Colors in minerals caused by Intervalence Charge Transfer (IVCT)
What is Intervalence Charge Transfer (IVCT):
A simplified view of the intervalence interaction is that of a transfer
of an electron between two adjacent metal ions. Consider the case
of a mineral structure with Fe2+ and Fe3+
adjacent cation sites.
Under the stimulation of light, the following reaction can occur:
Fe2+ (in site 1) + Fe3+ (in
2) goes to Fe3+ (in site
1) + Fe2+ (in site 2)
1) Interactions between ions of a single metallic element in different
Fe2+ - Fe3+ interactions are the kind most
found in terrestrial minerals. They are a major cause of color in a
of common and uncomon minerals. The Fe2+ - Fe3+
typically results in absorption of light in the red portion of the
Consequently, most minerals with this interaction are either blue or
when thin. Often, when thick, these minerals appear black. The iron
is an example of a mineral which, by stoichiometry, contains both
Fe2+ and Fe3+. In thin
section, the dark green color caused by the IVCT interaction
is apparent when the direction of the linerally polarized light is in
the direction of the chains of Fe atoms.
IVCT interactions are commonly strongly polarized (oriented in the
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
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,
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
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.
2) Interactions between ions of a different metallic elements.
The Fe2+ - Ti4+ interaction is commonly observed
in dravite, a member of the tourmaline
group, and produces a brown color when present at moderate
In the clinopyroxenes such as the ADOR
meteorite (50K) Fe2+ - Ti4+ IVCT produces
this brownish-red color.
Another example of the Fe-Ti IVCT interaction is found in neptunite
which is black when thick, but has a red-orange color in thin sections.
In all of these examples, light in the blue end of the spectrum is
preferentially absorbed with a resultant emphasis on the yellow, orange
and red portions of the spectrum. Fe-Ti IVCT is a common feature of
many common minerals such as micas, amphiboles and tourmalines and is
often a component of the color of these minerals.
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
Fe2+ - Fe3+ and Fe2+ - Ti4+
IVCT interactions occur.
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