Molecular Hydrogen in Natural Mayenite

Abstract

In the last 15 years, zeolite-like mayenite, Ca12Al14O33, 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: {Ca12Al14O32}2+(O2) → {Ca12Al14O32}2+(e−)2. 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 (Ca12.09Na0.03)∑12.12(Al13.67Si0.12Fe3+0.07Ti4+0.01)∑12.87O31.96 [F2.02Cl0.02(H2O)3.22(H2S)0.15□0.59]∑6.00, show the presence of an unusual band at 4038 cm−1, 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−1 corresponding to stretching vibrations of H2 is at lower frequencies in comparison with positions of analogous bands of gaseous H2 (4156 cm−1) and H2 adsorbed at active cation sites of zeolites (4050–4100 cm−1). This points out relatively strong linking of molecular hydrogen with the fluorkyuygenite framework. An appearance of H2 in the fluorkyuyginite with ideal formula Ca12Al14O32[(H2O)4F2], which formed after fluormayenite, Ca12Al14O32[□4F2], 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.