Lets Talk Gemstones
By Edna B. Anthony, Gemologist
(Contact the author for permission to reproduce this
article in any form.)
P.O.# 62653; COLORADO SPRINGS, CO. 80962
THE GARNET GROUP
[A NESOSILICATE]
AN INTRODUCTION TO THE GARNETS
The garnets are a complex group of nesosilicates of the silicate class of
minerals. In nesosilicates, only ionic bonds formed with interstitial cations
(positive charged atoms) connect the isolated SiO4 tetrahedra. The size and
the charge of these cations generally determine the structures of these minerals.
An equidimensional crystal habit and a lack of distinct cleavage planes are
the result of the independence of the SiO4 tetrahedra. Dense atomic packing
causes the characteristic high specific gravity and hardness of the structures.
In the twentieth edition of the Manual of Mineralogy by Cornelis Klein and
Cornelius S. Hurlbut, Jr. after J. D. Dana, we are told that garnets “crystallize
in the hex octahedral class of the isometric crystal system.” The most common
crystal habits for this class are the cube and the octahedron. The arrangements
of the atoms in their structures are such that these habits are rare in garnet.
(It is interesting to note that only pyrope occasionally exhibits cubes with
curved faces.)
Garnets usually occur in the dodecahedral and trapeszohedral forms or combinations
of these forms. The dodecahedral form is so typical that the dodecahedron
was once known as a garnetohedron. Richard M. Pearl states in his Garnet,
Gem and Mineral that “Twinning of garnet crystals shows only in the effect
of double refraction.” This anomalous double refraction may indicate internal
strain that may cause complex or sector twinning. (“Sector twins consist
of 12, 24 and 48 pyramids meeting at the center of the crystal.” -quoted
from Pearl) However, some mineralogists think that such double refraction
in garnet may be evidence of crystallization in the tetragonal crystal system.
In the Color Encyclopedia of Gemstones, Dr. Joel Arem presents an excellent
diagram of the relationships between the garnets. Under the formulas for
the garnet species, one is instructed to note “Henritermierite: Ca3(Mn,Al)(SiO4)2(OH)4.
Tetragonal, very garnetlike, often twinned.”
A3B2(SiO4)3 can represent the structural formula of garnet. The 8 coordinated
cationic sites represented by A are occupied by rather large divalent cations.
B represents 6 coordinated cationic sites occupied by smaller trivalent cations.
In garnets, the A cationic sites can be occupied by the large divalent atoms
of calcium, magnesium, iron or manganese. The 6 cationic sites represented
by B are occupied by smaller trivalent cations of aluminum, chrome, or iron.
The chemical compositions of the garnets allow them to be grouped in two
series known as isomorphous series. One of the series is composed of garnets,
where calcium atoms occupy the A sites. This series includes uvarovite, grossular,
and andradite and is referred to as the ugrandites. Arem notes that the large
atoms of calcium in the structure of the ugrandites cause them to exhibit
birefringence. X-ray data reveals that the ugrandites can crystallize in
the orthorhombic and, perhaps, in the monoclinic crystal systems. The occupation
of certain crystallographic sites by specific cations may cause such crystallization.
Twinning occurs frequently in andradite and grossular garnets, and color
zoning is the norm. Hydrogrossular is formed when tetrahedral (OH4) groups
(hydroxyl) replace some of the SiO4 tetrahedra in the grossular composition.
The water content of some hydrous garnets may be as much as 8.5%. Melanite,
the black variety of andradite, develops when sodium replaces calcium and
Ti4+ enters the B cationic sites.
In the other garnet series, no calcium is present (magnesium, iron, or manganese
atoms occupy the A sites) and the B sites are occupied by aluminum cations.
This series is known as the pyralspites and includes pyrope, almandine, and
spessartine. Pure end members of either series are seldom found. Extensive
substitution occurs in each of the series, but there is little solid solution
between the two series. Richard M. Pearl mentions such a combination called
spandite. It is a link, which involves titanium, between spessartite and
andradite. He also makes the statement that, “As long as any chemical element
can fit into the atomic structure because its size is right, the composition
of garnet is variable.” Phosphorus, vanadium, yttrium, and zirconium are
other elements that sometimes replace atoms in garnet’s structure.
