Beryl

Beryl is a mineral composed of beryllium aluminium silicate with the chemical formula Be3Al2(SiO3)6. Well-known varieties of beryl include emerald and aquamarine. Naturally occurring hexagonal crystals of beryl can be up to several meters in size, but terminated crystals are relatively rare. Pure beryl is colorless, but it is frequently tinted by impurities; possible colors are green, blue, yellow, pink, and red (the rarest). It is an ore source of beryllium.

Beryl

General

Category Cyclosilicate

Formula Be3Al2(SiO3)6

IMA symbol Brl

Strunz classification 9.CJ.05

Crystal system Hexagonal

Crystal class Dihexagonal dipyramidal (6/mmm)

H-M symbol: (6/m 2/m 2/m)

Space group P6/mcc

Unit cell a = 9.21 Å, c = 9.19 Å; Z = 2

Identification

Formula mass 537.50 g/mol

Color Green, blue, yellow, colorless, pink, and others

Crystal habit Prismatic to tabular crystals; radial, columnar; granular to compact massive

Twinning Rare

Cleavage Imperfect on {0001}

Fracture Conchoidal to irregular

Tenacity Brittle

Mohs scale hardness 7.5–8.0

Luster Vitreous to resinous

Streak White

Diaphaneity Transparent to translucent

Specific gravity 2.63–2.92

Optical properties Uniaxial (−)

Refractive index nω = 1.564–1.595

nε = 1.568–1.602

Birefringence δ = 0.0040–0.0070

Pleochroism Weak to distinct

Ultraviolet fluorescence None (some fracture-filling materials used to improve emerald's clarity do fluoresce, but the stone itself does not). Morganite has weak violet fluorescence.

Etymology

The word beryl – Middle English: beril – is borrowed, via Old French: beryl and Latin: beryllus, from Ancient Greek βήρυλλος bḗryllos, which referred to various blue-green stones, from Prakrit veruḷiya, veḷuriya 'beryl' which is ultimately of Dravidian origin, maybe from the name of Belur or Velur, a town in Karnataka, southern India. The term was later adopted for the mineral beryl more exclusively.

When the first eyeglasses were constructed in 13th-century Italy, the lenses were made of beryl (or of rock crystal) as glass could not be made clear enough. Consequently, glasses were named Brille in German (bril in Dutch and briller in Danish).

Classification

Already in the outdated 8th edition of the mineral classification according to Strunz, beryl belonged to the mineral class of “silicates and germanates” and there to the division of “ ring silicates (cyclosilicates)” (with six-membered rings [Si 6 O 18 ] 12−), where it formed the name giver of the “beryl series” with the system no. VIII/C.06a and the other members bazzite and indialite within the “beryl-cordierite group” (no. VIII/C.06).

In the Lapis mineral catalog by Stefan Weiß, last revised and updated in 2018, which, out of consideration for private collectors and institutional collections, still follows this old form of Karl Hugo Strunz 's system, the mineral was given the system and mineral number VIII/E.12-10. In the "Lapis Systematics," this also corresponds to the "Ring Silicates" section, with minerals whose structure consists of six-membered rings [Si 6 O 18 ] 12− being classified in groups VIII/E.12 to 21. Beryl forms an independent, but unnamed group here, together with bazzite, bunnoite, cordierite, ferroindialite, indialite, pezzottaite, sekaninaite, and stoppaniite. 

The 9th edition of Strunz's mineral classification, valid since 2001 and last updated by the International Mineralogical Association (IMA) in 2009, also classifies beryl in the division of "ring silicates". However, this division is further subdivided according to the structure of the silicate rings, so that the mineral, according to its structure, can be found in the subdivision "[Si 6 O 18 ] 12− six-membered simple rings without island-like, complex anions ", where it also forms the eponymous "beryl group" with the system number 9.CJ.05 and the other members bazzite, indialite, pezzottaite and stoppaniite.

Dana's classification of minerals, which is predominantly used in English-speaking countries, also classifies beryl in the class of "silicates and germanates" and within the division of "ring silicates: six-membered rings." Here, it is also found as the namesake of the "beryl group" with the system number 61.01.01, along with the other members bazzite, indialite, stoppaniite, and pezzottaite, within the subdivision " ring silicates: six-membered rings with Si 6 O 18 rings; possible (OH) and Al substitution."

Chemistry

In 100% pure form, which can only be produced synthetically, beryl consists of around 19% aluminum oxide (Al 2 O 3), 14% beryllium oxide (BeO) and 67% silicon dioxide (SiO 2).

Natural beryl can contain various impurities, including rubidium oxide (Rb 2 O) and cesium oxide (Cs 2 O). Water of crystallization (H 2 O, up to 3%), as well as argon and helium, can also be embedded in the channels of the ring silicate structure. 

Other possible impurities include lithium and sodium as well as various oxides and hydroxides, halides and/or fluorides. 

