The crystal structure and refined formula of mountainite, KNa2Ca2[Si8O19(OH)] · 6 H2O

Abstract

AbstractMountainite was described as a new mineral in 1957 with formula (Ca,Na2,K2)16Si32O80· 24 H2O; 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,P2/c,a= 13.704(2),b= 6.5760(10),c= 13.751(2) Å,β= 105.752(10)°,V= 1192.7(3) Å3,Z= 2,Dcalc= 2.28 g/cm3. The crystal structure was solved by direct methods and refined toR(F) = 0.0639 for 1186 unique reflections withI> 3σ(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 aTOTblock formed by two SiO-layers [Si8O18(O,OH)2] (T-layers) and zig-zag columns of edge-sharing CaO5(H2O) 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 neighboringTOTblocks is filled by Na cations and H2O molecules. The crystal-chemical formula is: KNa2Ca2{Si8O18[O(OH)]} · 6 H2O, the simplified formula is: KNa2Ca2Si8O19(OH) · 6 H2O.