1993/01/01 by Kenny Ståhl · 11 citations
Materials Science · Environmental Science · Chemistry · #Clay minerals and soil interactions #Chemical Synthesis and Characterization #Glass properties and applications #Neutron diffraction #Powder diffraction #Chemistry #Materials science #Crystallography #Nuclear chemistry #Chemical engineering #Crystal structure
paper · doi:10.1127/ejm/5/5/0845
published in European Journal of Mineralogy 5(5), 845-850 (Elsevier BV)
openalex publication_date 1993/01/01 · openalex created_date 2025/10/10 · openalex updated_date 2025/11/06
Fluorapopbyllite, KCa 4 Si 8 O 20 F.nH 2 O, n(H 2 O) = 8, was dehydrated at 473 K to an average of n(H 2 O) = 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(H 2 O) ≃ 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(H 2 O) ≃ 7 and the amorphous form. The frequently observed step-wise dehydration behaviour is explained as a kinetic (diffusion controlled) effect. Partially dehydrated fluorapophyllite, n(H 2 O) = 6.9(1), is tetragonal, space group P4/mnc, Z = 2, with unit cell constants of a = 8.9639(9) and c = 15-754(2) A at 295 K. With the exception of the water content, the crystal structure is not significantly different from that found by Stabl et al. (1987)