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)Ca-4[Si8O20](F,OH)(.)8H(2)O, 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 degrees 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.

Automated summary

Porous mineral interaction with liquids, especially water, is crucial in many technological and geochemical processes. The behavior of apophyllite, a mixed layered and framework microporous silicate, was studied under high pressure and high temperature conditions using in situ Raman spectroscopy. Results showed that the composition of the pressure medium and temperature influenced its behavior. Regardless of the compressing medium, the lattice vibrational modes of apophyllite showed consistent pressure shifts at high temperature. The behavior of apophyllite was similar in both penetrating and nonpenetrating media, indicating no pressure-induced overhydration. These findings emphasize the importance of structural factors such as limited water mobility and strong contacts between silicate layers in determining the tolerance towards the penetration of additional molecules. Elevated temperature did not affect the mobility of interlayer water molecules for pressure-induced intercalation in apophyllite.