Beryl

Beryl
Crystal structure of beryl
General
Category	Silicate minerals, cyclosilicate subclass, 6-membered single ring series
Chemical formula	Be3Al2(SiO3)6
Strunz classification	09.CJ.05
Dana classification	61.01.01.01
Crystal symmetry	6/m 2/m 2/m
Identification
Molar mass	537.50 g/mol
Color	colorless when pure, but see text
Crystal habit	massive to well crystallized
Crystal system	hexagonal
Cleavage	Imperfect on the [0001]
Fracture	Conchoidal
Mohs scale hardness	7.5-8
Luster	vitreous
Streak	White
Diaphaneity	Transparent to opaque
Specific gravity	Average 2.76
Optical properties	Uniaxial (-)
Refractive index	nω = 1.564-1.595, nε = 1.568-1.602
Birefringence	δ = 0.0040-0.0070
Ultraviolet fluorescence	None (some fracture filling materials used to improve emerald's clarity do fluoresce, but the stone itself does not)
References

The mineral beryl is a beryllium aluminium cyclosilicate with the chemical formula Be₃Al₂(SiO₃)₆. The hexagonal crystals of beryl may be very small or range to several meters in size. Terminated crystals are relatively rare. Pure beryl is colorless, but it is frequently tinted by impurities; possible colors are green, blue, yellow, red, and white. The name comes from the Greek beryllos which referred to a precious blue-green color-of-sea-water stone. The term was later adopted for the mineral beryl more exclusively.

Deposits

Three varieties of beryl: morganite, aquamarine and heliodor

Beryl of various colors is found most commonly in granitic pegmatites, but also occurs in mica schists in the Ural Mountains, and limestone in Colombia. Beryl is often associated with tin and tungsten ore bodies. Beryl is found in Europe in Norway, Austria, Germany, Sweden (especially morganite), and Ireland, as well as Brazil, Colombia, Madagascar, Russia, South Africa, the United States, and Zambia. U.S. beryl locations are in California, Colorado, Idaho, Maine, Connecticut, New Hampshire, North Carolina, South Dakota, and Utah.

New England's pegmatites have produced some of the largest beryls found, including one massive crystal from the Bumpus Quarry in Albany, Maine with dimensions 5.5 m by 1.2 m (18 ft by 4 ft) with a mass of around 18 metric tons; it is New Hampshire's state mineral. As of 1999, the largest known crystal of any mineral in the world is a crystal of beryl from Madagascar, 18 meters long and 3.5 meters in diameter.

Varieties

Varieties of beryl have been considered gemstones since prehistoric times:

Aquamarine and maxixe

Aquamarine (from Lat. aqua marina, "water of the sea") is a blue or turquoise variety of beryl. It occurs at most localities which yield ordinary beryl, some of the finest coming from Russia. The gem-gravel placer deposits of Sri Lanka contain aquamarine. Clear yellow beryl, such as occurs in Brazil, is sometimes called aquamarine chrysolite. When corundum presents the bluish tint of typical aquamarine, it is often termed Oriental aquamarine. The deep blue version of aquamarine is called maxixe. Its color fades to white when exposed to sunlight or is subjected to heat treatment, though the color returns with irradiation.

The pale blue color of aquamarine is attributed to Fe²⁺. The Fe³⁺ ions produce golden-yellow color, and when both Fe²⁺ and Fe³⁺ are present, the color is a darker blue as in maxixe. Decoloration of maxixe by light or heat thus may be due to the charge transfer Fe³⁺ and Fe²⁺. Dark-blue maxixe color can be produced in green, pink or yellow beryl by irradiating it with high-energy particles (gamma rays, neutrons or even X-rays).

In the United States, aquamarines can be found at the summit of Mt. Antero in the Sawatch Range in central Colorado. In Wyoming, aquamarine has been discovered in the Big Horn mountains, near Powder River Pass. In Brazil, there are mines in the states of Minas Gerais, Espírito Santo and Bahia. The Mines of Colombia, Zambia, Madagascar, Malawi, Tanzania and Kenya also produce aquamarine.

The biggest aquamarine ever mined was found at the city of Marambaia, Minas Gerais, Brazil, in 1910. It weighed over 110 kg, and its dimensions were 48.5 cm long and 42 cm in diameter.

Bixbite

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Red beryl (also known as "bixbite", "red emerald", or "scarlet emerald") is a red variety of beryl. It was first described in 1904 for an occurrence, its type locality, at Maynard's Claim (Pismire Knolls), Thomas Range, Juab County, Utah, USA. The old synonym bixbite is deprecated from the CIBJO, because of the risk of confusion with the mineral bixbyite (also named after the mineralogist Maynard Bixby). The dark red color of bixbite is attributed to Mn³⁺ ions.

Red beryl is rare and has only been reported from a handful of locations including: Wah Wah Mountains, Beaver County, Utah; Paramount Canyon, Sierra County, New Mexico; Round Mountain, Sierra County, New Mexico; and Juab County, Utah. The greatest concentration of gem-grade red beryl comes from the Violet Claim in the Wah Wah Mountains of mid-western Utah, discovered in 1958 by Lamar Hodges, of Fillmore, Utah, while he was prospecting for uranium.

