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 Al14.61Fe2.842+Fe1.443+Mg1.40Ti1.00Zn0.60(Ga,Mn,Na)0.11O30(OH)2. 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 Mo 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 2(Fe,Zn,Mg)2[4]Al4[6]O8 and a nolanite-like structure with composition 2(Mg,Al,Fe)[4](Al,Fe,Ti)4[6]O7(OH).

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.