Chrysoberyl
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| Chrysoberyl |
 |
| General |
| Category |
Oxide minerals |
Formula
(repeating unit) |
BeAl2O4 |
| Strunz classification |
04.BA.05 |
| Crystal symmetry |
Orthorhombic 2/m2/m2/m dipyramidal |
| Unit cell |
a = 5.481 Å, b = 9.415 Å, c = 4.428 Å; Z = 8 |
| Identification |
| Color |
Various shades of green, yellow, brownish to greenish black, may be
raspberry-red under incandescent light when chromian; colorless, pale
shades of yellow, green, or red in transmitted light |
| Crystal habit |
Crystals tabular or short prismatic, prominently striated |
| Crystal system |
Orthorhombic |
| Twinning |
Contact and penetration twins common, often repeated forming rosette structures |
| Cleavage |
Distinct on (110), imperfect on (010), poor on {001} |
| Fracture |
Conchoidal to uneven |
| Tenacity |
Brittle |
| Mohs scale hardness |
8.5 |
| Luster |
Vitreous |
| Streak |
White |
| Specific gravity |
3.5 - 3.84 |
| Optical properties |
Biaxial (+) |
| Refractive index |
nα=1.745 nβ=1.748 nγ=1.754 |
| Pleochroism |
X = red; Y = yellow-orange; Z = emerald-green |
| 2V angle |
Measured: 70° |
| References |
[1][2][3] |
| Major varieties |
| Alexandrite |
Color change; green to red |
| Cymophane |
Chatoyant |
The
mineral or
gemstone chrysoberyl is an
aluminate of
beryllium with the formula
BeAl2O4.
[3] The name chrysoberyl is derived from the
Greek words χρυσός
chrysos and βήρυλλος
beryllos, meaning "a gold-white spar". Despite the similarity of their names, chrysoberyl and
beryl
are two completely different gemstones. Chrysoberyl is the
third-hardest frequently encountered natural gemstone and lies at 8.5 on
the
hardness scale, between
corundum (9) and
topaz (8).
[4]
An interesting feature of its crystals are the
cyclic twins called
trillings. These twinned crystals have a
hexagonal
appearance, but are the result of a triplet of twins with each "twin"
oriented at 120° to its neighbors and taking up 120° of the cyclic
trilling. If only two of the three possible twin orientations are
present, a "V"-shaped twin results.
Ordinary chrysoberyl is yellowish-green and transparent to
translucent.
When the mineral exhibits good pale green to yellow color and is
transparent, then it is used as a gemstone. The three main varieties of
chrysoberyl are: ordinary yellow-to-green chrysoberyl, cat's eye or
cymophane, and
alexandrite. Yellow-green chrysoberyl was referred to as "chrysolite" during the
Victorian and Edwardian eras, which caused confusion since that name has also been used for the mineral
olivine ("
peridot" as a gemstone); that name is no longer used in the
gemological nomenclature.
Alexandrite, a strongly
pleochroic (trichroic) gem, will exhibit
emerald green, red and orange-yellow colors depending on viewing direction in partially
polarised light.
However, its most distinctive property is that it also changes color in
artificial (tungsten/halogen) light compared to daylight. The color
change from red to green is due to strong absorption of light in a
narrow yellow portion of the spectrum, while allowing large bands of
blue-greener and red
wavelengths
to be transmitted. Which of these prevails to give the perceived hue
depends on the spectral balance of the illumination. Typically,
alexandrite has an emerald-green color in daylight (relatively blue
illumination of high
color temperature) but exhibits a raspberry-red color in
incandescent light (relatively yellow illumination).
Cymophane is popularly known as "cat's eye". This variety exhibits pleasing
chatoyancy or opalescence that reminds one of an
eye of a cat. When cut to produce a
cabochon, the mineral forms a light-green specimen with a silky band of light extending across the surface of the stone.
Occurrence
Chrysoberyl forms as a result of
pegmatitic processes. Melting in the
Earth's crust produces relatively low-density molten
magma which can rise upwards towards the surface. As the main
magma
body cools, water originally present in low concentrations became more
concentrated in the molten rock because it could not be incorporated
into the
crystallization
of solid minerals. The remnant magma thus becomes richer in water, and
also in rare elements that similarly do not fit in the crystal
structures of major rock-forming minerals. The water extends the
temperature range downwards before the magma becomes completely solid,
allowing concentration of rare elements to proceed so far that they
produce their own distinctive minerals. The resulting rock, igneous in
appearance but formed at a low temperature from a water-rich melt, with
large crystals of the common minerals such as
quartz and
feldspar, but also with elevated concentrations of rare elements such as
beryllium,
lithium, or
niobium, often forming their own minerals, is called a
pegmatite.
The high water content of the magma made it possible for the crystals
to grow quickly, so pegmatite crystals are often quite large, which
increases the likelihood of gem specimens forming.
Chrysoberyl can also grow in the
country rocks near to pegmatites, when Be- and Al-rich fluids from the pegmatite react with surrounding minerals. Hence, it can be found in
mica schists and in contact with metamorphic deposits of
dolomitic
marble. Because it is a hard, dense mineral that is resistant to
chemical alteration, it can be weathered out of rocks and deposited in
river sands and gravels in alluvial deposits with other gem minerals
such as
diamond,
corundum,
topaz,
spinel,
garnet, and
tourmaline. When found in such
placers, it will have rounded edges instead of sharp, wedge-shape forms. Much of the chrysoberyl mined in
Brazil and
Sri Lanka is recovered from placers, as the host rocks have been intensely weathered and eroded.
