| Summary: | In the last 15 years, zeolite-like mayenite, Ca<sub>12</sub>Al<sub>14</sub>O<sub>33</sub>, has attracted significant attention in material science for its variety of potential applications and for its simple composition. Hydrogen plays a key role in processes of electride material synthesis from pristine mayenite: {Ca<sub>12</sub>Al<sub>14</sub>O<sub>32</sub>}<sup>2+</sup>(O<sup>2</sup>) → {Ca<sub>12</sub>Al<sub>14</sub>O<sub>32</sub>}<sup>2+</sup>(e<sup>−</sup>)<sub>2</sub>. A presence of molecular hydrogen in synthetic mayenite was not confirmed by the direct methods. Spectroscopy investigations of mayenite group mineral fluorkyuygenite, with empirical formula (Ca<sub>12.09</sub>Na<sub>0.03</sub>)<sub>∑12.12</sub>(Al<sub>13.67</sub>Si<sub>0.12</sub>Fe<sup>3+</sup><sub>0.07</sub>Ti<sup>4+</sup><sub>0.01</sub>)<sub>∑12.87</sub>O<sub>31.96</sub> [F<sub>2.02</sub>Cl<sub>0.02</sub>(H<sub>2</sub>O)<sub>3.22</sub>(H<sub>2</sub>S)<sub>0.15</sub>□<sub>0.59</sub>]<sub>∑6.00</sub>, show the presence of an unusual band at 4038 cm<sup>−1</sup>, registered for the first time and related to molecular hydrogen, apart from usual bands responding to vibrations of mayenite framework. The band at 4038 cm<sup>−1</sup> corresponding to stretching vibrations of H<sub>2</sub> is at lower frequencies in comparison with positions of analogous bands of gaseous H<sub>2</sub> (4156 cm<sup>−1</sup>) and H<sub>2</sub> adsorbed at active cation sites of zeolites (4050–4100 cm<sup>−1</sup>). This points out relatively strong linking of molecular hydrogen with the fluorkyuygenite framework. An appearance of H<sub>2</sub> in the fluorkyuyginite with ideal formula Ca<sub>12</sub>Al<sub>14</sub>O<sub>32</sub>[(H<sub>2</sub>O)<sub>4</sub>F<sub>2</sub>], which formed after fluormayenite, Ca<sub>12</sub>Al<sub>14</sub>O<sub>32</sub>[□<sub>4</sub>F<sub>2</sub>], is connected with its genesis. Fluorkyuygenite was detected in gehlenite fragments within brecciaed pyrometamorphic rock (Hatrurim Basin, Negev Desert, Israel), which contains reduced mineral assemblage of the Fe-P-C system (native iron, schreibersite, barringerite, murashkoite, and cohenite). The origin of phosphide-bearing associations is connected with the effect of highly reduced gases on earlier formed pyrometamorphic rocks.
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