Volatiles in arrojadite: combining single-crystal XRD and FTIR microspectroscopy

Arrojadites are complex phosphates typically found in granitic pegmatites or hydrothermal veins, although their occurrence in metamorphic rocks suggests wider conditions of formation. Re-examination by EMP, LA-ICP-MS and singlecrystal XRD analysis of a set of samples from various occurrences lead to revise the structure, the formula and the nomenclature of the group [1,2]. The correct structural model for arrojadite implies three OH-groups: two of these (W1 and W2) have similar local environment (they bridge three octahedra), crystallographic orientation and hydrogen-bond system. W3 is connected with the apical oxygen of a newly defined tetrahedron in the structure, and is involved in a bifurcated hydrogen bridge with surrounding oxygen atoms. Raman spectra reported in [1] show two higher-frequency, intense and convoluted bands which were assigned to specific local environments of W1 and W2; an additional lowfrequency, broad and weak feature in the spectra could not be assigned with certainty. We report in this work on a FTIR study of various samples studied in [1,2]. Chemical zoning of the volatile components was checked by FTIR imaging using an FPA detector fitted on a Bruker Hyperion 3000 microscope. In contrast with the Raman spectra, the FTIR patterns show a very intense and broad absorption extending from 3500 to 2900 cm-1. Single-crystal XRD has shown that F is ordered at the W1 site, and this generates a significant modification of the FTIR pattern, as observed on holotype fluoro-arrojadite-(BaFe) from Sidi Bou Kricha (Morocco). The FTIR spectrum of holotype arrojadite-(KNa) from Rapid Creek shows also an intense doublet at 3190-3087 cm-1 which can be assigned to NH4 + groups, as later confirmed by EMPA. The orientation of the absorber (i.e. the O-H bond) with respect to the crystallographic axis can be determined from polarized-light measurements along the principal optical direction [4]. In all the samples, no evidence of molecular water is present in the NIR 4000-6000 cm-1 region. [1] Cámara et al. (2006) Am. Mineral. 91, 1249-1259. [2] Chopin et al (2006) Am. Mineral. 91, 1260-1270. [3] Libowitzky & Rossman (1996) Phys. Chem. Miner. 23, 319- 327.