Garnet is a common mineral distributed worldwide. It occurs as crystals,
in massive and granular forms, and as tumbled pebbles. It can form under
a wide variety of geological conditions, but high temperatures are essential
for its development. It is of major importance as a rock-making mineral in
igneous, metamorphic, and sedimentary rocks. It alters frequently to chlorite,
serpentine, and talc. Chemical stability and resistance to weathering permit
excellent crystals to be found in alluvial deposits. It is known that inhabitants
of the American southwest still recover crystals from the desert sands and
ant hills there. Garnet has imperfect cleavage, but it can exhibit an unusual
angular fracture. This ability allows it to retain sharp cutting
edges. Industry takes advantage of this property and its hardness to produce
abrasive papers and cloths that are two to six times more efficient than
those of quartz. The large crystals of almandine recovered at Gore Mountain
in New York provide a major source for this industrial use. The physical
properties of pyrope (its elasticity and heat conductivity) make it ideal
for bearings used in the manufacture of very accurate watches, clocks, and
other fine instruments.
The use of garnet as a gemstone is historic. Before the technique of faceting
was developed, material from the underside of well-formed domed crystals
was often removed to facilitate the transmission of light through the stone.
It is known that garnet was used before 3400 B.C. in Predynastic Egypt and
in Sumeria as early as 2350 B.C. Artisans of the Bronze Age (2000 to 1000
B.C.) in Sweden incorporated garnet in their works. Caravan traders with
sources in Africa brought to Carthage garnets that were highly prized in
Rome. Pliny, quoted from an early Hebrew writing, said, “for the traveler
the well formed image of a lion, if engraved on a garnet will protect and
preserve honors and health, cures the traveler of all diseases, brings him
honor and guards him from all perils incurred in traveling.”
The Persians frequently carved images of their great men on garnets. The
inhabitants of the Middle East regions and Egypt obtained garnets from India
as early as 1000 B.C. through trade with Arabia. A garnet was one of the
twelve gems mounted in the breastplate of Aaron, sacred to Jews, which symbolized
the twelve tribes of Israel. Some of the peoples of Asia used garnets as
“magic” bullets. They believed such missiles were more accurate and lethal.
Relics of the Aztecs in central Mexico show they used garnets frequently.
In the American southwest, the Pueblo Indians began to use them as gems in
their later works. Garnets collected by the Comanche Indians at Jaco Lake
in Chihuahua, Mexico have been found at the Pueblo of Picuris in New Mexico.
We know this pink glossularite recovered from white marble deposits at Xalostoc,
Lake Jaco, and Morelos, Mexico as xalostocite, rosolite, and landerite.
In Europe, pyrope from deposits in Bohemia supplies some of the finest gems
to jewelers. The Victorian era is renowned for the use of these gems. Melanite
was used extensively in “mourning” jewelry during this period.
Attempts to synthesize garnet for industrial purposes began during the 1960’s.
These materials possess the structure of natural garnet but differ in chemical
composition, and they have no counterpart in nature. YAG (yttrium-aluminum-garnet)
is produced in a range of colors and colorless. Its dispersion exceeds that
of diamond. Faceted YAG is frequently used as a diamond substitute. Twenty-eight
of its trade names are listed on page 234 of the second edition of the Color
Encyclopedia of Gemstones by Dr. Joel Arem. GGG (gadolinium-gallium-garnet)
also serves as a diamond imitation. YIG (yttrium-iron-garnet) is opaque and
black with a metallic luster and is sometimes used by the trade to imitate
hematite.
Most sources indicate the name garnet is derived from the Latin word granatus
meaning “like a grain.” Before the science of mineralogy developed, most
red gems (including garnet) were known as carbuncles, also from the Latin
meaning “a live or burning coal.” Natural garnet is known by numerous appellations.
Some are more familiar than others. The glossary compiled and published by
Richard M. Pearl is enlightening. It seems appropriate to present the alphabetized
list, to which has been added information deemed pertinent, at the end of
this article, rather than fragmented in the separate articles concerning
the origin and specific properties of the species and varieties of garnet.
