Improvement of the crystal structural model of tinaksite

Tinaksite from ultralkaline agpaitic Murun massif (Russia) has been investigated. The name reflects its composition: titanium (Ti), sodium (Na), potassium (K) and silicon (Si). Its structure was reported for the first time in 1971 by Petrunina [1]. To the best of author's knowledge, tinaksite is the only silicate based on {hB, 21 }[3Si6O17(SiO2)] hybrid anion [2]. The structural model of Tinaksite proposed by Petrunina [1] was validated and improved by von Bissert [3] in 1980, who observed a more regular tetrahedra conformation. The tinaksite investigated by von Bissert [3] is triclinic with chemical composition: NaK2Ca2Ti[3Si7O19]OH. The compound here investigated has chemistry more complex, ((Na0.855K1.993Ca 2.008Ti0.792Mg0.083Fe3+0.096Mn0.100Zr0.027Sr0.013Zr0.027Zn0.008Ni0.007Cr0.005)[3Si7O19]OH), compared with Petrunina's [1] and von Bissert's [3] ones. It has been refined in P1, using the following lattice parameters: a = 7.0565(1) Å, b = 10.3750(1) Å, c = 12.1885(2) Å, = 92.802(1)°, = 90.763(1)°, = 99.241(1)°. In this work, the cation partition derived from SCXRD and EPMA data, is presented. The structure has been anisotropically refined up to an R(F) = 0.025 (for 5238 observed reflections with Fo > (Fo)). Tinaksite exhibits straight channels parallel to [001] direction, formed by double chains of silicate. The channels are stretched approximately along [110] direction, with maximum dimension 10.972(2) Å. The highest peak in the Fourier difference map is related to some structural disorder inside the Ti octahedron. This disorder likely can be ascribed to the entrance of the large K substituting the six-fold coordinated Na atom. As a matter of fact, the replacing of Na for K changes the coordination number of the polyhedron from 6 to 7, which seems to influence the neighbouring Ti atom position. Another point concerns the K1 and K2 atoms located inside the channels. According to von Bissert [3], the higher K1 thermal parameter with respect to the K2 one indicates the site filled by K1 is disorder. The structural refinement shows that K1 atom is located over 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, respectively. The bond distances for the three K atoms, related to K1, show these atoms are not always bonded to the same oxygens. Indeed, K1 can move inside a cage formed by 14 oxygen atoms with a volume of about 90 Å3. This volume results to be much larger even than the one of the ideal twelve-fold coordinated K atom (~18 Å3)