Crystal chemical and structural characterization of an unusual CO3-bearing sodalite-group mineral

We report the crystal chemical and structural characterization of an unusual CO3-bearing sodalite-group mineral. The sample was found in metasomatic ejecta from Mount Vesuvio, Italy. The mineral is characterised by an a cell parameter of 9.0352(2) Å and by a formula, recalculated from EMPA, of (Na5.28K1.40Ca1.23Fe0.01)Σ7.92[Si5.93Al6.07O24.01][(SO4)0.77Cl0.94 (CO3)0.30]Σ2.01. The structure refinement, conducted in P – 43n space group, indicates that the cations are distributed among three independent sites M1, M2, and M3, each one with partial occupancy. The Cl anion is located exactly at the centre of the cages (site Cl) along with the carbon atom of the CO3 group. The sulphur atom of the sulphate group is located in a site slightly displaced from the centre of the cage, similarly to some haüyne samples (site S). The sulphate group shows two different statistical orientations with probabilities of 2/3 and 1/3. Despite the very contrasted dimensions of the different anions/anionic groups involved, the framework is fully ordered with Si-O and Al-O bond distances of 1.6156(6) and 1.7341(6) Å respectively, without splitting of the framework oxygen sites (O1 and O2). This behaviour is different from that reported for nosean, lazurite and some haüyne samples.