INTRACRYSTALLINE RARE-EARTH ELEMENT DISTRIBUTIONS IN APATITE - SURFACE STRUCTURAL INFLUENCES ON INCORPORATION DURING GROWTH

Intracrystalline REE distributions in fluorapatite have been studied u sing cathodoluminescence and synchrotron X-ray fluorescence microanaly sis (SXRFMA). In the apatite samples studied the {<10(1)over bar 0>} f ace grows by the spiral mechanism. As a result, polygonized growth hil locks with three vicinal faces occur on {<10(1)over bar 0>} Face symme try, m, constrains the [011] and [01(1) over bar] growth steps to be e quivalent. However, [001] steps are not symmetry related. REE and Mn c athodoluminescence qualitatively indicates a differential distribution of Mn2+, Sm3+, Eu3+, Dy3+, and Eu2+ between subsectors associated wit h symmetrically nonequivalent vicinal faces (intrasectoral zoning). SX RFMA shows that the concentrations of ail the REEs analyzed (La, Ce, P r, Nd, Sm, Eu, Gd, Dy, Ho, and Y), except Eu, exhibit a sharp step fun ction, with the change in concentration correlated directly to the bou ndary between nonequivalent vicinal faces and associated subsectors. L anthanum, cerium, and praseodymium are enriched in the [001] vicinal s ubsector, and REEs heavier than Pr, except Eu, are enriched in the [01 1] vicinal subsectors. The distribution trend observed for Eu may be d ue to its presence in both the 2+ and 3+ oxidation states.The absence of evidence for anisotropies in step velocity that could influence inc orporation supports the proposal of Rakovan and Reeder (1994) that dif ferences in the atomic structure between [001] and (011) steps lead to differential incorporation in apatite. Atomic scale models of the str ucture of these steps show qualitative differences that are consistent with this proposal. The reversal in the REE partitioning trend betwee n Pr and Nd suggests an ion size effect on the partitioning between no nequivalent steps. The differential distribution of REEs indicates non equilibrium partitioning. Site specific controls on the partitioning o f REEs indicate that partition coefficients can be a function of surfa ce structure and dependent on the spatial segregation of nonequivalent surface sites. Hence, different REE partition coefficients can exist for incorporation at structurally different growth steps.