CRYSTAL-STRUCTURE CRYSTAL-CHEMISTRY RELATIONSHIPS IN THE ZEOLITES ERIONITE AND OFFRETITE

This study clarifies the crystal structure variations and relationship s in the zeolites erionite and offretite. The crystal structure analys es used Rietveld analysis of X-ray powder diffraction data, obtained b oth by synchrotron radiation and conventional X-ray sources, and on di ffraction patterns obtained by transmission electron microscopy. The f ramework Al atoms in erionite are preferentially located in the single six-membered ring of tetrahedra (T2 site), whereas the Si-Al distribu tion is essentially disordered on the tetrahedral framework sites in o ffretite. In both zeolites, the center of the cancrinite cage is alway s occupied by K cations in similar amounts in both minerals. The erion ite cage in erionite is occupied by Ca and Na atoms distributed on fou r distinct cation sites. Mg ions can be present up to 0.8 atoms per ce ll and are located between the Ca1 and Ca3 sites and very close to the Ca2 site on the symmetry axis. In offretite, the Mg site is located o n the trigonal axis of the gmelinite cage and the Mg atoms are bonded to a variable number (five or six) of H2O molecules, depending on the site population. The structural data and TEM analysis clearly show tha t the crystal chemistry of the extraframework cations, Mg in particula r, is a major factor controlling whether erionite or offretite crystal lizes, the Si-Al distribution in the framework, and the possible stack ing intergrowths of the two minerals.