Composition, Structure, and Conditions of Formation of Fluorine-Bearing Sodalite: Experimental Evidence

Fluoro-sodalite was synthesized for the first time at temperatures of 400–800°C and H 2 O pressures of 1–2 kbar in the Si–Al–Na–H–O–F system. X-ray diffraction and infrared spectroscopic investigations showed that fluorine is incorporated in the sodalite structure as anionic octahedral groups, [AlF 6 ] 3– , the number of which can vary from 0 to 1. Correspondingly, the end-members of the F-sodalite series are Na 7 (H 2 O) 8 [Si 5 Al 7 O 24 ] and Na 8 (AlF 6 )(H 2 O) 4 [Si 7 Al 5 O 24 ]. Depending on the composition of the system, F-sodalite associates at 500–650°C with nepheline, albite, cryolite, and villiaumite, which are joined by analcime below 500°C and aluminosilicate melt above 650°C. Fluorine-bearing sulfate–chlorine-sodalite was found for the first time in a pegmatite sample from the Lovozero massif. The highest fraction of the fluorine end-member in natural sodalite is 0.2. The incorporation of F into the sodalite structure requires much more energy compared with Cl – and SO 4 2- , because it is accompanied by a structural rearrangement and a transition from tetrahedral Al to octahedral Al.