The crystal structure of an synthetic meionite
Aitken, Bruno G.; Evans, Howard T.; Konnert, Judith A.
Neues Jahrbuch für Mineralogie - Abhandlungen Band 149 Heft 3 (1984), p. 309 - 324
25 Literaturangaben
veröffentlicht: Aug 2, 1984
DOI: 10.1127/njma/149/1984/309
ArtNo. ESP154014903005, Preis: 29.00 €
Abstract
The structure of a synthetic meionite has been refined in space group I4/m (a = 12.179(1) Å, c = 7.571(1) Å, Z = 2), by using 1618 observed diffractometer data measured with MoKα radiation. The composition, obtained by microprobe analysis of the cations, is (Ca3.40Na0.64) (Al5.43Si6.59)O24(CO3O). For this crystal of 84 percent meionite, no trace of X-ray reflections indicating lower symmetry was found. The configuration of the CO3 group was studied principally by means of electron density maps and difference (Δρ) maps. These showed the group to lie strictly in the mirror plane, and always disordered about the 4-fold axis. Attempted refinements of the position of a single CO3 group (repeated 4 times) did not converge. A model in which the CO3 group lies with equal probability on either side of the Ca-Ca diagonal axis in the Ca4 square was found to improve substantially the goodness of fit, as measured by the average root-mean-square Δρ value within the cavity. Thus, the best model was obtained by least-squares analysis of 6 parameters defining a CO3 group constrained to an equilateral triangle and disordered over 8 positions about the 4-axis and the Ca4 square pseudosymmetry. The rms (Δρ) value was further reduced by replacing CO32- by O2- (or OH- or H2O) to the extent of 12 mol. percent. The final model has Ca-O bond lengths with the CO3 group ranging from 2.15 Å to over 3 Å. Application of an empirical bond-length/bond-number relationship shows that the bond lengths correlate well with the need to satisfy the formal valency of each atom, both in the framework and in the cavity.
Schlagworte
Scapolite group • meionite • synthetic • crystal structure • carbonate ion