Powder neutron diffraction studies of a carbonate fluorapatite
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Powder neutron diffraction studies of a carbonate fluorapatite Th. Leventouri Physics Department, Florida Atlantic University, Boca Raton, Florida 33431
B.C. Chakoumakos Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6393
H.Y. Moghaddam Physics Department, Florida Atlantic University, Boca Raton, Florida 33431
V. Perdikatsis Institute of Geology and Mineral Exploration, Messogion 70, 11527 Athens, Greece (Received 19 June 1999; accepted 9 November 1999)
Atomic positional disorder of a single-phase natural carbonate fluorapatite (francolite) is revealed from analysis of the atomic displacement parameters (ADPs) refined from neutron powder diffraction data as a function of temperature and carbonate content. The ADPs of the francolite show a strong disturbance at the P, O3, and F sites. When it is heat treated to partially or 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 consistent with a substitution mechanism whereby the planar carbonate group replaces a phosphate group and lies on the mirror plane of the apatite structure.
I. INTRODUCTION
Interest in crystal structure–property relationships in apatite is motivated from diverse disciplines: biology, materials science, geology, and engineering.1 Because apatite is a major component of bone, teeth, and pathological calcifications in vertebrates, atomistic models for solubility, ion exchange, and crystal growth are important, particularly in research on the mechanism of bone mineralization and caries.2 The inherent biocompatibility of apatite makes it a prominent biomaterial and the physical and physiological behaviors of apatite ceramics for prosthetic implants and thin-film coatings are an active area of research.3 Apatite is also a ubiquitous mineral in all of the major rock types, and it is often used for fission-track age-dating studies.4 Moreover, the volatile content of apatite is an indicator of fluid composition during magmatism, metamorphism, and sedimentary diagenesis.5 Apatite is a major component of ancient and modern marine phosphate deposits (phosphorites), and their isotopic study yields paleoclimatic variations.6 Phosphorites are also the principal source of phosphate fertilizers worldwide; consequently, weathering mechanisms and dissolution kinetics are important to understand phosphorus uptake by plants. Apatite is also a component of sedimentation within industrial cooling systems, so understanding of nucleation and crystal J. Mater. Res., Vol. 15, No. 2, Feb 2000
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growth is of interest. Another engineering concern is the precipitation of phosphate minerals from wastewater treatment systems,7 which is an essential process to prevent eutrophication of local receiving waters. Calcium phosphate apatites are compounds of the formula Ca5(PO4)3X where X can be an F − ion
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