Sulfur K X-ray absorption near edge structure spectroscopy on the photochrome sodalite variety hackmanite

Abstract

Hackmanite (Na₈[AlSiO₄]₆Cl_2–y–z(S_x^2–)_y□_z) is a rare photochrome variety of sodalite (Na₈Al₆Si₆O₂₄Cl₂) that shows orange colored photoluminescence in UV light, exhibits a purple-red color subsequently to UV irradiation or after cracking, bleaches within minutes to hours in visible light, and usually recovers color in darkness. Part of the Cl^– anion is replaced by S in some hackmanites described in the literature and in all of our samples from Afghanistan, Canada, and Russia. Up to about 10% of Cl is replaced by S for the Russian hackmanites. Sulfur has been identified as sulfate and as reduced sulfur (most probably polysulfide radical ions) from S K X-ray absorption near edge structure (XANES) spectroscopy in hackmanites from Afghanistan and Canada. The Russian hackmanites from different sites of Lovozero massif (Kola peninsula) are free or almost free of sulfate. The spectral features of reduced sulfur in hackmanite significantly differ from that of other reduced sulfur containing minerals and compounds with sodalite structure like lazurite and ultramarine by a sharp pre-edge peak at 2465.2 eV and a shift of XANES features (singlet and triplet) by 2.52 eV towards higher energies. The pre-edge peak is related to sulfides donating electrons, triggered by UV radiation, to vacant sodalite cages creating sulfide radicals and color centers (F-centers). Its increase during X-ray irradiation favors radicals involved in the process in the way that X-rays are both, activator and probe of the partly- or un-occupied electronic states of the radicals. Prolongated exposure to X-rays changes the electronic configuration of hackmanite significantly indicated by two new pre-edge peaks.