Nesosilicate – Garnet Group

This article is one of a series from "Let's talk Gemstones" written by Edna B. Anthony and it talks about the complex Neososilicate, the Garnet Group.

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.

The Pyralspites

It has been noted earlier that the garnets are sometimes classified as the Ugrandites, those species where calcium occupies the A site in the general structural formula A3B2(SiO4)3], and the Pyralspites, those species where the B site is occupied by aluminum. Little solid solution occurs between these two categories, but solid solution series are formed between members of the same category. Pyrope, almandine, and spessartine comprise the pyralspite category. Solid solution series are formed between almandine and pyrope, pyrope and spessartine, and between almandine and spessartine. A simple way to illustrate the relationship of these species is to draw a triangle with lines connecting the points representing the pure "end members" of the three species. Progression along the line connecting the end members of two species indicates a ratio change of the atoms in the elements that compose the chemical compositions from the solid solution series' end members. Such a change in the ratio often affects the physical and optical properties of the varieties formed by the change. Pure end members of the various series rarely occur. Therefore, the intermediate varieties provide an important portion of the material for gemstones. Such varieties often bear trade names that give no clue to their relationship to the species. This "relationship triangle" provides the basis for the sequence in which the pyralspite species and their varieties are presented.

Almandine [Almandite]

Fe3Al2Si3O12 - Iron Aluminum Silicate

This most common of the garnet species (an end member of two solid solution series) occurs in a wide variety of locations and environments. Regional metamorphism of clay sediments has made it an abundant constituent of metamorphic rocks worldwide. Igneous rocks, alluvial deposits, and less frequent occurrences in pegmatic granite are other sources. The major sources of gem material occur in India, Sri Lanka, Australia and Brazil. Exceptionally large gem crystals are produced from mines in Madagascar. Excellent material exhibiting asterism is found in Idaho in the United States. [The Smithsonian Institution in Washington D. C. has a 175-carat star gem from Idaho in its collection.] Slate deposits near Wrangell, Alaska yield unusually well formed crystals.

Because pure almandine is rare, the term almandine is used when the iron in its chemical composition exceeds that of magnesium in its solid solution series with pyrope. The same is true of manganese in its solid solution series with spessartine. Its iron content causes it to fuse at 3-3.5 to a magnetic globule. Spectral examination always reveals a distinct and diagnostic pattern. From 3 to 5 wide bands and a possible 2 or more lines create a vivid spectrum. The presence of elements associated with pyrope and spessartine produce the range of almandine's hues from red, violet-red, orange-red, and reddish-brown to brown.

The asterism of the star garnets is caused by asbestiform pyroxene and amphibole inclusions. Translucent and transparent almandine material frequently houses short, stubby, low-relief crystals of biotite, ilmenite, spinel, quartz, apatite, monazite, haloed metamict zircon crystals, rod-like hornblende, and asbestiform needles of augite and acicular rutile crossed at 70 and 110 degrees. The iron content and inclusions can cause variations of density from 3.95 to 4.30. Simon & Schuster's Guide to Gems and Precious Stones states "the density increases parallel to the (refractive) index". Refractive indices between 1.785-1.83 are the norm. Readings below 1.78 indicate an intermediate member of the almandine-pyrope series. Joel Arem lists the dispersion as 0.027 in his Color Encyclopedia of Gemstones, and it is shown as 0.024 in Walter Schumann's Gemstones of the World. The vitreous luster sometimes borders on adamantine. Despite its lesser hardness (6.5 - 7.5), almandine's distinctive fracture makes it much more effective than quartz for use in abrasive materials. The major source for manufacture of these abrasives is the extensive garnet deposit at Gore Mountain in New York. The condition of the huge crystals there, extremely shattered by nature, precludes their use as gemstones.

