Crystal structure and composition of hiärneite, Ca₂Zr₄Mn³⁺SbTiO₁₆, and constitution of the calzirtite group

Abstract

The crystal structure of hiärneite has been refined from single-crystal X-ray diffraction data (λ = 0.71073 Å) on type material from Långban, Värmland, Sweden. The refinement converged to R 1 = 0.046 based on 1073 reflections with F 2 > 4σ(F 2 ). The tetragonal unit cell, space group I4 1 /acd, has the parameters a = 15.2344(6) Å and c = 10.0891(6) Å with Z = 8. The mineral is isostructural with calzirtite, ideally Ca 2 Zr 5 Ti 2 O 16 , with a fluorite-derivated structural topology. In hiärneite, Mn 3+ is ordered at a 4-to 8-fold coordinated site (with a distorted polyhedral coordination figure), without the atom splitting encountered at the corresponding Zr-dominated site of calzirtite. The end-member formula for hiärneite is established as Ca 2 Zr 4 Mn 3+ SbTiO 16 . The calzirtite group, with calzirtite, hiärneite and tazerhanite (cubic ZrO 2-x ), has been approved by IMA-CNMNC.