Surface-structure-controlled sectoral zoning of the rare earth elements in fluorite from Long Lake, New York, and Bingham, New Mexico, USA

The concentration and distribution of rare earth elements (REE) in sectorally zoned fluorite crystals from Long Lake, New York, and the Hansonburg Mining District, Bingham, New Mexico, have been studied using cathodoluminescence and synchrotron X-ray fluorescence microanalysis (SXRFMA). In cubo-octahedral samples from Long Lake, New York, Ce, Nd, Gd, Dy, Ho, Er, and Tm are preferentially partitioned into the |111| sector relative to the |100| sector. Partition coefficients (K_d = concentration in |111| sector/concentration in |100| sector) range between 3.5 for Ce, to 1.4 for Tm, with a general decrease in K_d as elements deviated from the ionic radius of Ca²⁺, for which REE substitute in fluorite. Diffusion of the REE has occurred, as evidenced by gradual changes in composition over distances of 0.2 to 0.3 mm at sector boundaries.In Bingham samples, three different partition coefficients were determined for Dy: K_d|100|/|111| = 2.83, K_{d |100|/|110|} = 1.77, and K_{d |110|/|111|} = 1.60. These are mean K_d values for a 95% confidence interval. In another sample from the same deposit, Dy, Er, and Gd were found to be preferentially incorporated into the |100| sector relative to the |210| sector with average K_{d |100|/|210|} of 3.1, 2.4, and 2.9, respectively. In a third sample, Nd was found to be preferentially incorporated into the |110| sector relative to the |321| sector with an average K_{d |110|/|321|} value of 2.3.Compositional heterogeneities in a given sector (concentric zoning) have been resolved using SXRFMA but are significantly less than the concentration difference across sector boundaries. Often fluorite exists in a wide variety of morphologies, as is the case in the Hansonburg Mining District of Bingham. We suggest caution when using the REE as petrogenetic indicators because fluorite trace element chemistry can vary greatly among crystals within a deposit depending on the internal morphology of a particular crystal.