Single-crystal growth experiments in the system Na2O–K2O–CaO–SiO2 resulted in the formation of two previously unknown K–Na–Ca-silicates with the following chemical compositions as determined from electron probe micro-analysis: phase I, Na1.54(3)K0.50(3)Ca6.01(2)Si3.98(1)O15 (idealized: Na1.5K0.5Ca6Si4O15 or Na1.5K0.5Ca6[SiO4]2[Si2O7]) and phase II, Na1.41(1)K0.19(1)Ca2.20(1)Si1.993(5)O7. The low diffraction quality of the single crystals of phase II allowed only a determination of the monoclinic unit-cell parameters: a = 10.45 Å, b = 10.42 Å, c = 12.24 Å, β = 91.5° and V = 1333.8 Å3. Diffraction data collected on very small single-crystalline fragments of phase I at the X06DA beamline of the Swiss Light Source, Paul Scherrer Institute, Villigen, Switzerland yielded for this compound : monoclinic symmetry, space group Pc, a = 9.0150(17) Å, b = 7.3350(10) Å, c = 11.0050(18) Å, β = 107.815(11)°, V = 692.8(2) Å3, Z = 2. Structure determination was based on direct methods. Subsequent least-squares refinements converged at R(|F|) = 0.033 for 2520 reflections and 249 parameters. Phase I is a mixed-anion silicate and contains insular [SiO4] tetrahedra as well as [Si2O7] dimers in the ratio 2:1. The cation distribution of the sodium, potassium and calcium ions among the eight different non-tetrahedral M-sites has been studied by simultaneously minimizing the differences between observed and calculated total site scattering values (as determined from single-crystal structure analysis), bond valence sums and bond lengths. Chemical composition was used as an additional constraint during the optimization process. The crystal structure can be also described as a mixed tetrahedral-octahedral network based on [SiO4] and [CaO6] polyhedra containing cavities where the remaining cations are located for charge compensation. Phase I is isostructural to β-Na2Ca6Si4O15, a high-temperature polymorph of this sodium calcium silicate. A detailed analysis of the similarities between the two compounds has been performed. Finally, a comparison between the powder diffraction patterns of different A2Ca6Si4O15 compounds is presented that may be helpful for the identification of these phases in residuals of biomass combustion.

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