Summary: Three new analyses of kalsilite are given, with optical and X-ray data; two (K/(K + Na) = 0·95. 0·98) are from rocks where it is closely associated with nepheline. An analysis of one of the nephelines is also given (K/(K + Na) = 0·31). The cell dimensions vary appreciably with sodium content, and the composition of kalsilite may be determined from the equation: 100 K/(K + Na) = 100−227·27(Δ 6·264), where Δ is the difference, in degrees, between the diffraction angles 2ϑ for 101¯2 and 101¯1, using Cu-Kα radiation.