CO₂ distribution in CO₂–rich melanophlogite from Fortunillo, Tuscany, Italy

Abstract

CO₂ distribution in M¹² and M¹⁴ cages of CO₂–rich melanophlogite from Fortunillo, Tuscany, Italy was studied using synchrotron powder X–ray diffraction. Original and two heat–treated samples at 500 and 1000 °C were studied at room temperature. The diffraction patterns of these samples can be indexed as a cubic cell (Pm3n). For the non–heated sample, CO₂ occupancy for M¹⁴ cage is close to unity, whereas about 0.85 for M¹² cage. For the 500 °C–heated sample, the occupancies for M¹⁴ and M¹² cages are reduced to 0.79 and 0.57, respectively. Present study showed that CO₂ has preference to M¹⁴ cage, but substantial CO₂ occupies M¹² cage. The electron distributions obtained by MEM analysis clearly reveal orientationally ordered CO₂ distribution in M¹² and M¹⁴ cages. Present result is also used to clarify a recently proposed interpretation for the splitting of CO₂ Raman vibrational peak for the heat–treated melanophlogite.