Reappraisal of the crystal structure of Tinaksite from Murun massif (Russia): cation partition andstructural disorder

Tinaksite is a widespread mineral in the rocks of charoite complex of the Murun massif (Russia). The structure of tinaksite was reported for the first time in 1971 by Petranunia [1]. Until now, it is the only one silicate known based on a hybrid anion. According to Libeau [2], the latter can be described briefly by {hB, 21 }[3Si6O17(SiO2)], which represents the joint of an unbranched dreier single chain with a loopbranched dreier single chain. In 1980 von Bissert [3] confirmed the general structure architecture of tinaksite proposed by Petranunia [1], but found a more regular tetrahedral conformation. According to von Bissert [3], tinaksite is triclinic with the following chemical composition: NaK2Ca2Ti [3Si7O19]OH. The compound here investigated has been solved in P1, a=7.0565(1) Å, b=10.3750(1) Å, c=12.1885(2) Å, 92.802(1)°, , but differently from those studied by Petranunia [1] and von Bissert [3], it shows a more complex chemistry ((Na0.855K1.993Ca2.008Ti0.792 Mg0.082Fe3+ 0.142Mn0.100Ba0.006Sr0.006Zr0.007Zn0.004Ni0.003Cr0.002) [3Si7 O19]OH) and a more interesting structural details. In this work, the cation partition, derived by means of SCXRD and EMPA measurements, is presented. The structure has been anisotropically refined to a R(F) = 0.025 (for 5238 observed reflections with Fo > 3(Fo)). The highest peak in the Fourier difference map seems due to some structural disorder inside the Ti octahedron. This disorder likely is related to the entrance of the large K in place of six-fold coordinated Na atom. The replacing of Na by K seems to change the coordination number of the polyhedron from 6 to 7, so perturbing the neighbouring Ti atom position. Another point of interest concerns the K1 and K2 potassium atom located inside the system of one-dimensional channels of the structure, formed by double chains of silicate. According to von Bissert [3], the higher K1 thermal parameters with respect to the K2 one indicates the disorder on K1 site. The structural refinement shows that in the first case K occupies at least three different positions. A charge distribution analysis, performed by means of the Chardi-It software [4], has returned an ECON number [5] of 11 and 9 for the K1 and K2 site respectively. The bond distances for the three K atoms concerning K1 show these atoms are not always bonded to the same oxygens. In fact, K can move inside a cage formed by 14 oxygen atoms with a volume of about 90 Å3. This volume results to be much larger than of one assigned to K atom.