Abstract
Mountainite was described as a new mineral in 1957 with formula (Ca,Na~2~,K~2~)~16~Si~32~O~80~· 24 H~2~O; its crystal structure was not solved up to now. We studied the structure of mountainite from the Yubileinaya pegmatite, Lovozero alkaline complex, Kola Peninsula, Russia. Mountainite is monoclinic,P__2/c,a= 13.704(2),b= 6.5760(10),c= 13.751(2) Å,β= 105.752(10)°,V= 1192.7(3) Å^3^,Z= 2,D~calc~= 2.28 g/cm^3^. The crystal structure was solved by direct methods and refined to__R(F) = 0.0639 for 1186 unique reflections with__I__> 3__σ__(I). Rietveld refinement on powder data completely confirmed the model obtained using a single crystal. Mountainite is a phyllosilicate, representative of a new structure type. The most specific feature of the mountainite structure is a__TOT__block formed by two SiO-layers [Si~8~O~18~(O,OH)~2~] (T-layers) and zig-zag columns of edge-sharing CaO~5~(H~2~O) octahedra sandwiched between them (O-layer). K cations occupy 10-fold polyhedra and are located between the columns of Ca-centered octahedra. The interlayer space between the neighboring__TOT__blocks is filled by Na cations and H~2~O molecules. The crystal-chemical formula is: KNa~2~Ca~2~{Si~8~O~18~[O(OH)]} · 6 H~2~O, the simplified formula is: KNa~2~Ca~2~Si~8~O~19~(OH) · 6 H~2~O.