Under the stimulation of light, the following reaction can occur:
IVCT interactions are commonly strongly polarized (oriented in the direction of the metal-metal axis). They cause strong pleochroism in minerals. An example is ilvaite. Two orientations of the crystal are shown in linerally polarized light. The dark, intense color is observed when the direction of the polarizer is parallel to the direction in which the electron is transfered between the two metal ions in the crystal.
One of the classic examples is the mineral vivianite shown here in thin section. This mineral is pale green (colorless when thin) but turns vivid blue upon partial oxidation. The blue color results from Fe2+ - Fe3+ IVCT
Here is an interesting example of synthetic acmite with sector zonation caused by IVCT. Acmite is pale green color due to Fe3+. In one zone of the monoclinic crystal, there is a minor amount of Fe2+ which generates the The Fe2+ - Fe3+IVCT and causes the much darker green color.
Ti3+ - Ti4+ interactions are found in
meteoritic
minerals. A example of the blue color caused in some minerals by this
interaction is the color
of the fine-grained, titanium-containing, calcium aluminum oxide
mineral, hibonite, in an inclusion known as the Blue Angel in the Murchison metorite. This mineral absorbs red
light but lets blue light pass. The optical
spectrum of this mineral has a band with a maximum absorption hear
690 nm. Better examples include blue hibonite in the Murray meteorite and the
hibonite-bearing inclusions in the Vigarano
CV3 chondrite meteorite.
Mn2+ - Ti4+ interactions are rarely observed, but have been documented in tourmaline. The band of pale yellow color in elbaite (28K) from Nepal arises from the the Mn2+ - Ti4+ interaction. The light and dark brown zones are from the Fe2+ - Ti4+ interaction.
In some minerals such as kyanite (21 K) sapphire (70 K) , both the Fe2+ - Fe3+ and Fe2+ - Ti4+ IVCT interactions occur.
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