Differential incorporation of trace elements and dissymmetrization in apatite: the role of surface structure during growth

Apatite crystals from Llallagua, Bolivia, and Minas Gerais, Brazil, show heterogeneous distributions of trace elements that correspond directly to coeval portions of symmetrically nonequivalent vicinal faces of growth hillocks on both {101̄0} and {0001} faces. The same samples also exhibit sectoral zoning. Synchrotron X-ray fluorescence microanalysis reveals differences up to a factor of 2 in Y and 1.5 in Sr concentrations between subsectors associated with symmetrically noneqivalent vicinal faces of trigonal growth hillocks on {101̄0} faces. Variations exist between pyramidal vicinal faces and surfaces parallel to {0001}, which are found at the summits of many of these hillocks. Such variations indicate nonequilibrium partitioning and suggest that there is no unique value of K_D that describes the actual partitioning for a given mineral-fluid system; rather, effective K_D values differ for structurally distinct regions of a crystal surface. -from Authors