different c(o) lattice parameters. The octahedral B site of norsethite differs distinctly from that of dolomite, due to its sm aller size and higher distortion. Norsethite shows a greater distance from octahedral B layer to O layer, in spite of its lower B-O bond lengths with respect to dolomite.