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

Abstract

AbstractThe 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) Å3, 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.