If the pegmatite fluid is rich in
beryllium, crystals of
beryl
or chrysoberyl could form. Beryl has a high ratio of beryllium to
aluminium, while the opposite is true for chrysoberyl. Both are stable
with the common mineral quartz. For alexandrite to form, some
chromium
would also have had to be present. However, beryllium and chromium do
not tend to occur in the same types of rock. Chromium is commonest in
mafic and
ultramafic rocks in which beryllium is extremely rare. Beryllium becomes concentrated in
felsic
pegmatites in which chromium is almost absent. Therefore, the only
situation where an alexandrite can grow is when Be-rich pegmatitic
fluids react with Cr-rich country rock. This unusual requirement
explains the rarity of this chrysoberyl variety.
Alexandrite
The alexandrite variety displays a color change (alexandrite effect)
dependent upon the nature of ambient lighting. Alexandrite effect is the
phenomenon of an observed color change from greenish to reddish with a
change in source illumination due physiological response of the human
eye in a particular part of the visible spectrum. This color change is
independent of any change of hue with viewing direction through the
crystal that would arise from
pleochroism. Alexandrite results from small scale replacement of aluminium by
chromium
ions in the crystal structure, which causes intense absorption of light
over a narrow range of wavelengths in the yellow region of the
spectrum.
[citation needed]
Alexandrite from the
Ural Mountains in
Russia is green by daylight and red by incandescent light. Other varieties of alexandrite may be yellowish or pink in daylight and a
columbine or
raspberry red by incandescent light.
Alexandrite step cut cushion, 26.75 cts.
According to a popular but controversial story, alexandrite was discovered by the Finnish mineralogist
Nils Gustaf Nordenskiöld (1792–1866), and named alexandrite in honor of the future
Tsar Alexander II of Russia.
Nordenskiöld's initial discovery occurred as a result of an examination
of a newly found mineral sample he had received from Perovskii, which
he identified as emerald at first.
[5] The first emerald mine had been opened in 1831.
Alexandrite up to 5 carats (1,000 mg) and larger were traditionally
thought to be found only in the Ural Mountains, but have since been
found in larger sizes in Brazil. Other deposits are located in
India (
Andhra Pradesh),
Madagascar, and
Sri Lanka. Alexandrite in sizes over three carats are very rare.
Some gemstones described as lab-grown (synthetic) alexandrite are actually
corundum laced with trace elements (e.g.,
vanadium) or color-change
spinel
and are not actually chrysoberyl. As a result, they would be more
accurately described as simulated alexandrite rather than synthetic, but
are often called
Czochralski alexandrite after the process that grows the crystals.
[6]
Cymophane
Fine color Cymophane with a sharp and centered eye.
Translucent yellowish
chatoyant chrysoberyl is called
cymophane or
cat's eye.
Cymophane has its derivation also from the Greek words meaning 'wave'
and 'appearance', in reference to the chatoyancy sometimes exhibited. In
this variety, microscopic tubelike cavities or needlelike inclusions
[7] of
rutile
occur in an orientation parallel to the c-axis producing a chatoyant
effect visible as a single ray of light passing across the crystal. This
effect is best seen in gemstones cut in
cabochon form perpendicular to the c-axis. The color in yellow chrysoberyl is due to Fe
3+ impurities.
Although other minerals such as
tourmaline,
scapolite,
corundum,
spinel and
quartz
can form "cat's eye" stones similar in appearance to cymophane, the
jewelry industry designates these stones as "quartz cat's eyes", or
"ruby cat's eyes" and only chrysoberyl can be referred to as "cat's eye"
with no other designation.
Gems lacking the silky inclusions required to produce the cat's eye
effect are usually faceted. An alexandrite cat's eye is a chrysoberyl
cat's eye that changes color. "Milk and honey" is a term commonly used
to describe the color of the best cat's eyes. The effect refers to the
sharp milky ray of white light normally crossing the cabochon as a
center line along its length and overlying the honey colored background.
The honey color is considered to be top-grade by many gemologists but
the lemon yellow colors are also popular and attractive. Cat's eye
material is found as a small percentage of the overall chrysoberyl
production wherever chrysoberyl is found.
Cat's eye really became popular by the end of the 19th century when the
Duke of Connaught
gave a ring with a cat's eye as an engagement token; this was
sufficient to make the stone more popular and increase its value
greatly. Until that time, cat's eye had predominantly been present in
gem and mineral collections. The increased demand in turn created an
intensified search for it in
Sri Lanka. Early 20th century prices could go up as high as $8000 for a cut stone.
[8]
See also
References
- ^ Handbook of Mineralogy
- ^ Mindat.org
- ^ a b Webmineral data
- ^ Klein, Cornelis; and Cornelius S. Hurlbut, Jr. (1985). Manual of Mineralogy (20th ed. ed.). New York: Wiley. ISBN 0-471-80580-7.
- ^ ""Alexandrite or Diaphanite?". In Alexandrite Tsarstone Collectors Guide.". 2006. Retrieved 2007-07-09.
- ^ "Alexandrite Synthetics and Imitations, In Alexandrite Tsarstone Collectors Guide.". 2006. Retrieved 2007-07-09.
- ^ "Mitchell, T. E. and Marder, J. M., "Precipitation in Cat's-Eye Chrysoberyl," Electron Microscopy Soc. Proceedings, 1982.
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