113-7 Towards a Quantitative Calibration of Hydroxyl Content in Apatite by Raman Spectroscopy

Session: Mineralogical Characterization of Economic Resources: From Critical Minerals to Gemstones (Posters)

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Abstract:

The hydroxyl (OH) content of apatite [Ca₅(PO₄)₃(F, OH, Cl)] is of potential value for mineral thermobarometry and for assessing volatile contents of magmas, but is notoriously difficult to measure. Measurements rely on either ion probe analysis or by determining F and Cl contents by EPMA and subtracting their molar fractions from one. Here, we explore the potential to use Raman spectroscopy to quantify OH contents in situ at the micron scale in apatite. For this project we gathered apatites of varying chemistry, including Fluorapatite (Durango), Hydroxylapatite (Holly Springs, Sapo), “Chlorapatite” (Dashkezan), and apatites of mixed chemistry (Ontario, Skardu). Each apatite was ground coarsely (≥100 µm) and grains were mounted in 1” epoxy rounds to randomize orientation. Each grain was analyzed on a Horiba LabRAM Raman microscope with an 1800 lines/mm grating, using either a 442 (blue) nm or 633nm (red) laser, ranging in power from 25-100% depending on fluorescence and peak intensities. Each spectrum had three acquisitions with a duration of five seconds across a range from 400-4000 cm^-1 resulting in a total acquisition time between five and ten minutes depending on laser and power. Fluorescence is a chronic problem that cannot be completely mitigated for many apatites, nonetheless two robust results emerge from our data. First, orientation affects the intensity of the OH band and consequently the peak height ratio of the OH band relative to the v1 PO₄ band. The OH peak intensity and OH/v1 PO₄ are highest when the beam is parallel to the c-axis and are lowest when the beam is perpendicular to the c-axis. Second (fortunately), crystal orientation also affects the v3/v1 PO₄ peak intensity ratios and splitting of the v2 (c. 450 cm^-1), v4 (c. 600 cm^-1), and v3 (c. 1050 cm^-1) PO₄ regions – greater splitting occurs when the beam is perpendicular to the c-axis. Using these characteristics (peak intensity ratios and variable peak splitting) a calibration should be possible that simultaneously corrects for crystal orientation and quantifies OH content. Further development using different wavelength lasers and a wider range of apatite chemistries seems warranted.  

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8103

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