Structure of calcinaksite KNa[Ca(H2O)][Si4Ol0], the first hydrous member of the litidionite group of silicates with [Si8O20]8−tubes

Abstract

Calcinaksite, KNa[Ca(H2O)][Si4Ol0], 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 P \bar 1. The structure model was determined by the `charge-flipping' method and refined toR= 0.0527 in anisotropic approximation using 3057I> 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 CaO5Ø (Ø = H2O) octahedra while Na forms NaO5square 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) Å2. In calcinaksite, there are three short distances between the water molecule and oxygen atoms, Ow...O3 [2.844 (5) Å], Ow...O9 [2.736 (4) Å] and Ow...Ow[2.843 (7) Å]. These distances correspond to three hydrogen bonds detected by IR data (the bands at 3340, 3170 and 3540 cm−1).