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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  &gt; 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.

The crystal structure and refined formula of mountainite, KNa 2 Ca 2 [Si 8 O 19 (OH)] · 6 H 2 O