Powder neutron diffraction studies of a carbonate fluorapatite

Atomic positional disorder of a single-phase natural carbonate fluorapatite (francolite) is revealed from analysis of the atomic displacement paramete rs (ADPs) refined from neutron powder diffraction data as a function of tem perature and carbonate content. The ADPs of the francolite show a strong di sturbance at the P, O3, and F sites. When it is heat treated to partially o r completely remove the carbonate, the ADPs as well as the other structural parameters resemble those of a fluorapatite (Harding: pegmatite) that was measured under the same conditions. The various structural changes are cons istent with a substitution mechanism whereby-the planar carbonate group rep laces a phosphate group and lies on the mirror plane of the apatite structu re.