EVALUATION OF NATURAL APATITES AS FTIR STANDARDS
Crystals were selected from Durango, Mexico; Eagle County, Colorado; Holly Springs, Georgia; and the Lake Baikal region, Russia (possibly near Slyudyanka). Sections were cut parallel to (001) and mapped with unpolarized radiation using a mechanical stage under an IR microscope with a 50 mm analytical area. Thickness was determined directly from each spectrum, relying on the linear relationship between area of the phosphate combination bands and thickness (Tacker et al. 2010, G.C.A, 74, Sup. 1, A1018).
Naturally occurring apatites are not suitably homogeneous for use as FTIR standards. The analyzed apatites, including Durango apatite, were found to be heterogeneous for both the OH stretching region and the carbonate ν3 domain. The Durango apatite has been used previously as an analytical standard for OH, despite an unassigned absorbance at ~3482 cm-1. Although Durango may be useful as an intralaboratory reference, its use as an interlaboratory basis for comparison is precluded by our data. Holly Springs hydroxylapatite has been well-described with crystal structural refinements. Holly Springs hydroxylapatite is also heterogeneous, and the complexity of the OH stretching region and the lack of availability limit its utility.
In the absence of naturally occurring standards, other approaches are needed. The original research on Durango apatite was conducted with a homogenized powder made from several large crystals. An alternative approach combines the mathematics and approach of Sambridge et al. (2008, Am.Min., 93, 751) and Kovacs et al. (2008, Am.Min., 93, 765) with analysis of multiple randomly oriented grains from a homogenized powder. However, this does not address the need for a standard for single crystal analysis. Another option is well-characterized synthetic standards. Given the complexity of the OH stretching and the carbonate ν3 regions, synthetic standards may be the best strategy to target specific substitutions.