Almandine's name is a corruption of the name of the ancient city of Alabanda, located in what is now southwest Turkey, near where it was discovered. It is a gem with an ancient history. Noah is reputed to have hung garnet in the ark to disperse light. It was used in windows of temples and cathedrals and is believed to have been one of the gems representing the twelve tribes of Israel decorating the "breastplate of Aaron". The finest almandines from the alluvial gemstone deposits in Sri Lanka are often called "Ceylon rubies". A careful examination of its physical and optical properties is sometimes needed to distinguish almandine from red spinel. Paste (glass) is used as a simulant. Almandine is not produced synthetically for use as gems. The expense of production by the flux growth method is prohibitive, and the high temperature of the melt method causes the conversion of the necessary ferric iron to ferrous iron in the chemical composition with unsatisfactory results.

Intermediate Members of the Almandine - Pyrope Solid Solution Series

The chemical composition of the intermediate members of the series grades almandine to pyrope as the presence of iron decreases and magnesium increases. Without chemical analysis, the major criterion for determination and designation of an intermediate member of the series is the refractive index. The refractive index for the series exhibits a range from 1.785-1.830 for almandine to 1.714-1.742 for pyrope. The presence of chrome can usually be discovered by spectral analysis.

Rhodolite

This gem garnet variety has increased enormously in popularity within the last few years. Its purplish undertones have led to the use of the term "raspberry garnet" in the trade. In The Illustrated Encyclopedia of Minerals and Rocks, Dr. J. Kourimsky indicates the distinctive color "is caused by the presence of iron and chromium". Upon spectral examination, it exhibits the almandine spectrum. Determination of its position in the series requires chemical analysis. Without such analysis, most gemologists apply the name rhodolite if the refractive index reading is 1.77 or below. In addition to the inclusions common to garnets, apatite crystals are sometimes present.

Chrome Pyrope

The presence of chrome creates the especially vivid red of this intermediate member of the series. Its position in the series lies closer to pyrope than does that of rhodolite as the ratio of magnesium increases in the chemical composition.

Pyrope

The red hues of the pyrope-almandine series nearest the end member pyrope are commonly called pyrope. The Greek word pyropos, meaning "fiery-eyed", gives them the name. Volcanic rock and alluvial deposits in Argentina, Australia, Brazil, Myanmar, Scotland, Switzerland, Tanzania, and the desert sands of the southwest in the United States are sources of this popular garnet. Pyrope often is associated with and appears as inclusions in diamonds in pipes located in Africa and Canada. Pyropes are usually pea-sized or smaller. The slopes of the Bohemian Highlands in Czechoslovakia are famous for the production of especially fine pyropes called "Bohemian garnets". Dr. Kourimsky tells that Anselmus Boetius de Boot, in his Gemmarum et lapidum historia, "writes not only about the garnet the size of a pigeon's egg but about other large pyropes" found there in the sixteenth century and that "pyropes the size of a hazelnut were valued as highly as rubies". Pyropes from this source were used extensively in Victorian jewelry. "Cape ruby" is a name applied to the pyrope found in deposits at the Cape of Good Hope in South Africa. Although pyropes are notably free of inclusions, sometimes octahedra, tiny needles, and rounded irregularly outlined quartz crystals (snowballs) are present.

Pyrope

Mg3Al2Si3O12 - Magnesium Aluminum Silicate

This end member of the two solid solution series almandine- pyrope and pyrope-spessartine would be colorless in the pure state. Its refractive index range of 1.714 -1.742 is the lowest of all the garnets. The existence of pure pyrope as a gem is unknown.

It is confusing that the term "pyrope" is used as a designation for an intermediate member of the almandite-pyrope solid solution series. Neither iron nor manganese is present in the chemical composition of this end member of the two solid solution series, almandine-pyrope and pyrope-spessartite. However, it is a component of the intermediate members of the two series and is present with spessartite and grossular in the spessartite-grossular-pyrope type of color-change garnet. The pure state of pyrope (it would be colorless) is not known to exist in nature. A possible refractive index of 1.714 is the lowest exhibited by a garnet. Joel Arem lists its physical and optical properties as follows: possible variation of the refractive index of 1.730 to 1.766, a dispersion of 0.022 and a variation of specific gravity of 3.65 to 3.87.

