Rare-earth-element ordering and structural variations in natural rare-earth-bearing apatites

High-precision crystal structure refinements (R < 0.02) were undertaken on natural rare-earth-element-bearing apatites that contain 1.26, 0.85, 0.62, and 0.33 rare-earth atoms per ten Ca sites in the unit cell. Electron and ion microprobe analyses of the REE-substituted apatites indicate that charge balance for the altervalent REE³⁺ ⇋ Ca²⁺ substitution is maintained by a combination of REE³⁺ + Si⁴⁺ ⇋ Ca²⁺ + P⁵⁺ and REE³⁺ ⇋ Na⁺ ⇋ 2Ca²⁺.

Variations in individual and mean interatomic distances reflect the cumulative effect of the following substitutions: OH⁻ ⇋ F⁻ in the column anion sites, Si⁴⁺ ⇋ P⁵⁺ in the tetrahedral sites, REE³⁺ ⇋ Ca²⁺ in Ca sites, and Na⁺ ⇋ Ca²⁺ in Ca sites. Mean Ca(1)-O distances in the crystals studied range from ~2.55 Å to ~2.57 Å and define regular, but not linear, increasing trends as a result of substitution of larger REE and Na atoms for Ca. Interatomic distances in the Ca(2) polyhedron are affected by both cation and anion substitutions, and thus variations are considerably less regular than those observed for Ca(1). Mean tetrahedral bond distances range from 1.536 Å to 1.542 Å and reflect the substitution of Si⁴⁺ for P⁵⁺.

All four apatites studied are distinctly enriched in the light REEs. Structure refinements demonstrate that the light REEs (La → Sm), taken as a g.roup, show a marked preference for the Ca(2) site in the apatite structure. Values of REE_Ca(2)/REE_Ca(1), calculated per individual site to account for the different multiplicity of the two Ca sites, vary between 1.76 and 3.00 over the range of compositions studied. Bond valence calculations for substituent REEs in the apatite structures indicate that the preference of REEs for the Ca(2) site may be a function of differing preference of elements within the light REE group; these calculations suggest that La → Pr should preferentially substitute in the Ca(2) site, whereas Pm → Sm should selectively substitute at the Ca(1) site. The calculations also indicate that Nd³⁺ can readily substitute in either Ca site. These preferences can explain the variation in partition coefficients of the REE ³⁺ in apatite.