The garnets constitute a group of minerals which share a
common structure. Common end member compositions are:
- Almandine ---------- Fe2+3Al2(SiO4)3
- Andradite ----------- Ca3Fe3+2(SiO4)3
- Grossular ----------- Ca3Al2(SiO4)3
- Pyrope -------------- Mg3Al2(SiO4)3
- Spessartite ---------- Mn3Al2(SiO4)3
- Uvarovite ----------- Ca3Cr2(SiO4)3
Visible Spectra (generally 350 - 1100 nm)
As would be expected from a chemically complex group of
minerals, garnets owe their color to a variety of causes.
Absorption by Cr3+, V3+,
Fe3+
(6-coordinated site), Fe2+ (8-coordinated site),
Mn2+,
and Fe2+ - Ti4+
intervalence charge
transfer can all be found in the spectra of garnets.
Spectra of representative garnets
Colorless garnets
When devoid of transition metals such as
Mn, Fe, Cr, and V, grossular and pyrope garnets are colorless.
Dark red to pink garnets with Fe2+
Almandine GRR 892, India. 1.0 mm thick
Almandine GRR
1040, North River Garnet Company, Warren Co., NY, [Fe1.35
Mg1.30 Ca0.26 Mn0.03]
Al1.98(SiO4)3;
1.54 mm thick; data file
Almandine, GRR 1056, Wrangell, Alaska, [Fe2.03Mg0.67Ca0.18Mn0.07]Al2.0(SiO4)3,
colored light reddish-purple by Fe2+ ,
normalized to 1.0 mm thickness; data file
Almandine, GRR
1776, dark
reddish-purple gemstone, [Fe2.5Mg0.3Mn0.2Ca0.1]Al2.0(SiO4)3,
probably from India, Graphed at 1.0 mm, normalized from 0.213 mm
thickness. Visible region
only; data file
Pyrope, GRR
83, rhodolite variety, Umba River Valley,
Tanzania, colored pink by Fe2+; 0.78 mm
thick; data file
Pyrope, GRR
131, unknown locality, probably Tanzania,
colored pink by Fe2+ ;
Approximate 8-coordinated site composition: Mg1.82 Fe0.99 Ca0.04 Mn0.04. Plotted as 2.007 mm thick; data
file
Pyrope, GRR
1121i, Wesselton, South Africa, Mg2.32
Mn0.01 Fe0.43 Ca0.27 Al1.95 Cr0.02, 1.693 mm thick; data file
Pyrope, GRR
1121f, Wesselton kimberlite diamond mine, South Africa. Mg2.09 Mn0.02 Fe0.67 Ca0.26 Al1.89 Cr0.03 Ti0.03. Plotted as 0.547 mm thick; data file; data
file
Pyrope-spessartine, GRR
1267,
Kenya, sold commercially as "Malaya Garnets". 1.686 mm thick; data
file
Pyrope, Mon-9,
Monastery Mine, South Africa, 1.37 mm thick; data file . Showing the
three near-IR Fe2+ bands and weaker broad OH
bands that are visible in the 2800 nm region.
Orange garnets with Mn2+
- Spessartine, GRR
72, Rutherford Mine, VA, colored
orange by Mn2+ 3.45 mm thick; data
file
- Spessartine, GRR
43, Rutherford Mine, Amelia, VA,
colored orange by Mn2+ ,
expanded view from a
2.165 mm thick sample. data file
- Andradite-spessartine, GRR 149, Franklin, NJ, colored orange
due to the mixture of Fe, Mn and Ti. Approximate compositon: Ca2.23,
Mn0.72, Fe1.48, Al0.44, Ti0.02. Plotted as 1.0 mm thick.
Orange to black garnets with Fe2+, Fe3+
and
Ti4+
- Andradite, GRR
568, variety (black when thick) melanite ,
San Benito Mountains, California, with both Fe3+
and Ti4+, 0.102 mm thick. Approximate omposition in the 6-coordinated site: Fe1.16, Ti0.74, Al0.007. data
file
- Andradite (variety melanite), GRR
755, Santa Rita Peak,
San Benito Mountains, California. The garnet crystals are
black, but when
thinned, it becomes dark brown-orange. The sharp peak near 441
nm is
from Fe3+; approximate composition in the 6-coordinated site: Fe1.69, Ti0.27, Al0.04. 0.045 mm thick, data file
- Grossular, GRR 53, (orange-brown) variety hessonite, . Jeffrey Mine, Asbestos (now Val-des-Sources), Québec, Canada
- Grossular, GRR
1113,
(orange-brown), North Hill, Riverside, California. (Ca2.86
Fe0.12 Mn0.02)
[Al1.52 Fe0.43 Ti0.05],
with both Fe2+ and Fe3+, 0.336
mm thick. data
file
- Garnet, GRR3197,
(orange) Tunduru, Tanzania ,
2.084 mm thick. Data file.
