Tokkoite, K2Ca4[Si7O18(OH)](F,OH), a new mineral from Murun massif, South Yakutia, USSR, is triclinic with unit-cell dimensions a = 1.0438(3) nm, b = 1.2511(3) nm, c = 0.7112(2) nm, α = 89.92(2)°, β = 99.75(2)°, γ = 92.89(2)°, space group P[unk], Z = 2. The crystal structure has been determined by the Patterson method and difference Fourier maps followed by a least-squares refinement to an R factor of 0.032 for 906 (I > 1.96 σI) reflections measured on an automatic single-crystal X-ray diffractometer.
The structure consists of corrugated walls of calcium octahedra parallel to (100), between which bands of silicon – oxygen tetrahedra are situated. One of the non-bridging end anions is a OH group. K atoms are located within zig-zag channels formed by curved silicate bands.
Tokkoite and tinaksite, K2Ca2NaTi [Si7O18(OH)]O, are isostructural. These two structures are related by the substitution 2Ca2+ ⇌Ti4+ + Na+ and (F,OH)− ⇌ O2− which occurs simultaneously in kationic and anionic parts of minerals. Common structural details of tokkoite, miserite and canasite are also considered.
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