Crystal habit and structure

Beryl belongs to the hexagonal crystal system. Normally beryl forms hexagonal columns but can also occur in massive habits. As a cyclosilicate beryl incorporates rings of silicate tetrahedra of SiO3–O (the connected O is from another SiO3) that are arranged in columns along the C axis and as parallel layers perpendicular to the C axis, forming channels along the C axis. These channels permit a variety of ions, neutral atoms, and molecules to be incorporated into the crystal thus disrupting the overall charge of the crystal permitting further substitutions in aluminium, silicon, and beryllium sites in the crystal structure. These impurities give rise to the variety of colors of beryl that can be found. Increasing alkali content within the silicate ring channels causes increases to the refractive indices and birefringence.

Beryl crystallizes hexagonally in the space group P 6 /mcc (space group no. 192) with lattice parameters a = 9.22 Å and c = 9.20 Å and two formula units per unit cell. The crystal structure of beryl consists of six-membered single rings with the structural formula [Si 6 O 18 ] 12−, which are concentrically stacked on top of each other in the direction of the c-axis and each rotated by 30°. Due to the concentric arrangement of the rings, open channels with a diameter of a few Ångströms are formed. The various impurities are interchangeably embedded in these hollow channels. The aluminum and beryllium ions are located between the rings, with aluminum being surrounded by six oxygen ions and beryllium by four oxygen ions. Therefore, aluminum and beryllium are also referred to as and coordination, respectively. 

Characteristics

Morphology

The crystal morphology of beryl is predominantly simple and is characterized by the hexagonal prism {10 1 0} and the terminal pinacoid {0001}. In addition, hexagonal-dipyramidal forms in the first and second positions {11 2 1} and {10 1 1} as well as the dihexagonal-dipyramidal full form (holohedry) are occasionally found. 

The habit can be short- to long-prismatic, with the prism surfaces often longitudinally striated. Occasionally, stem-like and granular to firm masses are also found. 

Physical and chemical properties

The melting point of beryl is 1650 °C. Beryl is insensitive to various acids and only slightly soluble in hydrogen fluoride (HF). However, it is sensitive to alkaline solutions and is therefore soluble in, among others, sodium hydroxide (NaOH, caustic soda) and potassium hydroxide (KOH, caustic potash). 

Varieties

The colorless pure form of beryl, like the pure form of quartz (rock crystal), has its own name and is known as goshenite. Colorless beryls, however, are very rare.

Far better known as colored varieties are the green emerald, the blue aquamarine, the yellow golden beryl (also heliodor) and the pink morganite, which was named after the New York banker John Pierpont Morgan (not to be confused with moganite from the silicic acid family).

A pinkish, caesium-containing beryl variety is called vorobyevite or worobieffite or simply caesium beryl.

Also very rare is the variety Red Beryl, whose outdated name Bixbit is one of the undesirable trade names according to the CIBJO regulations due to the clear risk of confusion with the mineral Bixbyite.

Aquamarine and maxixe

Aquamarine (from Latin: aqua marina, "sea water") is a blue or cyan variety of beryl. It occurs at most localities which yield ordinary beryl. The gem-gravel placer deposits of Sri Lanka contain aquamarine. Green-yellow beryl, such as that occurring in Brazil, is sometimes called chrysolite aquamarine. The deep blue version of aquamarine is called maxixe (pronounced mah-she-she). Its color results from a radiation-induced color center.

The pale blue color of aquamarine is attributed to Fe2+. 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 between Fe3+ and Fe2+.

In the United States, aquamarines can be found at the summit of Mount Antero in the Sawatch Range in central Colorado, and in the New England and North Carolina pegmatites. Aquamarines are also present in the state of Wyoming, aquamarine has been discovered in the Big Horn Mountains, near Powder River Pass. Another location within the United States is the Sawtooth Range near Stanley, Idaho, although the minerals are within a wilderness area which prevents collecting. In Brazil, there are mines in the states of Minas Gerais, Espírito Santo, and Bahia, and minorly in Rio Grande do Norte. The mines of Colombia, Skardu Pakistan, Madagascar, Russia, Namibia, Zambia, Malawi, Tanzania, and Kenya also produce aquamarine.

Emerald

Emerald is green beryl, colored by around 2% chromium and sometimes vanadium. Most emeralds are highly included, so their brittleness (resistance to breakage) is classified as generally poor.

The modern English word "emerald" comes via Middle English emeraude, imported from modern French via Old French ésmeraude and Medieval Latin esmaraldus, from Latin smaragdus, from Greek σμάραγδος smaragdos meaning 'green gem'.

Emeralds in antiquity were mined by the Egyptians and in what is now Austria, as well as Swat in contemporary 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 rarest emeralds come from the two main emerald belts in the Eastern Ranges of the Colombian Andes: Muzo and Coscuez west of the Altiplano Cundiboyacense, and Chivor and Somondoco to the east. 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 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 millimetre (0.039 in) per month, a typical seven-month growth run producing emerald crystals of 7 mm of thickness. The green color of emeralds is widely attributed to presence of Cr3+ ions. Intensely green beryls from Brazil, Zimbabwe and elsewhere in which the color is attributed to vanadium have also been sold and certified as emeralds.

