Supporting Information
X-ray Absorption Near Edge Structure (XANES) spectroscopy at the Fe K-edge
Fe K-edge XANES spectra from an intact needle oriented vertically to the beam plane have been obtained for different rotation angles (with 45° steps) around the longitudinal axis of the needle. The rotation angle variation does not affect the edge position of Fe K-edge XANES spectra but results in some variations in the shape of the maximum of the main peak (Fig. S1). The spectra of the different rotation angles of the vertical orientation were merged.
Fig. S1. Fe K-edge XANES spectra of a needle vertically oriented to the beam plane for five different rotation angles (45° steps) around the longitudinal axis of the needle (dashed lines), and the averaged spectrum (solid line offset for comparison).
Sb L-edge XANES spectroscopy
Sb XANES spectroscopy at the L3- and L2-edges has been applied to determine the Sb valence. Spectra were measured on an additional acicular crystal mounted on a Kapton tape. Energy was calibrated from the first inflection point in the L3 XANES spectrum of a Sn foil to 3929 eV. The Sb Lα1 and Lα2 fluorescence emission and Lβ1 fluorescence emission were used to record the L3 edge and the L2-edge spectra, respectively.
The Sb L3-edge spectrum is disturbed by the Ca K-edge as can be seen in the Sb L3 pre-edge background which is modulated by the Ca K-edge EXAFS signal (Fig S2). Therefore, the Sb L2-edge, which is similar to the shape of the Sb L3-edge, is compared to available reference L3-edge spectra of senarmontite Sb2O3 for Sb(III) and bahianite Al5Sb3O14(OH)2 for Sb(V) (Fig. 5). The sample spectrum matches in its edge position and shape with the bahianite spectrum indicating Sb(V).
Fig. S2. Sb L3- and L2-edges XANES spectra of on an additional acicular crystal. The Sb L3-edge XANES spectrum is disturbed by a Ca K-edge signal. Shift of the undisturbed Sb L2 XANES spectrum (inlet, black, solid) to the Sb L3-edge position (inlet, grey, solid) proofs their similarity and justifies the use of the Sb L2-edge spectrum instead of the disturbed Sb L3-edge spectrum.
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