THE REFINEMENT OF THE CRYSTAL-STRUCTURE OF STILLWELLITE - RE[BSIO5]

The X-ray intensity data of stillwellite were collected from a fragmen t of a crystal from Bassano Romano, Viterbo (Latium-Italy), using a PW 1100 Philips single crystal diffractometer. Stillwellite is trigonal (a = 6.844, c = 6.700 angstrom, space group P3(1), Z = 3). The first d etermination of the crystal structure by VORONKOV & PYATENKO (1967) co nverged to R(hkO) = 0.092 and R (okl) = 0.128. Our refinement, which c onverged to R = 0.015 for 1070 unique reflections, showed that the cry stal fragment used for the data collection was actually a twin (ratio between volumes of the twin individuals: 0.6/0.4). The structure is ch aracterized by helical [001] chains of threefold rings of two boron te trahedra (mean B-O distance 1.477 angstrom) and one silicon tetrahedro n (mean Si-O distance 1.624 angstrom). Each RE atom is bonded to nine oxygens (mean RE-O distance 2.572 angstrom) belonging to four differin g chains. The common occurrence of [100] twins may be related to the m arked pseudosymmetry in space group P3(1)21 shown by the majority of t he atomic sites.