Tuhualite Visible Spectrum (300 - 2500 nm)
Tuhualite, (Na,K)2Fe22+Fe23+Si12O30, is a rare silicate mineral found at Mayor Island, New
Image of the tuhualite absorption spectrum
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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