Structural disorder in tokkoite from Murun massif (Russia)

Tokkiote is a new mineral discovered in 1986 by Lazebnik et al. [1] on Murun massif (Russia). It belongs to group of alkali-calcium silicates containing hydroxyl and fluorine anions. The SCXRD investigation on this mineral dates back to 1989 [2]. Tokkoite is triclinic with general formula K2a4(Si7O18)(OH,F)2 [2] and is a Ca-bearing compound isostructural to tinaksite [3]; both minerals contain the rare [Si7O18(OH)]9- silicate radical. In this work a re-appraisal of tokkoite structure was undertaken and a cation partition, obtained by means of SCXRD and EPMA measurements, is derived. The structure has been refined in space group P1 (R1 = 4.45%), using the following lattice parameters: a = 10.4222(5) Å, b = 12.5023(6) Å, c = 7.1146(3) Å, = 89.904(2)°, = 99.714(2)°, = 92.979(2)°. The structure consists of layers of calcium octahedra interconnected along (100), between which the silicate-oxygen chains are located. K atoms are located inside the curved shape channels formed by silicon chains. Structural refinement showed that Si-tetrahedra are slightly distorted, but more regular if compared to [2]. Ca(1), Ca(3) and Ca(4) are octahedrally coordinated and appear more distorted than [2], with = 2.199, = 2.368, = 2.371, while in Ca(2) polyhedron Ca-ion seems to be bond to seven oxygen with = 2.860 Å. The highest peak (3.56 e/Å3) in the Fourier difference map is related to structural disorder at the Ca(1) octahedron, which could be due to a changing of the coordination number from 6 to 7 of Ca(2) polyhedron, which shares two edges with Ca(1) octahedron. According to [2], one of two potassium atoms in the unit cell is weakly linked and evidences some positional disorder not discussed by [2]. Structural refinement shows that K1 atom is distributed among at least three different positions having occupancy 0.658, 0.211 and 0.122, respectively. Similar behaviors were been observed also in tinaksite structure [4]