• journal_title：Mineralogical Magazine
• Contributor：I. V. Rozhdestvenskaya ; T. Kogure ; E. Abe ; V. A. Drits
• Publisher：Mineralogical Society of Great Britain and Ireland
• Date：2009-
• Format：text/html
• Language：en
• Identifier：10.1180/minmag.2009.073.2.883
• journal_abbrev：Mineralogical Magazine
• issn：0026-461X
• volume：73
• issue：5
• firstpage：883
• section：Letter

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 = 7.16 Å, β = 96.3° and V = 4517 ų. The space group inferred from systematic absences and HREM images is P2₁/m. 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 SiO₄ 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 H₂O molecules are probably located inside the tubular silicate radicals. From these results, a general formula is derived: K_6-7(Ca,Na)₁₈[(Si₆O₁₇)(Si₁₂O₃₀)(Si₁₈O₄₅)](OH,F)₂·nH₂O with two formula units in the unit cell (Z = 2).