Beitrag
The crystal structure of the orthorhombic calzirtite from Val Malenco (Italy)
Callegari, A.; Mazzi, F.; Ungaretti, L.
Neues Jahrbuch für Mineralogie - Monatshefte Jg. 1997 Heft 10 (1997), p. 467 - 480
8 Literaturangaben
veröffentlicht: Nov 24, 1997
DOI: 10.1127/njmm/1997/1997/467
ArtNo. ESP156199710002, Preis: 29.00 €
Abstract
Calzirtite Ca2Zr5Ti2O16 is described in the literature as tetragonal, α = 15.094, c = 10.043 Å, and its crystal structure has been determined (PYATENKO & PUDOVKINA 1961) and refined (ROSSELL 1982, SINCLAIR et al. 1986) in space group 14l/acd. However, calzirtite from Rocca Castellaccio, Val Malenco (Sondrio, Italy), α = 15.207, c = 10.114 Å, shows also measurable X-ray reflections with h+k+l = 2n+1, which are forbidden in the above tetragonal space group, and whose intensities are compatible with the orthorhombic space group Pbca. The tetragonal crystal structure, in which eight Zr atoms statistically occupy half of sixteen equivalent sites, may be converted into an orthorhombic model by ordering the Zr atoms on eight orthorhombic equivalent sites. As this ordering is compatible with both Pbca and Pcab subgroups of the tetragonal 14l/acd space group, two sets of structural domains are formed, which may be also interpreted as due to (110) twinning. Apart from the ordering of Zr and some consequent very slight shifts in the coordinates of the other atoms, the orthorhombic structure is topologically identical to the tetragonal structure. The X-ray diffraction analysis of two fragments of the same sample of Italian calzirtite showed that, besides the ordered orthorhombic fraction with different volume ratios of its twin individuals (0.54/0.46 and 0.72/0.28, respectively), about 17-20 per cent in volume of the tetragonal disordered fraction is also present. The crystal structure refinements carried out on the two fragments converged respectively to R = 0.04S-0.029 for the "observed" reflections with I/σ(I) >3 and R = 0.068-0.044 for all unique reflections. Increasing amounts of disordered tetragonal calzirtite have been obtained by repeated heating of the ordered orthorhombic phase in the temperature range of 700-1350 °C.
Schlagworte
Calzirtite • order-disorder • twinning