Strontium in the apatite structure: Strontian fluorapatite and belovite-(Ce)

Strontium is one of the most common substituents in apatite; the presence a nd behavior of Sr in apatite-group phases are of considerable significance in geology, materials science and biology. The atomic arrangements in stron tian fluorapatite (1.18 Sr atoms per 10 Ca sites) and belovite-(Ce) [ideall y Sr-6(REE2Na2)(PO4)(6)F-2] have been refined to R values of 0.016 and 0.01 4, respectively, in order to examine the incorporation of Sr in apatite and apatite-group minerals. In strontian fluorapatite, Sr is ordered at the Ca (2) site, and no dissymmetrization from P6(3)/m results from the substituti on; expansion of the Ca(2) site accommodates the Sr. In belovite-(Ce), the symmetry of the atomic arrangement degenerates to P(3) over bar as the apat ite Ca(1)-equivalents split to form separate sites for Na and the REE: Sr i s ordered at the apatite-equivalent Ca(2) sites. The symmetry reduction res ults principally from shifts in O(3) apatite equivalents [which are split t o O(3) and O(4) in belovite] to accommodate Na and REE in distinct sites, s ites that are equivalent to Ca(1) sites in P6(3)/m apatite. The ordering of Sr and REE in belovite suggests that it may be possible to tailor the emis sion characteristics of apatite phosphors and lasers by controlling the dis tribution of luminescence-activating lanthanides between the two Ca sites w ith Sr codoping.