Golden beryl and heliodor

Golden beryl can range in colors from pale yellow to a brilliant gold. Unlike emerald, golden beryl generally 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 2,054-carat (410.8 g) stone on display in the Hall of Gems, Washington, D.C., United States.

Goshenite

Colorless beryl is called goshenite. The name originates from Goshen, Massachusetts, where it was originally discovered. In the past, goshenite was used for manufacturing eyeglasses and lenses owing to its transparency. Nowadays, it is most commonly used for gemstone purposes.

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 resulting color depends on the content of Ca, Sc, Ti, V, Fe, and Co impurities.

Morganite

Morganite, also known as "pink beryl", "rose beryl", "pink emerald" (which is not a legal term according to the new Federal Trade Commission Guidelines and Regulations), and "cesian (or caesian) 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.

Red beryl

Red variety of beryl (the "bixbite") was first described in 1904 for an occurrence, its type locality, at Maynard's Claim (Pismire Knolls), Thomas Range, Juab County, Utah. The dark red color is attributed to Mn3+ ions. Old synonym "bixbite" is deprecated from the CIBJO because of the possibility of confusion with the mineral bixbyite (both named after mineralogist Maynard Bixby). Red "bixbite" beryl formerly was marketed as "red" or "scarlet emerald", but these terms involving "Emerald" terminology are now prohibited in the US.

Red beryl is very rare and has only been reported from a handful of North American locations: Wah Wah Mountains, Beaver County, Utah; Paramount Canyon, Round Mountain, Juab County, Utah; and Sierra County, New Mexico, although this locality does not often produce gem-grade stones. The bulk of gem-grade red beryl comes from the Ruby-Violet Claim in the Wah Wah Mts. of midwestern Utah, discovered in 1958 by Lamar Hodges, of Fillmore, Utah, while he was prospecting for uranium. Red beryl has been known to be confused with pezzottaite, a caesium analog of beryl, found in Madagascar and, more recently, Afghanistan; cut gems of the two varieties can be distinguished by their difference in refractive index, and the rough crystals easily by their differing crystal systems (pezzottaite trigonal, red beryl hexagonal). Synthetic red beryl is also produced. Like emerald and unlike most other varieties of beryl, the red ones are usually highly included.

While gem beryls are ordinarily found in pegmatites and certain metamorphic stones, red beryl occurs in topaz-bearing rhyolites. It is formed by crystallizing under low pressure and high temperature from a pneumatolytic phase along fractures or within near-surface miarolitic cavities of the rhyolite. Associated minerals include bixbyite, quartz, orthoclase, topaz, spessartine, pseudobrookite and hematite.

Deposits

Beryl is a common mineral, and it is widely distributed in nature. It is found most commonly in granitic pegmatites, but also occurs in mica schists, such as those of the Ural Mountains, and in limestone in Colombia. It is less common in ordinary granite and is only infrequently found in nepheline syenite. Beryl is often associated with tin and tungsten ore bodies formed as high-temperature hydrothermal veins. In granitic pegmatites, beryl is found in association with quartz, potassium feldspar, albite, muscovite, biotite, and tourmaline. Beryl is sometimes found in metasomatic contacts of igneous intrusions with gneiss, schist, or carbonate rocks. Common beryl, mined as beryllium ore, is found in small deposits in many countries, but the main producers are Russia, Brazil, and the United States.

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 by 1.2 m (18.0 by 3.9 ft) with a mass of around 18 tonnes (20 short tons); it is New Hampshire's state mineral. As of 1999, the world's largest known naturally occurring crystal of any mineral is a crystal of beryl from Malakialina, Madagascar, 18 m (59 ft) long and 3.5 m (11 ft) in diameter, and weighing 380,000 kg (840,000 lb).

Use as a gemstone

Beryls of all color varieties are processed into gemstones if they are of good quality. However, emerald was one of the first varieties to be used for this purpose and mined in large quantities. The oldest mines date back to around 1300 BC.

Clear gemstones are usually faceted. However, the significant pleochroism exhibited by some beryl varieties must be taken into account when cutting.

Translucent or opaque stones are given a cabochon cut. Larger mineral aggregates are sometimes processed into decorative objects.

Use as a raw material

Beryl is the main source of the toxic light metal beryllium, which is used, among other things, in aerospace technology as a component of special alloys. More than 80 percent of the world's annual production comes from the USA. In the Middle Ages, beryl was also ground into lenses, which were used in eyeglasses, giving them their name.

Human health impact

Beryl is a beryllium compound that is a known carcinogen with acute toxic effects leading to pneumonitis when inhaled. Care must thus be used when mining, handling, and refining these gems.

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