Cathodoluminescence and microprobe study of rare-earth elements in apatite

The cathodoluminescence (CL) spectrum was scanned from 400 to 700 nm for each of fourteen apatite crystals from carbonatites, pegmatites, nelsonites, and granites. Peaks due to activation by Sm³⁺, Dy³⁺, Tb³⁺, Eu³⁺, Eu²⁺, and Mn²⁺ were identified in the CL spectra. Proton (pixe) and electron-microprobe analyses were used to determine the major (Ca, P,F) and minor elements (As, Si, Th, Mn, Fe, Sr, Na, Cl, REEs) in the apatite crystals. The microprobe analysis of two of the apatite samples (Durango and Huddersfield) compare favorably with analyses reported in the literature for apatite samples from the same localities. The rare-earth-element (REE) content varies from 0.2 to more than 15 wt% and is generally proportional to Na + Si, suggesting charge compensation for substitution of REE³⁺ for Ca²⁺. The chondrite-normalized distribution of the REEs shows light REE enrichment for the majority of samples with the Ce/Y ratio varying from 30 to 0.1. The relative height of the Sm³⁺ and Dy³⁺ peaks in the CL spectrum was found to be a good guide to the relative light to heavy REE enrichment. Apatites from more alkaline environments tended to be richest in the light REEs. The CL spectrum for an apatite from the Llallagua tin deposit in Bolivia shows a broad Eu²⁺ peak at a short wavelength and narrow Eu³⁺ peaks superimposed on a broad Mn²⁺ peak at a longer wavelength. The peak assignment was confirmed using an ultraviolet source and by comparison with spectra from synthetic apatite. The chondrite-normalized REE distribution for this sample shows a large positive Eu anomaly, which is similar to the positive anomaly shown for an apatite from the Panasqueira tin deposit in Portugal.

A series of glasses doped with Eu and heated at 1400°C at a series of oxygen fugacities was used to show the effect of oxygen fugacity on the relative height of the Eu²⁺ and Eu³⁺ peaks in the CL spectrum. It is concluded from these spectra that the positive Eu anomaly in the apatite from Llallagua and Panasqueira was not due to the preferential incorporation of Eu²⁺ by the apatite structure, but rather to the crystallization of the apatite from a reservoir strongly enriched in Eu.