Structure Refinement and Crystal Chemistry of Tokkoite and Tinaksite from the Murun Massif (Russia)

The structures of tokkoite, K2Ca4 [Si7O18 OH](OH, F) and tinaksite, K2Ca2NaTi[Si7O18OH]O from the Murun massif (Russia) were refined from single-crystal X-ray diffraction data in the triclinic space group P (1) over bar Average crystallographic data are a approximate to 10.423, b approximate to 12.477, c approximate to 7.112 angstrom, alpha approximate to 89.92 degrees, beta approximate to 99.68 degrees, gamma approximate to 92.97 degrees, V approximate to 910.5 angstrom(3) for tokkoite; a approximate to 10.373, b approximate to 12.176, c approximate to 7.057 angstrom, alpha approximate to 90.82 degrees, beta approximate to 99.22 degrees, gamma approximate to 92.80 degrees, V approximate to 878.5 angstrom(3) for tinaksite. The substantial similarities between the geometrical parameters of the tokkoite and tinaksite structures led us to conclude that the two minerals are isostructural. However, major differences of tokkoite with respect to tinaksite are larger lattice constants, especially concerning the b parameter, longer < M-O > distances, especially < M1-O >; larger values of the M1-M3 and O20-O2 bond lengths, and a stronger distortion of the M1 polyhedron. Mossbauer analysis showed that significant trivalent iron is present, Fe-VI(3+) 40.0(7)% in tokkoite and 12.8(3)% in tinaksite. It is confirmed that 2Ca(2+) ((M1+M2)) + (F, OH)(-)((O20)) <-> Ti-(M1)(4+) + Na+ ((M2)) + O-(O20)(-) is the exchange reaction that describes the relation between tokkoite and tinaksite. In addition, this exchange reaction causes local stress involving mainly the M1 site and its interaction with the M2 and M3 sites.