Colors from metal ions in minerals
All of the examples of colored minerals on this page have
color due to metal ions. Ions of the first row transition
elements (Ti to Cu) are normally responsible for color in these
minerals. These ions have electrons in the five 3d
orbitals. In the crystallographic sites found in minerals, the 3d
orbitals split into different energies. Visible light interacts
with these electrons and causes them to be excited to higher
energy orbitals. The wavelengths that cause these transitions are
subtracted from the incident light resulting in color.
First Row Metals
- Ca2+, Sc3+, and Ti4+ by
themselves all cause no color in minerals. They have no electrons in
d-orbitals. Titanium can interact with other metal ions to cause color
as discussed in the section on intervalence charge transfer.
- Ti3+ by itself is not a factor in the coloration of
most terrestrial minerals. Through intervalence charge transfer and
band gap process it can contribute to the color of a few
extraterrestrial minerals such as hibbonite and pyroxene and the rare
blue examples of the terrestrial TiO2 minerals, anatase and
rutile. Chains of edge-sharing octahedra in Ti3+ - Ti4+-bearing
clinopyroxene from the Allende
meteorite give rise to a olive-green color. Originally it was
argued by Dowty and Clark (1973) that features from both Ti3+ and
Ti3+ - Ti4+ intervalence charge transfer
contribute to the color, although later Burns (1973) argued that only Ti3+
was responsible for the color.
- V3+ in grossular
garnet (tsavorite variety from Kenya) causes the green color. In zoisite (tanzanite variety) it
contributes to the color which varies depending upon the direction in
which you view the crystal (pleochroism).
- VO2+ causes bright blue color in a few minerals. Cavansite from India shows the typical blue color of
this ion. Synthetic
clinopyroxene grown from lithium vanadate flux can commonly
incorporate this ion and cause blue color. In apophyllite the color is more green than blue.
- Cr3+ causes red and green colors. Cr3+
causes green color in emerald, synthetic orthopyroxene
and jadeite. Red color from Cr3+ is
seen in spinel from Burma and synthetic ruby.
- Mn3+ causes red and green colors in octahedral sites. Muscovite mica from Brazil containing
is red as is Mn3+ in beryl from
orthopyroxene, and piemontite
from Whitewater, California. Andalusite containing Mn3+ is
green. In the amphibole, tremolite,
from New York, it produces a violet color.
- Mn2+ usually results in a pink color in octahedral
sites. Rhodonite from Minas Gerais, Brazil, is
a pyroxenoid containing Mn2+and has the typical pink color
of Mn2+ minerals. Rhodocrosite
from Colorado has a high concentration of Mn2+ and a bright red color. At
lower concentrations, Mn2+ causes pale pink color.
When the Mn2+ is in a tetrahedral site, then yellow-green
color results such as is the case with willemite.
- Fe2+ in forsterite
from San Carlos, Arizona, and in phosphophyllite from Bolivia is the ion responsible for
the green color. In some minerals with high concentrations of Fe2+,
such as fayalite or orthopyroxene,
the color is brown.
- Fe2+ in the square planar site of gillespite or eudialyte produces a rasberry red color.
- Fe2+ in the eight-coordinated site of pyrope garnet from Tanzania
produces the near-red color.
- Fe3+ in octahedral sites causes only pale color when
the Fe3+ ions are isolated from each other by intervening
silicate ions, etc. Pale purple color is found in phosphates such as strengite and sulfates such as coquimbite. Yellow-green can be found in ferric
silicates such as andradite
garnet from Italy.
- Fe3+ is in the tetrahedral site of plagioclase feldspar from Lake
County, Oregon, produces a pale yellow color. In an unusual variety of diopside containing Fe3+
in a tetrahedral site, it produces bright orange color in thin section.
- Co2+ in synthetic
olivine and cobaltian calcite from the
Kakanda Mine, Zaire, causes a typical reddish color. In tetrahedral
sites, Co2+ causes blue color such is found in some spinels.
- Ni2+ in synthetic
olivine has the green color typical of Ni2+ in an
octahedral site. If all the nickel is forced in to the larger M2 site
by appropriate chemical substitution (in this case in a LiScSiO4
olivine), the color is yellow, typical of Ni2+ in large,
- Cu2+ usually occupies sites distorted from octahedral
geometry. It produces blue and green color in minerals such as azurite, malachite,
aurichalcite and the blue elbaite tourmaline from Paraiba, Brazil.
Other Metal Ions
- Rare-earth elements (Ce, Pr, Nd) are occasionally factors in the
color of minerals. They have narrow lines in the absorption spectra
when they are in the normal 3+ oxidation state. Minerals with abundant
rare earths often have brown to orange-brown colors. Rare earths can be
seen in the spectra of many minerals.
The rare-earth elements have distinctive colors in the 3+
oxidation state and distinctive absorption spectra that change
little with different hosts. Here are some examples of synthetic
rare-earth garnets and phosphates.
Several of the garnets illustrated show
evidence of contamination by other rare-earths - better examples
to come in the future
- The UO22+ ion is responsible for the
brilliant yellow color of many uranium minerals such as carnotite and autinite.
- U4+ gives blue color to zircon.
Often asked questions about color in minerals
- What about lithium? It is widely believed that Li causes
various colors in minerals. Li+ does not cause color;
it has no electrons in the d-orbitals. It frequently accompanies
other ions such as Mn which do cause color.
- What about cesium? It is widely believed that Cs causes
various colors in minerals. Cs+ does not cause color;
it also has no electrons in d-orbitals. It frequently accompanies
other ions such as Mn which do cause color.
Back to the list
of causes of color
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Mineral Spectroscopy home page
last updated: 30-Apr-2006