- Grossular GRR
1386, (brown), Lalatema, Moshi, Kenya , 4.126 mm thick. Data file
Yellow-green garnets with Fe3+
- Andradite, GRR
48, Val Malenco, Sondrio Province, Italy ,
colored
greenish-yellow by Fe3+, nesr end-member comopsition. Plotted as 1.03 mm
thick; data
file.
- Andradite, GRR 1268, San Benito Mountains, California. Greenish-yellow crystals.
- Andradite, GRR
1765, Ural Mountains, Russia ,
colored by Fe3+, 0.737 mm thick; data
file.
- Grossular, GRR
1934b
from Sierra de las Cruces, near Lake Jaco, Chihuahua, Mexico, 2.60 mm
thick.
This is a yellow-green rim on a crystal with a small black
core. Data
file
- Andradite, green
GRR 2185 plotted as 2.38 mm thick and brownish GRR 2185,
plotted as 2.38 mm thick, from Farm Tubussis, Erongo Moutains,
Namibia.
These garnets vary from green to brownish. Data files: green;
brown
- Andradite, GRR
3137 from the Antetezajbato Mine, Ambanja, Madagascar,
plotted as 1.00 mm thick. This light green garnet
has the spectrum with the least deviation from pure andradite. Data file
Green garnets with V3+ and Cr3+
- Uvarovite, GRR
1019, Jacksonville, California, colored green by Cr3+ . Approximate composition of the 6-coordiated site: Cr1.19 Fe0.57 Al0.15 Ti0.04 V0.01. Plotted as
0.029 mm thick. data file
- Grossular, variety Tsavorite, GRR
229, Tanzania ,
colored green by 0.11 wt% V3+. Plotted as 0.758 mm thick. data
file
- Andradite, bright green variety demantoid, GIA
2952, Russia ,
2.5 mm thick.The spectrum of the slab
has both the features of Fe3+ and the features
of Cr3+. Data
file.
- Andradite, variety demantoid, GRR 2666 &
GRR 2667, Kladovka Mine, Ural Mountains, Russia. Slabs made
from two 0.639 mm thick crystals; orange as mined and
green after
heat-treatment which
is used to remove the brown component of color
often found in the
natural stones. Data files: GRR 2666 - orange;
GRR 2667 - green
Red garnets with Cr3+
- Pyrope, GRR 332,
Ugelvik, Oteroy, Norway. Upper mantle sample colored by Cr3+.
Plotted as 0.80 mm thick.
- Red pyrope, GRR
1055, Moses Rock, Utah ,
Chromian-pyrope, Ca0.33 Mn0.02
Mg2.13 Fe0.54 Al1.95
Cr0.07, 0.840 mm thick; data file
- Pyrope, GRR 1121, Wesselton kimberlite diamond mine, South Africa. Mg2.09 Mn0.02 Fe0.67 Ca0.26 Al1.89 Cr0.03 Ti0.03. Plotted as 0.547 mm thick; data file.
Red and pink garnets with Mn3+
- Dark pink grossular GRR1890, from
Sierra de
Cruces Range, Coahuila, Mexico, 1.17 mm thick. These garnets are often
called often called raspberry
grossular .
They contain about 1 to 1.5% manganese oxide. Only about 10 percent of
the manganese, which is in the 3+ oxidation state, is
resonsible
for the pink color. Data
File.
Blue to purple garnets
- Pyrope, GRR
2225, Tunduru, Tanzania. This is a color-change
pyrope-spessartine garnet that
goes from pale purplish-blue to light purple depending upon
the
illumination. It contains about 0.6 wt% vanadium. Thickness: 2.426 mm. Data
file.
Luminescence Spectra of Garnets
- Grossular,
GRR 1386, 365 nm excitation, nearly colorless garnet from the
Merelani Hills, Tanzania . Data files; RT,
LN2
- Grossular,
GRR 1893, 365 nm excitation, green Cr-containing crystal from Asbestos, Ontario,
Canada. Data file: RT
- Grossular, GRR
2067, 365 nm excitation, green tsavorite , from the Lelatema Mountains, Tanzania. Data files: RT, -78 C
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revised 25-Feb-2024