Intermediate members of the Pyrope - Spessartite solid solution series

As the chemical composition of the end member pyrope grades towards that of the end member spessartite in the pyrope-spessartite series, the physical and optical properties again change. The presence of manganese becomes more pronounced. Intermediate members of this series can closely resemble the hessonite variety of grossular garnet, but the granular "treacly" appearance of hessonite under the microscope is absent. Brown, red, orange, and yellow hues dominate the color range with peach and pink tones appearing as the chemical make-up approaches the chemical composition of pure spessartite. A dispersion of 0.027 combines with the high refractive index [1.79-1.81] to make well cut gems of the intermediate members of this series especially bright.

Malaya - [Pyrospessartite]

The name of this variety of the pyrope-spessartite series is not related in any way to the area of southeast Asia known as Malaya or its people. It is derived from the word of the Swahili tribe in Africa meaning prostitute. Joel Arem in the Color Encyclopedia of Gemstones attributes it to the Bantu tribe word for "outside the family" or "deceiver". According to unconfirmed reports, miners, frustrated in their search for a different mineral and disgusted by the frequent presence of such a similar material, bestowed the unflattering name on the similar substance. The sources in Africa provided the name, but, perhaps, the use of the term pyrospessartite combined with the designation of the place of origin, Tanzania, would create less confusion. As with most varieties of garnet, it is necessary to closely examine pyrospessartite's physical and optical properties and/or subject the stone to chemical analysis to make a positive identification. It can resemble hessonite of the grossular garnets, but the diagnostic roiled "scotch in water" appearance of hessonite under the microscope is absent. Colors range from brownish red to lighter orange and yellow hues as the chemical composition nears that of pure spessartite.

Spessartite - Mn3Al2Si3O12 - Manganese Aluminum Silicate

This end member of the two garnet series formed with pyrope and almandine was discovered in the 1800s in the Spessart area in northwest Bavaria (Germany). The deposit was small, and commercial exploitation of the material was limited. Subsequently, sources of attractive materials (intermediate between pure spessartite and almandite) were found in Sweden, Italy, Myanmar (Burma), Sri Lanka, Pakistan, Australia, Mozambique, Kenya, Tanzania, Zambia, Madagascar, Brazil, and the United States. The designation "spessartite" was applied to all and continues today. A "pure" spessartite was unknown in the jewelry trade until the discovery of the bright orange material in Namibia in 1991. With a high refractive index of 1.79 to 1.81, spessartite ranks second only to andradite in the wide range of indices exhibited by the garnet group. The dispersion of 0.027 equals that of almandite and grossular but is less than half that of andradite [0.057]. Specific gravity can vary from 4.12 to 4.18 with 4.15 being the norm. In addition to inclusions of tremolite, irregular "cobwebs" caused by dispersed drops of included liquid may be present.

Mandarin Garnet

The discovery of the iron-free, brilliant orange spessartite in Namibia in 1991 in commercial quantities created a sensation in the jewelry trade. The deposit lay in mica schist along the course of the Kunene river in the mountainous northwest area bordering Angola. Few inclusions marred the excellent crystals. The name "hollandine" was chosen for its introduction as a spectacular new gemstone. This was changed to Mandarin garnet when it was discovered that "hollandine" denotes a little-known metal. Within less than five years time, the deposit was depleted, and the area closed. The planned recovery of material from metamorphic bedrock in the rugged terrain surrounding the first find will be more difficult. Crystals recovered from the surface layer of this area contain numerous inclusions of tremolite, but cleaner material lies below. In 1999, a new source of the pinkish-orange gemstone was discovered in a remote area of Nigeria. Though the material was more yellow than the orange Namibian crystals, the size and abundance of the material made the public more aware of and increased the demand for spessartite gems. Despite great expectations, this deposit, too, was quickly depleted. Acquisition of material from the Nigerian area is sporadic and available only in small amounts from local native traders. Currently, developers are investigating a promising new deposit of a high (reputedly 90% or more) manganese content material located in the gem-rich Alto Mirador pegmatite dike of Paraiba in Brazil. Mandarin garnet brought spessartite wide acclaim. It is confusing that the terms "spessartite" "spessartine" and "mandarin garnet" are frequently applied to intermediate members of the spessartite-almandite garnet series.

