Garnets
| including varietal names and compositions |
| almandine |
Fe3Al2(SiO4)3 |
rhodolite = intermediate,
reddish py-al |
| andradite |
Ca3Fe2(SiO4)3 |
demantoid = green, Cr-containing
topazolite = amber, Ti-containing
melanite = black, Ti-containing |
| calderite |
Mn3Fe2(SiO4)3 |
|
| goldmanite |
Ca3V2(SiO4)3 |
|
| grossular |
Ca3Al2(SiO4)3 |
|
| hibschite |
Ca3Al2(SiO4)3-x(H4O4)x |
partial replacement of Si by H |
| katoite |
Ca3Al2(H4O4)3 |
full replacement of Si by H |
| kimzeyite |
Ca3(Zr,Ti)2(Si,Al,Fe)3O12 |
|
| knorringite |
Mg3Cr2(SiO4)3 |
|
| majorite |
Mg3(Al,Fe,Si)2(SiO4)3 |
6-coordinated and 4-coordinated Si |
| pyrope |
Mg3Al2(SiO4)3 |
|
| schorlomite |
Ca3(Ti)2(Fe2Si)3O12 |
Ti4+-containing, black garnet |
| spessartite |
Mn3Al2(SiO4)3 |
|
| uvarovite |
Ca3Fe2(SiO4)3 |
|
Here is a photogallery of garnets.
Almandine from Wrangel, Alaska, showing the typical color of
Fe2+ in the almandine eight-coordinated site..
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| Almandine from Salida, Colorado, covered with green chlorite. |
This grossular from Lake Jaco, Mexico is pink from a minor amount
of Mn3+
in the 6-coordinated site. |
The same grossular from a different angle. Garnets are
cubic, after all. |
Spessartite from Namibia shows the orange color of end-member
spessartite. When Fe2+ substitutes for Mn, the color gains
a red component.
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| Pale green grossular from the Wah Wah Mtns, Utah, USA |
Pale grossular from the Jeffrey Mine, Asbestos, Québec, Canada |
Grossular from Asbestos, Québec, Canada |
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|
| Mn3+-containing red grossular from the Sierra de Cruces
Range, Coahuila, Mexico. |
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| Grossulars of various color from the Lalatema district,
Tanzania |
Grossular from Belvedere, Vermont, USA. |
Colorless hibschite from Marienberg be Aussig, Bohemia, is an intermediate
hydrogrossular in the series: grossular - katoite. These garnets
form at low temperature and have octahedral morphology. The amber
core of the crystals is vesuvianite. The view is of a thin section under
a microscope.
Another hibschite from the limestone quarries near Crestmore,
California. This mineral seen in this thin section was originally
called plazolite. It consists of veins of fine-grained hydrogarnet
between gehlenite grains. In partially crossed polarizers (on the
right) the hibschite appears in the gray color comprising about 1/2 of
the field of view..
Andradite from Val Malenco, Italy, (left) and from Santa
Rita Peak, San Benito County, California (center). The yellowish-green
color or the Val Malenco garnet is typical of Fe3+. The
browner color of the San Benito County garnet is from its Ti content. With
even higher Ti-content, black andradites (right) known as melanite result.
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| Pyrope from the Dora Maira Massif, Italy, is the closest to
end-member composition of any pyrope in the world. The light pink
color is from a minor amount of Fe2+. Unlike most pyropes
which come from the mantle in diamond pipes, this one is found at the base
of a massive metamorphic complex. |
Intermediate composition Pyrope-Almandine (py al) from the Umba
River Valley, East Africa, has the typical red color of the rhodolite variety. |
Three Pyropes from Garnet Ridge, Arizona, USA. As their
chromium content increases clockwise from the upper right, the amount of
purple component increases. |
Uvarovite from
California. The green color is from Cr3+. Large
crystals of uvarovite are rarely found.
Synthetic Garnets
Because the garnet structure is so adaptable to a variety of elements,
many synthetic compositions can be made in the garnet structure.
Particularly important are the rare-earth garnets. In these garnets,
a trivalent rare-earth element replaces Ca2+ and a trivalent
element replaces Si4+.
Gadolinium Gallium Garnet: Gd3Ga2(GaO4)3
=
Gd3Ga5O12 GGG
Gadolinium contains more unpaired electrons than any other element,
so it has interesting magnetic properties. Bubble Memories
based
on magnetic domains in GGG have been used where information in RAM must
remain even if power is removed. Pictured above is the tip of a boule
grown from a melt in an iridium (Ir) crucible.
 |
Synthetic garnets are also used for gemstones. This is YAG, a
garnet of composition Y3Al2(AlO4)3
doped
with Er3+ for color |
.
