E N V I R O N M E N T , E A R T H A N D P L A N E T A R Y S C I E N C E S

S C I E N T I F I C H I G H L I G H T S

1 3 2 H I G H L I G H T S 2 0 2 3 I

X-ray absorption spectroscopy provides new insight into rare earth geochemical proxies in fluorapatite

Rare earth elements such as yttrium are widely used as geochemical proxies to reconstruct conditions associated with certain geological settings. X-ray absorption spectroscopy was used to determine the speciation of yttrium in mineral fluorapatite. The results further understanding of the geochemical conditions in which these deposits are formed.

Fluorapatite (FAp, Ca5(PO4)3F) is a mineral that hosts rare earth elements (REE, including yttrium), which are widely used as geochemical proxies to reconstruct conditions such as T, pH, Eh and the composition of crystallising fluids associated to sedimentary or hydrothermal- magmatic geological settings. REE are known to integrate the FAp lattice through substitution mechanisms in the crystallographic sites of Ca: the 9-fold coordinated Ca(1) site, and the 7-fold coordinated Ca(2) site [1], involving either Na, Si, or O as coupled substitutions.

However, depending on crystal-chemistry constraints as well as external factors (P, T and fluid composition), some authors (see review article [2] and references therein) suggest that, i) light rare earth (L)REE preferentially occupy the Ca(l) site while heavy (H)REE favour the Ca(2) site, ii) LREE and HREE preferentially occupy the Ca(2) and Ca(1) site, respectively, or iii) both LREE and HREE are only present as substitution at the Ca(2) site. According to [3], REE may also be found adsorbed on FAp crystals in natural marine environments. In contrast with previous studies, this work uses a direct method to determine the speciation of yttrium, a HREE proxy, employing extended X-ray absorption fine-structure (EXAFS) spectroscopy at beamline BM23 to probe the yttrium (Y) K-edge in fluorapatites.

The speciation of yttrium was characterised in both hydrothermal-magmatic FAp (H-Fap; Durango, Mexico) and sedimentary FAp (S-FAp; Morocco). Data reduction, based on Fourier transforms, wavelet analysis and multi- shell fits (Figure 106), show that:

i) in H-FAp, Y appears mainly 7-fold coordinated, corresponding to a substitution at the Ca(2) site (i.e., Y(2), see Figure 107a), together with the following potential and compatible coupled substitutions:

Y(2)3+ + Na+ = 2Ca2+ Eq. 1 Y(2)3+ + SiO44- = Ca2+ + PO43- Eq. 2 Y(2)3+ + O2− = Ca2+ + F− Eq. 3

ii) in S-FAp, Y shows an average coordination number 7.4, with an atomic landscape consistent with two distinct models: 1) a mixture of Y(2) (i.e., Eq. 1, Eq. 2 or Eq. 3) and Y-adsorbed located at the c-axis channel (Figure 107b), or 2) Y(2) associated to carbonate groups as coupled substitution (Figure 107c): Eq. 4 Y(2)3+ + □vacancy + 2CO32- + O2− = 2Ca2+ + PO43- + F−

Fig. 106: Wavelet transform of EXAFS spectra [4] with corresponding k3-weighted spectra and Fourier transform (black solid lines). Results of multi-shell fits are shown in shifted coloured dotted lines: a) H-FAp fitted based on a Y(2) model; (b) S-FAp fitted based on a Y(2) + Y-adsorbed mixture model (blue), and a Y(2)-carbonate model (red).