Iron-bearing Intermediate Members of Spessartite-Almandite Series

As the ratio of iron to manganese increases and the chemical composition grades from spessartite to almandite, the refractive index rises and brown tones appear. However, the substances retain a vivid color caused by the presence of the manganese. This is typical of gems from Madagascar and the other sources noted above. The orangy-red material produced in Amelia, Virginia and Ramona, California set the standard for the most desired color for such spessartite gems until the Namibian discovery. Opened in 1903, the Little Three and the Hercules mines situated in a pegmatite dike near San Diego, California were the best known sources for iron-bearing spessartite gems until the late twentieth century. The high cost of production caused the close of the mines in 1997. As noted above, the intermediate members of the spessartite-almandite solid solution series are frequently and incorrectly referred to as spessartite or spessartine or mandarin garnet. These terms are no longer exclusive to iron free spessartite.

Color-Change Garnets

It has been noted earlier that the chemical composition of garnet can vary widely, so long as the "size" of an atom can be accommodated within the structural lattice. An intermediate member of a garnet series is often said to contain a "mixture of the molecules" of the end members of the series. This is a simple way to say that the ratio of chemical elements in the chemical composition changes in a progression from one end member to the other. In the pyrope to spessartite series, the elements involved are magnesium and manganese. In the chemical composition of pure pyrope, there is no manganese. If manganese is present in the surrounding matter as a garnet crystal develops, then it may occupy an atomic site in the lattice that could be occupied by a magnesium atom. It is thus said that a manganese atom has replaced an atom of magnesium. As the ratio of manganese to magnesium increases, the series "grades" from the end member pyrope to the end member spessartite until manganese has completely "replaced" magnesium in the chemical composition. It is important to remember that atoms of other elements can occupy sites in the atomic lattice. The color-change garnets are excellent examples of the complexity of the chemical make-up of the garnets.

The color-change garnets found in East Africa involve "a mixture of the molecules" of spessartite and grossular, with a substantial amount of chrome and vanadium incorporated into the chemical composition. Spessartite is the major component, but grossular can be almost half the composition, with almandine or pyrope also in the mix. In the Color Encyclopedia of Gemstones, Joel Arem gives the following information. The spessartite-grossular-almandine color-change garnet has a refractive index of 1.773, with a density norm of 3.98. It appears greenish-yellow brown in transmitted fluorescent light. In reflected fluorescent light, it changes to purplish-red. In incandescent light, it exhibits a reddish-orange to red color.

The spessartite-grossular-pyrope color-change garnet exhibits a change of color from "light bluish green" in transmitted fluorescent light to purple in reflected fluorescent light. In incandescent light, the color is "light red to purplish red". The refractive index is 1.763 with a specific gravity norm of 3.89.

Again, the Color Encyclopedia of Gemstones is the source of the information concerning the pyrope-spessartite color-change garnet found in the Umba Valley of East Africa. Calcium and titanium are a part of its chemical composition. A refractive index of 1.757 and a specific gravity of 3.816 are normal. The spectrum exhibits "absorption bands at 4100, 4210, and 4300 that may merge to form a cutoff at 4350." A wide definite band at 5730 occurs in material that exhibits a strong change of color. Acicular rutile and hematite platelets are common inclusions. These gems change from reddish purple in tungsten light to greenish-blue in daylight.

Joel Arem mentions color-change garnet crystals of less than a carat that have been recovered in Norway. The material exhibits a refractive index of 1.747 with a density of 3.715. The color changes from "violet in daylight" to "wine-red in incandescent light." He also mentions "alexandrite-like garnets" with a color change from "violet-red to blue-green." He notes only that although they "are small, — a stone of 24.87 carats was sold in 1979".