Tuhualite Visible Spectrum (300 - 2500 nm)
Tuhualite, (Na,K)2Fe22+Fe23+Si12O30, is a rare silicate mineral found at Mayor Island, New
Zealand.
Image of the tuhualite absorption spectrum
- GRR
388 spectrum from Opo Bay, Mayor Island, North Island, New
Zealand 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 a published formula, our work indicates that
tuhualite is an anhydrous mineral .
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