Crystal structures of shlykovite and cryptophyllite: comparative crystal chemistry of phyllosilicate minerals of the mountainite family

Abstract

We solved the crystal structures of the phyllosilicates shlykovite, KCaSi 4O 9(OH)·3H 2O, and cryptophyllite, K 2CaSi 4O 10 ·5H 2O, two minerals recently discovered in the Khibiny alkaline complex (Kola Peninsula, Russia). Both minerals are monoclinic with close values of the a, b and s parameters but significantly different c parameters: shlykovite a -6.4897(4), b = 6.9969(5), c = 26.714(2) A, β = 94.597(8)°, V = 1209.12(15) A 3, sp. gr. P2 1/c, Z = 4; cryptophyllite a = 6.4934(14), b = 6.9919(5), c = 32.087(3) A, β = 94.680(12)°, V= 1451.9(4) A 3, sp. gr. P2 1/n, Z = 4. The structures were solved from synchrotron diffraction data collected on the same intergrown crystal and refined anisotropically to R(F) = 0.0960 for 1147 unique reflections with I > 2σ(I) (shlykovite) and R(F) = 0.0856 for 1667 unique reflections with I > 2σ(I) (cryptophyllite). Shlykovite and cryptophyllite are representatives of two new, closely related structure types. The main structural units of both minerals are TOT blocks consisting of tetrahedral Si layers (T) and an octahedral component (O), sandwiched between them. Each T-layer consists of 4- and 8-membered rings of SiO 4-tetrahedra and can be considered as a half of a double layer described in the structures of the members of the rhodesite mero-plesiotype series. A topologically closely related Si layer also forming T-fragments of TOT blocks was recently discovered in mountainite, KNa 2Ca 2[Si 8O 19(OH)]·OH 2O. The O-fragment of the TOT blocks in shlykovite and cryptophyllite is formed by columns of edge-sharing Ca-centred octahedra CaO 5(H 2O). In both minerals K cations are located in the voids of the Si layer. The content of the interlayer space is different in each mineral: only H 2O molecules in shlykovite, K atoms and H 2O molecules in cryptophyllite. Mountainite, shlykovite and cryptophyllite are closely related in configuration of the TOT blocks, their main structure unit, and could be used to define the mountainite structural family. ©2010 E. Schweizerbart'sehe Verlagsbuchhandlung, D-70176 Stuttgart.