While gem beryls are ordinarily found in pegmatites and certain metamorphic rocks, bixbite occurs in topaz-bearing rhyolites. It formed by crystallizing under low pressure and high temperature from a pneumatolitic phase along fractures or within near-surface miarolitic cavities of the rhyolite. Associated minerals include bixbyite, quartz, orthoclase, topaz, spessartine, pseudobrookite and hematite. The red color is thought to be from manganese substituting for aluminium in the beryl structure.

Emerald

Rough and faceted emeralds from the collection at the National Museum of Natural History

Emerald refers to green beryl, colored by trace amounts of chromium and sometimes vanadium. The word "emerald" comes from Latin smaragdus from Greek smaragdos - σμάραγδος, its original source being a Semitic word izmargad or the Sanskrit word, marakata, meaning "green". Most emeralds are highly included, so their brittleness (resistance to breakage) is classified as generally poor.

Emeralds in antiquity were mined by the Egyptians and in Austria, as well as Swat in northern Pakistan. A rare type of emerald known as a trapiche emerald is occasionally found in the mines of Colombia. A trapiche emerald exhibits a "star" pattern; it has raylike spokes of dark carbon impurities that give the emerald a six-pointed radial pattern. It is named for the trapiche, a grinding wheel used to process sugarcane in the region. Colombian emeralds are generally the most prized due to their transparency and fire. Some of the most rare emeralds come from three main emerald mining areas in Colombia: Muzo, Coscuez, and Chivor. Fine emeralds are also found in other countries, such as Zambia, Brazil, Zimbabwe, Madagascar, Pakistan, India, Afghanistan and Russia. In the US, emeralds can be found in Hiddenite, North Carolina. In 1998, emeralds were discovered in the Yukon.

Emerald is a rare and valuable gemstone and, as such, it has provided the incentive for developing synthetic emeralds. Both hydrothermal and flux-growth synthetics have been produced. The first commercially successful emerald synthesis process was that of Carroll Chatham. The other large producer of flux emeralds was Pierre Gilson Sr., which has been on the market since 1964. Gilson's emeralds are usually grown on natural colorless beryl seeds which become coated on both sides. Growth occurs at the rate of 1 mm per month, a typical seven-month growth run producing emerald crystals of 7 mm of thickness. The green color of emeralds is attributed to presence of Fe³⁺ and Fe²⁺ ions.

Golden beryl and heliodor

Golden beryl

Heliodor

Golden beryl can range in colors from pale yellow to a brilliant gold. Unlike emerald, golden beryl has very few flaws. The term "golden beryl" is sometimes synonymous with heliodor (from Greek hēlios - ἥλιος "sun" + dōron - δῶρον "gift") but golden beryl refers to pure yellow or golden yellow shades, while heliodor refers to the greenish-yellow shades. The golden yellow color is attributed to Fe³⁺ ions. Both golden beryl and heliodor are used as gems. Probably the largest cut golden beryl is the flawless 2054 carat stone on display in the Hall of Gems, Washington, D.C.

Goshenite

Colorless beryl is called goshenite. The name originates from Goshen, Massachusetts where it was originally described. Since all these color varieties are caused by impurities and pure beryl is colorless, it might be tempting to assume that goshenite is the purest variety of beryl. However, there are several elements that can act as inhibitors to color in beryl and so this assumption may not always be true. The name goshenite has been said to be on its way to extinction and yet it is still commonly used in the gemstone markets. Goshenite is found to some extent in almost all beryl localities. In the past, goshenite was used for manufacturing eyeglasses and lenses owing to its transparency. Nowadays, it is most commonly used for gemstone purposes and also considered as a source of beryllium.

The gem value of goshenite is relatively low. However, goshenite can be colored yellow, green, pink, blue and in intermediate colors by irradiating it with high-energy particles. The resoluting color depends on the content of Ca, Sc, Ti, V, Fe, and Co inmpurities.

Morganite

Morganite, also known as "pink beryl," "rose beryl," "pink emerald," and "cesian beryl," is a rare light pink to rose-colored gem-quality variety of beryl. Orange/yellow varieties of morganite can also be found, and color banding is common. It can be routinely heat treated to remove patches of yellow and is occasionally treated by irradiation to improve its color. The pink color of morganite is attributed to Mn²⁺ ions.

Discovery and naming

Morganite was first discovered together with other gemstone minerals, such as tourmaline and kunzite, at Pala, California, early in the twentieth century. This started a bonanza for these quite popular gemstones which drew the attention of gemologist George Frederick Kunz, who knew that pink beryl was quite a rarity. In 1911, Kunz suggested naming the pink variety of beryl "morganite" after financier J. P. Morgan.

The Rose of Maine

On October 7, 1989, one of the largest gem morganite specimens ever uncovered, eventually called "The Rose of Maine," was found at the Bennett Quarry in Buckfield, Maine. The crystal, originally somewhat orange in hue, was 23 cm long and about 30 cm across, and weighed (along with its matrix) just over 50 lbs (23 kg).

References

Further reading

• Hurlbut, Cornelius S.; Klein, Cornelis Manual of Mineralogy, 20th ed.; Wiley: New York, 1985. ISBN 0-471-80580-7.
• Sinkankas, John Emerald & Other Beryls; Geoscience Press, 1994. ISBN 0-8019-7114-4.

External links

• Minerals.net
• Mineral Galleries