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
Other, less common end-member compositions are:
Also used is a name for intermediate hydrogrossulars:
- Calderite ------------ Mn3Fe3+2(SiO4)3
- Goldmanite --------- Ca3V2(SiO4)3
- Katoite -------------- Ca3Al2(OH)4
- Kimzeyite ----------- Ca3(Zr,Ti)2[(Si,Al,Fe3+)O4]3
- Knorringite ---------- Mg3Cr2(SiO4)3
- Majorite ------------- Mg3(Fe,Al,Si)2(SiO4)3
- Morimotoite -------- Ca3Ti4+Fe2+(SiO4)3
- Schorlomite --------- Ca3Ti4+2[(Fe3+2Si)O4]3
- Hibschite ------------ Ca3Al2(SiO4)3-x(OH)4x
0.2 < x < 1.5
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+,
(6-coordinated site), Fe2+ (8-coordinated site),
and Fe2+ - Ti4+
transfer can all be found in the spectra of garnets.
Spectra of representative 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 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
131, unknown locality, probably Tanzania,
colored pink by Fe2+ ; 2.007 mm thick; data
83, rhodolite variety, Umba River Valley,
Tanzania, colored pink by Fe2+; 0.78 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, South Africa, 0.547 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
- Spessartine, GRR
43, Rutherford Mine, Amelia, VA,
colored orange by Mn2+ , expanded view from a
2.165 mm thick sample. data file
- Pyrope-spessartine, GRR 1267,
Kenya, 1.686 mm thick; data file
Orange to black garnets with Fe2+, Fe3+
- Andradite, GRR
568, variety (black when thick) melanite , San Benito Mountains, California, with both Fe3+
and Ti4+, 0.102 mm thick. data
- 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+; 0.045 mm thick, data file
- 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+, 1.03 mm thick; data
- Andradite, GRR 1268, San Benito Mountains, California. Greenish-yellow crystals.
- Andradite, GRR 1765, Ural Mountains, Russia , colored by Fe3+, 0.737 mm thick; data
- 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.
- 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+
GRR 1019, Jacksonville, California, colored green by Cr3+, 0.029 mm thick.
- Grossular, variety Tsavorite, GRR
229, Tanzania ,
colored green by V3+, 0.758 mm thick. data
- 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+
- 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
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
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