Single-crystal, polarized-light, FTIR spectroscopy of arrojadite

Arrojadites are extremely complex phosphates which are typically found in granitic pegmatites or hydrothermal veins. Their occurrence in metamorphic rocks [1] suggests wide conditions of formation for these minerals. The crystal structure and crystal chemistry of arrojadite have been debated for several decades, until [2] and [3] re-examined a set of samples from various occurrences and revised the structural model, the formula and the nomenclature of the group. One key aspect in the new crystal-structural model for arrojadite is the presence of three OH-groups [2]: two of these (O-H1 and O-H2) are very similar in local octahedral environment, crystallographic orientation and hydrogen-bond system. In contrast, O-H3 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. The Raman spectra reported in [2] show two higher-frequency, intense and convoluted bands which were assigned to specific local environments of OH1 and OH2; an additional low-frequency, broad and weak feature in the spectra (Fig. 1, left) could not be assigned with certainty. The FTIR spectra collected in the course of present work on various samples studied in [2], are similar to the Raman spectra in the higher-frequency 3600-3500 cm-1 range, with two intense and multi-component bands. In contrast, in the lower-frequency region, FTIR shows a very intense and broad absorption extending from 3500 to 2900 cm-1 (Fig. 1, right).The FTIR spectrum of arrojadite from Rapid Creek (Yukon) shows also an intense doublet at 3190-3087 cm-1 (Fig. 1 right, top) which can be assigned to NH4+ groups, as confirmed by EMPA only in this sample. 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]. For the arrojadite studied here, the data are consistent with assignment of the higher-frequency components to the O-H1 and O-H2 groups obtained from the Xray structure refinements even in the case of partial occupancy, while the assignment of the broad band to the H-O3 group proposed by [2] needs further work. No evidence of molecular water is present in the NIR 4000-6000 cm-1 region. [1] Demartin, F. et al. (1996) Can. Mineral., 34, 827-834. [2] Cámara, F. et al. (2006) Am. Mineral., 91, 1249-1259. [3] Chopin, C. et al (2006) Am. Mineral., 91, 1260-1270. [4] Libowitzky, E. & Rossman, G.R. (1996) Phys. Chem. Miner., 23, 319-327.