Crystal chemistry and IR spectroscopy of Cl- and SO4-bearing cancrinite-like minerals

Cl- and SO4-bearing cancrinite-like minerals from Italian localities were studied by means of EPMA, X-ray diffraction, and IR spectroscopy to relate their chemical composition, IR data, and structural features. The framework of the cancrinite-like minerals consists of six-membered rings of aluminosilicate tetrahedra stacked along c. Six types of subunits resulting from different stacking sequences can be identified: free channel, cancrinite, sodalite, losod, liottite, and giuseppettite cages. The free channel is possible only with an AB sequence; in fact, the insertion of a C layer interrupts the channel forming the various cages. The occurrence of these structural subunits seems to correspond to the different chemistries of the cancrinite-like minerals, especially their anion content. According to this model, the SO4 group seems to play a major role. With the exception of giuseppettite, it tends to fill completely the available voids within the frameworks.