Abstract
Högbomite-8H from Central Australia has hexagonal symmetry, P63mc with unit-cell parameters of a = 5.734 (3)Å and c = 18.389(8)Å. Its unit-cell composition, as determined from microprobe analysis (combined with information from the structure determination), is . The correct structural model was predicted with the aid of geometric principles previously derived from a structure determination for the related mineral, nigerite-24R. The structure was refined to an R value of 0.032 (Rw = 0.031) for 1425 observed reflections, with F ≥ 6σ(F), collected using MoKα radiation. The model is based on a closest-packed anion framework with an 8-layer mixed stacking sequence along c given by ABCABACB, i.e. (ccchccch), and with cations ordered into 6 tetrahedral and 16 octahedral sites per unit cell. The structure may be described as a 1:1 intergrowth of a spinel-like structure with composition and a nolanite-like structure with composition .
