The comparative compressibility of reedmergnerite, danburite and their aluminium analogues

Abstract

The compressibilities of reedmergnerite (NaBSi 3 O 8 ) and danburite (CaB 2 Si 2 O 8 ) have been determined between 0 GPa and 5 GPa by single crystal X-ray diffraction, using a Merrill-Bassett diamond anvil cell. Further compressibility studies on anorthite (CaAl 2 Si 2 O 8 ) between 0 GPa and 2.55 GPa and the use of previously published data for albite (NaAlSi 3 O 8 ) enabled the Murnaghan equation of state to be fitted to the volume, pressure data and a bulk modulus obtained for each crystal structure. For a straight comparison of unit cell volume changes with pressure it was necessary to fix K' at 4. For sections involving the change of bulk modulus with increasing Al/Si disorder K' was fixed at four separate values to confirm that the trends observed were real. Comparison of changes in unit cell volume with pressure allows a number of conclusions to be made : The extra framework cation M″ plays a major role in determining structure compressibility, substitution of B for Al results in a increase in bulk modulus, and increasing Al/Si disorder leads to a decrease in the bulk modulus of the anorthite structure. The comparison of strain ellipsoids for reedmergnerite and albite confirm that the major structural changes with increasing pressure are driven by changes in T-O-T bond angles