THE CRYSTAL CHEMISTRY OF GADOLINITE-DATOLITE GROUP SILICATES

Abstract

Abstract This contribution reviews and affords new insights into the crystal chemistry of gadolinite-datolite group (GDG) silicates by re-examination of published and unpublished analytical data. More than 200 electron microprobe analyses (EMPA) and data from both single-crystal and powder X-ray diffraction of 53 samples of GDG silicates were collected to study site occupancy effects on local and long-distance geometry of the crystal structure. Results were achieved by statistical analysis and bond-length and bond-angle calculations. Here, crystal-structure geometry and site occupancy dependence is manifested in the GDG chemical composition and lattice parameters. While the substitution of B for Be at the tetrahedral Z site has the greatest impact on the size of the a parameter, variations in W -site occupancy result in a change in the size of the b parameter. Moreover, the degree of variability in the occupancy of the W site appears to influence the layer-stacking order, which differs between well ordered datolite and weakly ordered gadolinite-subgroup (GSG) minerals. This is documented by c parameter variability. The relationship between the X -site occupancy and lattice parameters is explained by the Jahn-Teller effect, which induces bond-length and angular distortion of the X O 6 octahedron. In addition, the structural arrangement of the X site excludes large cation (such as Ca, Bi, and REE) occupancy without any significant impact on the size of the X O 6 octahedron. Consequently, the formula for minasgeraisite-(Y) with the originally proposed Ca occupancy of the X site is questionable, since no significant difference from hingganite lattice parameters was observed. Thus the mineral requires further structural refinement. The structural arrangement of GDG silicates suggests that two dominant substitution vectors control their chemical composition: CaB(REE) −1 Be −1 and Fe 2+ O 2 (OH) −2 . However, both substitutions are more widespread in GSG minerals than in datolite subgroup (DSG) minerals. This may result from different geochemical properties of their genetic environment. However, there can be structural factors concerning X , W , and Z site occupancy which differentiate the far more chemically variable GSG from DSG.