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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2017: 2.645

CiteScore 2017: 2.31

SCImago Journal Rank (SJR) 2017: 1.440
Source Normalized Impact per Paper (SNIP) 2017: 1.059

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1945-3027
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Volume 89, Issue 10

Issues

Accommodation of the carbonate ion in apatite: An FTIR and X-ray structure study of crystals synthesized at 2–4 GPa

Michael E. Fleet / Xiaoyang Liu / Penelope L. King
Published Online: 2015-03-28 | DOI: https://doi.org/10.2138/am-2004-1009

Abstract

Carbonated hydroxylapatite (C-OHAp) and carbonate apatite (CAp; x ≥ 0.5) in the composition series Ca10(PO4)6-y[(CO3)x+(3/2)y(OH)2-2x], x = 0.0-0.7, y = 0.0-0.6, have been synthesized at 2-4 GPa, and studied by FTIR spectroscopy and single-crystal X-ray diffraction. Three structural locations for the carbonate ion have been identified: (1) apatite channel, oriented with two oxygen atoms close to the c-axis (type A1); (2) close to a sloping face of the PO4 tetrahedron (type B); and, (3) stuffed channel position (type A2). Type A1 and B carbonate are equivalent to type A and B CAp of bone and enamel, whereas type A2 is a high-pressure feature. In type A CAp, ordering of type A1 carbonate within the apatite channel results in space group P3̅ ; all other apatites studied have average structures with P63/m symmetry. Results for three new structures are: type A C-OHAp, x = 0.14, y = 0.0, a = 9.4468(4), c = 6.8806(4) Å, and R (residual index of structure refinement) = 0.025; type B C-OHAp, x = 0.0, y = 0.17, a = 9.4234(2), c = 6.8801(3) Å, and R = 0.025; and type A-B CAp, x = 0.7, y = 0.5, a = 9.4817(6), c = 6.8843(3) Å, and R = 0.025. A fourth structure analysis suggests that the type A-B CAp exchanges some of its channel carbonate with OH- during room-temperature storage in nujol oil, with x and y reduced to 0.6 and 0.4, respectively. Local structural adjustments to accommodate the carbonate ion in the c-axis channel of OHAp include dilation of the channel, contraction of the Ca1On polyhedron, and rotation of the PO4 tetrahedron about the P-O1 bond. The progressive increase in the a unit-cell edge length with increase in carbonate content of type A CAp is readily attributed to the dilation of the apatite channel. Carbonate-for-phosphate substitution in OHAp (type B CAp) requires displacement of O3 along ±[001] and, thus, results in expansion of c (and contraction of a).

About the article

Received: 2003-10-08

Accepted: 2003-05-03

Published Online: 2015-03-28

Published in Print: 2004-10-01


Citation Information: American Mineralogist, Volume 89, Issue 10, Pages 1422–1432, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-2004-1009.

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© 2015 by Walter de Gruyter Berlin/Boston.

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