HALOGEN ANALYSIS OF APATITE USING ION CHROMATOGRAPHY

Ion chromatography (IC) has numerous applications in a large spectrum of analytical sciences, but its use in geological sciences has been limited. To our knowledge IC has not been used to analyze the chemical composition of natural apatite. We are introducing a new method to evaluate the concentration of halogens, phosphate, and alkali and alkali-earth cations in apatite using IC on acid digested apatite crystals. In addition to rock-forming apatite, this novel technique may be applied to bone and tooth apatite and its synthetic equivalents.

A combination of methods is currently required to chemically characterize apatite, because of its complex formula - X₅(PO₄)₃Z, where X stands for Ca, partly substituted by REEs, Sr, Pb, Na, Mg etc. and Z stands for F, substituted by OH, Cl, Br, etc. Elements such as Cl and Br may fall below the detection limits of a typical electron microprobe. Only a few studies report Br, but the existence of a synthetic bromapatite endmember indicates that Br concentrations in apatite may have been overlooked. We analyzed apatite of igneous origins, including samples from granitic and alkali pegmatites previously investigated by EMP, FTIR, and SIMS (Tacker, Amer. Min. v.89, 2004). The sample purity was confirmed using a laser Raman microprobe.

We dissolved pre-cleaned, powdered apatite in 7.3 mM high-purity sulfuric acid at a minimum dilution factor of 1667, just below saturation. Two separate replicates were injected in a DX-320 Dionex chromatograph with suppressed conductivity detector for the analysis of anions and cations, respectively. The apparent phosphate recovery ranged between 75 and 100 %, justified by substitutions of heavy metals on the X site and/or incomplete sample digestion. The F concentrations in apatite ranged from 0.2 to 4.0 wt %, Cl was found to be as high as 0.3 % in certain samples, and Br reached 355 ± 43 ppm. To our knowledge, this may be the highest Br value reported for natural apatites. For comparison to other analytical methods, the calculated method detection limit (MDL) for F was 30 ppb which translates to a level of 50 ppm in the apatite. The Cl and Br MDLs in apatite were 180 and 138 ppm, respectively. Because it is relatively fast and inexpensive, IC may be of future interest in the chemical analysis of halogens in natural and synthetic apatites.