Synthesis and crystal structure of Na₂MnSi₄O₁₀: relationship with the manaksite group

摘要

Na₂MnSi₄O₁₀ has been hydrothermally synthesized from gel kept in a Teflon-lined autoclave at 225°C for 10 days. Its P [`1]P bar{1} crystal structure has been solved from single-crystal X-ray diffraction data and anisotropically refined to R = 0.025. Being the structure isotypic with that of NaKMnSi₄O₁₀ (manaksite), it is referred to a manaksite-like cell (a = 6.946 Å, b = 7.846 Å, c = 9.974 Å, α = 106.21°, β = 98.54°, γ = 115.74°) instead than to its reduced cell (a = 6.946 Å, b = 7.846 Å, c = 9.885 Å, α = 101.94°, β = 102.02°, γ = 115.74°). The Na+ cation corresponding to K+ in manaksite is split on two positions. All known AA′MSi₄O₁₀ structures are triclinic, but show a strong monoclinic character. They are based on tubes delimited by elliptical six-membered rings of SiO₄ tetrahedra and connected, to form a heteropolyhedral framework, by dimeric units consisting of two edge-sharing tetragonal pyramids of M-coordination polyhedra. A and A′ cations are located in the silicate tubes and in channels that are delimited by four tetrahedra and two dimeric units. The bottlenecks of tubes and channels are however, too narrow to define the structure as microporous.