The structure of danalite at high temperature obtained from synchrotron radiation and Rietveld refinements

The structural behavior of danalite, ideally Fe-8[Be6Si6O24]S-2, at atmospheric pressure and from 33 to 1035degreesC on heating, was determined by using in situ synchrotron X-ray powder-diffraction data (lambda = 0.92249 Angstrom) and Rietveld refinement. The sample was heated at a rate of about 9.5degreesC/min., and X-ray traces were collected at about 18degreesC intervals. The unit-cell parameter for danalite increases linearly to 1035degreesC. The percent change in volume between 33 and 1035degreesC is 1.6(1)%. Other structural parameters show an abrupt change at about 926degreesC. Between 33 and 926degreesC, the Be-O and Si-O distances are nearly constant. Between 926 and 1035degreesC, the Be-O bond length decreases, whereas the Si-O bond distance increases, but the changes are small. From 33 to 926degreesC, the Be-O-Si angle increases by 1.12(3)degrees, and the total increase up to 1035degreesC is 1.21(3)degrees. Simultaneously, between 33 and 926degreesC, both the angles of rotation of the BeO4 (phi(Be)) and SiO4 (phi(Si)) tetrahedra decrease by about 0.71degrees, respectively. From 926 to 1035degreesC, both phi(Be) and phi(Si) increase by about 0.23degrees. The Fe-S bond length increases by 0.023(1) Angstrom from 33 to 926degreesC, and by 0.007(1) Angstrom from 926 to 1035degreesC. The Fe-O bond distance increases by 0.024(1) Angstrom from 33 to 926degreesC, and decreases by 0.001(1) Angstrom from 926 to 1035degreesC. Large displacement-parameters occur for the Fe and S atoms, and as the Fe-S bond expands with temperature, the Fe atoms move toward the plane of the six-membered rings in the framework, which causes the tetrahedra to rotate slightly and results in the expansion of the structure. The abrupt change in structural parameters probably occurs because of the oxidation of Fe2+ (and minor Mn2+) cations beyond 771degreesC, and the loss of S-2(g) beyond 1029degreesC.