Glossary
“Adelaide ruby”
Red garnet from South Africa
“African jade”
Massive green grossular garnet
“Alabandine ruby”
Almandine garnet
Allochroite
Andradite garnet from Switzerland (colorless)
“American ruby”
Pyrope garnet; also rose quartz
Aplome
Dark-brown, yellowish-green, or brownish-green andradite garnet (contains
manganese)
“Arizona ruby”
Pyrope garnet
“Arizona spinel”
Garnet
“Australian ruby”
Garnet
Black garnet
Andradite garnet
Blythite
Manganic manganese garnet
Bobrowka garnet
Demantoid-alluvials Nizhniy Tagil in Urals; parent rock banks Bobrovsk,
Sysertsk region
“Bohemian ruby”
Pyrope garnet; also rose quartz
Bredbergite
Magnesium andradite garnet from Sweden
Calderite
Manganous manganese-ferric iron garnet [Mn3Fe2(SiO4)3]
“California ruby”
Garnet
“Cape ruby”
Pyrope garnet (implies Cape of Good Hope or South African origin)
“Ceylonese ruby”
Almandine garnet
Cinnamon stone
Hessonite garnet
Colophonite
Resinous coarse cloudy yellow-brown andradite garnet; also non-gem
variety vesuvianite
“Colorado ruby”
Pyrope garnet
“Elie or ely ruby”
Pyrope garnet (implies Isle of Ely or Scotland origin)
Emildine or emilite
Spessartite garnet from South Africa
Fashoda garnet
Pyrope garnet
“Fashoda ruby”
Pyrope garnet
“Garnet jade”
Massive green grossular garnet
Garnetoid
Hydrogrossular garnet
Goldmanite
Vanadiferous andradite garnet [Ca3V2Si3O12] crystals tiny, dark green
Gooseberry stone
Grossular garnet
“Green garnet”
Enstatite
Guarnaccino
Yellowish-red garnet
Haplome
Andradite garnet (contains manganese); syn. Aplome
“Hematite garnet”
Synthetic iron-rich garnet
Hyacinth
Hessonite garnet; also zircon
Hydropyrope
A synthetic garnet
Hydrospessartine
A possible synthetic garnet
Jacinth
Hessonite garnet; also zircon
“Kandy spinel”
Almandine garnet
Kelyphite
Pyrope garnet inside chlorite
Kimseyite
Calcium-zirconium garnet from Arkansas [Ca3(Zr,Ti)2(Al,Si)3O12]
Knorringite
A chromiferous garnet [Mg3Cr2Si3O12]
Kollin garnet
Almandine garnet
Landerite
Pink grossular garnet from Mexico
Leuco-garnet
White garnet from Bohemia
Majorite
Purple garnet [Mg3(Fe,Al,Si)2Si3O12] found in a meteorite
“Montana ruby”
Red garnet
“Mountain ruby”
Red garnet
“Olivene”/“olevene”
Demantoid garnet
“Oregon jade”
Grossular garnet
Partschinite
Spessartite garnet from Rumania
Polyadelphite
Brownish-yellow andradite from New Jersey
Pyreneite
Grayish-black andradite garnet from France
Rhodolite
Intermediate pink, rose, or purplish to violet-red garnet between almandine
and pyrope
“Rock ruby”
Pyrope garnet
“Rocky Mountain ruby”
Pyrope garnet
Roddingite
Hydrogrossular garnet from New Zealand
Romanzonite
Dark-brown grossular garnet
Rose garnet
Pink grossular garnet; also rhodolite
Roselite or rosolite
Pink grossular garnet from Mexico
Rothoffite
Brown andradite garnet from Sweden
Schorlomite
Dark-brown to black andradite garnet [Ca3Ti2Fe3O12]
“Siberian chrysolite”
Demantoid garnet
Skiagite
Ferrous-ferric iron garnet from Scotland
“South African jade”
Massive green grossular garnet
Spalmandite
Intermediate almandine-pyrope garnet
Succinite
Amber-colored grossular garnet from Italy; also amber
Suriam garnet
Violet-red almandine garnet
Syriam/Syrian garnet
Violet-red almandine garnet
Topazolite
Greenish-yellow to yellow andradite garnet
Transvaal jade
Massive green grossularite from South Africa
Vermeille or vermeille garnet
Brownish-red almandine garnet; also pyrope garnet
“Vesuvian garnet”
Early name for leucite whose crystal form resembles that of garnet
“White garnet”
Translucent variety grossular garnet resembles white jade in appearance;
also Leucite
Wiluite
Green variety of grossular garnet; also a greenish variety of vesuvianite
Xalastocite
Pink grossular garnet from Mexico
Yamatoite
Manganese vanadium silicate (Mn3V2Si3O12)