SAPPHIRINE .2. NEUTRON AND X-RAY-DIFFRACTION STUDY OF (MG-AL)VI AND (SI-AL)IV ORDERING IN MONOCLINIC SAPPHIRINE

The crystal structure of a P21/a polymorph of sapphirine (a=11.286(3), b=14.438(2), c=9.957(2) Å, β=125.4(2) °) of composition [Mg3.7Fe0.12+Al4.1- Fe0.13+]IV[Si1.8Al4.2]IVO20 was refined using structure factors determined by both neutron and x-ray diffraction methods to conventional R factors of 0.067 and 0.031. respectively, for Fobs>2σ. The results of the two refinements agree reasonably well, but a half-normal probability plot (Abrahams, 1974) comparing the two data sets indicates that the pooled standard deviations of the atomic coordinates have been underestimated by a factor of two. The structure of sapphirine, solved initially by Moore (1969), consists of cubic closest packed oxygens with octahedral and predominantly tetrahedral layers alternately stacked along [100]. The layer in which 70% of the octahedral sites are occupied has an Mg-Al distribution characterized by Mg-rich octahedra sharing edges mainly with Al-rich octahedra. Mean octahedral bond lengths correlate well with Al occupancy determined by neutron site refinement if the relative number of shared octahedral edges is taken into account (see Table 1). The predominantly tetrahedral layer has 10% of the octahedral sites occupied by Al and 30% of the tetrahedral sites occupied by Al-Si in the ratio 2.33:1. There are single chains of Al-Si tetrahedra parallel to z with corner-sharing wing tetrahedra (T5 and T6) on either side in the (100) plane. The mean T-O distance is highly correlated with Al occupancy, XAl, as determined from the neutron site refinement: {Mathematical expression} Details of the neutron refinement are summarized below. © 1979 Springer-Verlag.