Structure of calcinaksite KNa[Ca(H 2 O)][Si 4 O l0 ], the first hydrous member of the litidionite group of silicates with [Si 8 O 20 ] 8− tubes

Calcinaksite, KNa[Ca(H 2 O)][Si 4 O l0 ], a new natural member of the litidionite group, was found in a calcic xenolith from alkaline basalt of the Bellerberg volcano, Eastern Eifel region, Rhineland–Palatinate, Germany. The crystal structure has been studied based on single‐crystal X‐ray diffraction data. Triclinic unit‐cell parameters are: a = 7.021 (2), b = 8.250 (3), c = 10.145 (2) Å, α = 102.23 (2), β = 100.34 (2), γ = 115.09 (3)°, space group . The structure model was determined by the `charge‐flipping' method and refined to R = 0.0527 in anisotropic approximation using 3057 I > 3σ( I ). Calcinaksite is a hydrous calcium‐dominant litidionite‐group mineral. The crystal structure of calcinaksite (like other litidionite‐group minerals and related compounds) is based on a heteropolyhedral framework and is characterized by the presence of several types of channels. Calcium forms distorted CaO 5 Ø (Ø = H 2 O) octahedra while Na forms NaO 5 square pyramids. Nine‐coordinated K atoms are located in a channel extending along [010]. Water molecules occupy a channel running along the [100] direction and are characterized by a rather high equivalent isotropic displacement parameter of 0.053 (2) Å 2 . In calcinaksite, there are three short distances between the water molecule and oxygen atoms, O w ...O3 [2.844 (5) Å], O w ...O9 [2.736 (4) Å] and O w ...O w [2.843 (7) Å]. These distances correspond to three hydrogen bonds detected by IR data (the bands at 3340, 3170 and 3540 cm −1 ).