Refined crystal structure of iron-rich triclinic astrophyllite

Crystal structure of Fe-rich triclinic astrophyllite K₂NaFe₇ [Ti₂Si₈O₂₆F](OH)₄ is refined (a Syntex P $bar 1$ automatic diffractometer, 3809 reflections, 2θ/θ scan, R = 0.041): a = 5.365(2), b = 11.88(1), c = 21.03(2) Å, α = 84.87(6)°, β = 92.25(5)°, γ = 103.01(4)°, sp. gr. A $bar 1$ , Z = 2, d _calcd = 3.29 g/cm³. The refined structure is identical to that reported earlier. The structure is built by three-layer TOT sheets in which an O layer of Fe-octahedra is sandwiched between the T layers consisting of Si-tetrahedra and Ti-octahedra. It is established that differently chosen unit cells of the mineral are interrelated.