K2CaSi4O10: A novel phase in the ternary system K2O–CaO–SiO2 and member of the litidionite group of crystal structures

The main goal of this work was to verify whether a phase with composition K₂CaSi₄O₁₀ exists in the ternary system K₂O‐CaO‐SiO₂. Therefore, a series of solid‐state reactions of stoichiometric mixtures of K₂CO₃, CaCO₃ and SiO₂ was performed at 800 and 900?C which, indeed, resulted in the formation of this previously unknown potassium calcium silicate. More detailed characterizations of this compound were based on single‐crystal X‐ray diffraction experiments. Basic crystallographic data are as follows: triclinic symmetry, space group P‐1, a = 7.0915(7) Å, b = 8.4211(9) Å, c = 10.2779(12) Å, α = 104.491(10)°, β = 100.570(9)°, γ = 113.738(9)°, V = 515.26(10) ų, Z = 2. Structure solution was performed by direct methods. Subsequent refinement calculations using anisotropic displacement parameters for all atoms converged to a residual of R(|F|) = 0.0355 for 1889 independent reflections with I > 2σ(I). From a structural point of view K₂CaSi₄O₁₀ belongs to the so‐called litidionite family of A′AMSi₄O₁₀ compounds for which several natural and synthetic representatives have been described in the literature. Actually, it is the first member where the A′‐ and A‐positions are exclusively occupied by K‐ions. Following the nomenclature for oxosilicates K₂CaSi₄O₁₀ can be allocated to the group of the tubular chain silicates. Fundamental building units are loop‐branched dreier double chains (running parallel to 100]) which can be described using the following structural formula: {lB,}³Si₈O₂₀]. Ca‐ions are coordinated by 5 nearest oxygen neighbors in form of distorted trigonal bipyramids. By sharing a common edge two adjacent bipyramids are linked into Ca₂O₈]‐dimers providing linkage between consecutive tubes in the direction of the c‐axis. Charge compensation is achieved by the incorporation of the larger potassium ions into cavities of the heteropolyhedral network. Powder X‐ray diffraction patterns of the bulk material of the synthesis products revealed that, additionally to K₂CaSi₄O₁₀, the 800°C ‐sample contained K₈CaSi₁₀O₂₅ and at least one further, yet unknown crystalline phase. This unidentified so‐called 22‐Å compound was also present in the 900 °C‐specimen together with K₂CaSi₄O₁₀ and K₂Ca₄Si₈O₂₁. Our proof of existence of K₂CaSi₄O₁₀ is a further step towards a better understanding of the ternary system K₂O‐CaO‐SiO₂ and provides a basis for identification and quantification of this compound in phase analysis. It corrects earlier phase‐analytical studies on this system which is of relevance for applied and technical mineralogy including different types of residual materials such as slags or ashes from biomass combustion. The results of our investigation show that even comparatively simple ternary oxide systems are not as well understood as expected.