The crystal structure of tokkoite and its relation to the structure of tinaksite

Abstract

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 σ_I) 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.