Comparative study of the HP-HT behavior of a layered silicate apophyllite in water and paraffin oil

The interaction of porous minerals with liquids, especially water, is involved in many technological and geochemical processes. Apophyllite (K,Na)Ca₄[Si₈O₂₀](F,OH)^.8H₂O, exhibiting mixed properties of layered and framework microporous silicates, is a promising object to model the interaction of a fluid with porous material. It was studied by the in situ Raman spectroscopy at simultaneously high pressure (HP) and high temperature (HT) up to 225°C and 9 GPa in order to clarify the effect of the composition of the pressure medium and the temperature on its HP behavior. Penetrating (water) and nonpenetrating (paraffin oil) media were used. Independently of the compressing medium, at HT, the lattice vibrational modes of apophyllite demonstrate regular pressure shifts. Almost uniform shift rates imply little differentiation between the compression of silicate layers and interlayer ion-molecular assemblage. Similar HP behavior of apophyllite in penetrating and nonpenetrating media indicates no pressure-induced overhydration within 1–9 GPa. Thereby, the significance of such structural factors, as a limited H₂O mobility and relatively strong contacts between silicate layers, determining low structure tolerance towards the penetration of additional molecules, is emphasized. The elevated temperature does not affect the mobility of the interlayer H₂O molecules to allow the pressure-induced intercalation in apophyllite.