A structural model for charoite

摘要

The crystal structure of charoite was investigated mainly by using selected-area electron diffraction (SAED), X-ray diffraction (XRD) and high-resolution electron microscopy (HREM). SAED and XRD patterns indicate that the structure has a monoclinic cell: a = 32.296, b = 19.651, c = 1.16 A, p = 96.3° and V= 4517 A~3. The space group inferred from systematic absences and HREM images is P2ilm. A model of the charoite structure is proposed that is based on the features of related Ca-alkaline silicate structures and HREM images. The structure of charoite consists of three different silicon-oxygen radicals (polymerized SiO_4 tetrahedra) which are located between Ca polyhedra. Two of these radicals form continuous tubular structures comprising pectolite-like tetrahedral chains. Calcium polyhedra are joined to form blocks, each of which consists of four columns sharing edges and apices. Potassium and H_2O molecules are probably located inside the tubular silicate radicals. From these results, a general formula is derived: K_(6-7)(Ca,Na)_(18)[(Si_6O_(17))(Si_(12)O_(30))(Si_(18)O_(45))](OH,F)_2-nH_2O with two formula units in the unit cell (Z = 2).