A neutron powder diffraction study of partially dehydrated fluorapophyllite, KCa4Si8O20F.6.9H2O
European Journal of Mineralogy Volume 5 Number 5 (1993), p. 845 - 850
published: Jan 1, 1992
manuscript accepted: May 19, 1993
manuscript received: Jan 5, 1993
ArtNo. ESP147050505013, Price: 29.00 €
Abstract Fluorapophyllite, KCa4Si8O20F.nH2O, n(H20) = 8, was dehydrated at 473 K to an average of n(H20) = 4. Rietveld analysis of neutron powder diffraction data from the partially dehydrated sample showed no evidence of a crystalline hemi-dehydrated form of fluorapophyllite. The dehydration process is described as a two step reaction. The first step leads to n(H2O) « 7. One water out of eight in the original K coordination sphere is expelled, the remaining water of a mirror related pair assumes a new position in the mirror plane. In the second step, continued dehydration then destabilizes the K and Ca coordinations, which connect the silicate layers, and a non-diffracting amorphous phase is formed. The sample under study is thus a mixture of the crystalline form with n(H2O) « 7 and the amorphous form. The frequently observed step-wise dehydration behaviour is explained as a kinetic (diffusion controlled) effect. Partially dehydrated fluorapophyllite, n(H2O) = 6.9(1), is tetragonal, space group PAImnc, Z = 2, with unit cell constants of a = 8.9639(9) and c = 15.754(2) Å at 295 K. With the exception of the water content, the crystal structure is not significantly different from that found by Stáhl et al. (1987).