Tuhualite is a rare silicate mineral found at Mayor Island, New Zealand. It is
Image of the tuhualite absorption spectrum
GRR 388 spectrum from Opo Bay, Mayor Island, North Island,
Plotted as 0.050 mm thick.
The crystal in this picture of a thin section (polarized E||b) is one of the original specimens collected by Hutton.
The purple color of this material comes from
Fe2+ - Fe3+ intervalence charge transfer.
The structure determination
indicates that the Fe2+ is in a highly distorted tetrahedral site (purple)
and the Fe3+ is in an octahedral site (green). These sites form a zig-zag chain of alternating
tetrahedra and octahedra that runs parallel to the c-axis.
In polarized light, the color of the mineral varies considerably with the direction in which it is observed.
Data Files: E|| alpha = a-axis;
E|| beta = b-axis;
E|| gamma = c-axis.
Contrary to the published formula, our work indicates that tuhualite is an anhydrous mineral.