Labradorite
is a variety of Anorthite
which is a member of the plagioclase Feldspars of the Feldspar Group of minerals that includes Albite,
Amazonite,
Andesine, Anorthite, Bytownite, Hyalophane, Labradorite,
Moonstone,
Oligoclase, Orthoclase, Sanidine
and Sunstone. Although
Labradorite is a variety of Anorthite, it is
actually a mixture of Albite and Anorthite with a ratio ranging from 30 : 70 to 50 : 50. It
is an intermediate member of the Albite-Anorthite Series.
Labradorite
is a plagioclase feldspar variation and is a mixture
of Albite and Anorithe, with a small percentage of Orthoclase.
Chemical analysis shows that Labradorite always has
an Anorithe contribution of 44 to 61 percent, which
in mineralogy is referred to as the ”Bøggild
Range”. Within the material, the twinned Anorithe and
Albite crystals are aligned in parallel lamellae of
varying thickness, usually according to the albite law.
Labradorite
is a member of the Plagioclase Feldspars of the Feldspar Group of minerals that also
includes Albite,
Amazonite,
Andesine, Anorthite, Bytownite, Hyalophane, Labradorite,
Moonstone,
Oligoclase, Orthoclase, Sanidine
and Sunstone. Labradorite
may be best known for the opaque variety with a color
play of iridescent colors that includes blues, greens,
gold, orange, yellow and purple. The transparent variety
ranges in color from colorless to yellow. It occassionally has
inclusions of small to microscopic particles of colloidal
copper which creates the effect of schiller. Schiller
is the reflection of light off the copper particles
suspended in the gem. This type of Labradorite with
schiller is called Sunstone.
Labradorite
was named in 1780 by Abraham Gottlob Werner (1749-1817)
from the occurrence at Ford Harbour, Paul Island, Labrador, Canada.
Labradorite is named after its location of discovery on the Isle of
Paul, near Nain, Labrador, Canada. It was discovered there in 1770 by a
Moravian missionary.
Several mines in Oregon produce transparent orange, yellow, red, blue,
green, and clear labradorite without labradorescence. These can be cut
into very nice faceted stones. Some of this material has platy
inclusions of copper in a common alignment that can produce an
aventurescent flash when played in the light. These materials are
marketed under the name "Oregon Sunstone" and have attracted a strong following from local designers and the tourist trade.
Ove
Balthasar Bøggild (1924), "On
the Labradorization of the Feldspars" Labradorite can display an iridescent optical effect, (or schiller) known as labradorescence.
Ove
Balthasar Bøggild (1872–1956) was Professor
of Mineralogy at the Mineralogical Museum of the University
of Copenhagen, Denmark.
The term
labradorization was first proposed in 1924
by Ove Balthasar Bøggild (1872–1956) in his book "On
the Labradorization of the Feldspars". He defined
labradorization as
"the peculiar reflection of the light from
submicroscopical planes orientated in one direction (rarely in two
directions); these planes have never such a position that they can be
expressed by simple indices, and they are not directly visible under the
microscope." His term labradorization later became labradorescence,
a current gemological term.
the
light entering the material and returned from its interior
is not a true optical reflection but is a diffusoin
of light spread over a range of angles.
reflection
and refraction on alternating Albite and Anorithe layers.
The reflected light waves can constructively or destructively
interfere with each other. The reason for this colour
play is that the material is made up from repeated,
microscopically thin twinned crystal lamellae, i.e.
thin layers. Such twinned structures can appear when
two types of crystal with similar structure inter-grow,
and both end up sharing the same crystal lattice. The
two different crystals form alternating, parallel layers
that are approximately 50 to 100 nm thick. On each of
these lamellae, light is partly reflected and partly
refracted – which, together with the thinness of the
layers, can cause significant interference colours.
In contrast to most other materials that exhibit this
effect, though, the interference colours are highly
directional in this case; this is one of the main characteristics
of labradorescence.
As
stated earlier, the phenomenon of Labradorite is not
limited to one particular reflection colour. In fact,
different zones of colour can usually be seen in a single
stone; it is rare to find stones with completely monochromatic
reflection patterns. The reason for this is that the
individual lamellae do not grow regularly, but that
the overall chemical composition of the mineral, and
therefore also the thickness of the individual lamellae,
usually changes slightly throughout the material.
Labradorite
often shows smooth transitions between colour zones,
while Spectrolite tends to have sharp edges between
them. However, no general rules can be given.
Gemstone varieties of labradorite exhibiting a high degree of labradorescence are called spectrolite.
One
remarkable property of Labradorescent minerals is that
their iridiscence colour hue can be predicted from their
chemical composition alone.
labradoresence a play of colors or colored reflections exhibited especially by labradorite and caused by internal structures that selectively reflect only certain colors
an iridescent play of colors similar to adularescence. This labradorescence, or schiller effect, is the result of light diffraction within the lamellar intergrowths – fine, adjacent layers of the separate materials (lamellae)
Labradorescent material is most often cut into cabochons. The
labradorescence phenomenon is best exhibited when the base of the
cabochon is parallel to the layers in the material that produce the
labradorescent flash. Careful study of the material is required so that
the finished stone will be oriented to produce a full "face up color."
If the stone is cut at any other angle, the layers that produce the
labradorescence will be inclined when the stone is viewed from directly
above. This will yield a labradorsecent flash that will appear to be
off-center.
Some specimens of sunstone are labradorite. Sunstone is a plagioclase gemstone in which tiny platelets of copper
or another mineral are arranged in a common orientation. These
platelets produce a reflective flash when incident light enters the
stone at a proper angle relative to the angle of observation.
Labradorite is the only mineral in the plagioclase series that exhibits strong labradorescence.
Some specimens of labradorite exhibit a schiller effect, which is a strong play of iridescent blue, green,
red, orange, and yellow colors as shown in the photographs above and at right. Labradorite is so well known for these
spectacular displays of color that the phenomenon is known as “labradorescence.
Labradorescence is not a display of colors reflected from the surface of
a specimen. Instead, light enters the stone, strikes a twinning
surface within the stone, and reflects from it. The color seen by the
observer is the color of light reflected from that twinning surface.
Different twinning surfaces within the stone reflect different colors of
light. Light reflecting from different twinning surfaces in various
parts of the stone can give the stone a multi-colored appearance.
Originally found at Ford Harbour, Paul Island, near Nain, off the east coast of Labrador, Canada.
Lynx
Eye is a variety of labradorite with green iridescence.
Labradorite
may be best known for the opaque variety with a color
play of iridescent colors that includes blues, greens,
gold, orange, yellow and purple. The transparent variety
ranges in color from colorless to yellow. It occassionally has
inclusions of small to microscopic particles of colloidal
copper which creates the effect of schiller. Schiller
is the reflection of light off the copper particles
suspended in the gem. This type of Labradorite with
schiller is called Sunstone.
Labradorite is named after the location
of its first discovery; Labrador, Canada. Plagioclase is from the Greek meaning oblique cleavage.
Feldspar is from the Swedish feldt + spat
meaning that it was found in fields overlying granite.
Labradorite
distribution: widespread. From Ford Harbour,
Pauls Island, Labrador, Newfoundland; at Lake St. John,
Quebec; and elsewhere in Canada. In the USA, especially
in northern New York, forming the Adirondack Mountains;
crystals from Sagebrush Flat, about 37 km north of Plush,
Lake County, Oregon; atop the San Marcos Mountains,
San Diego County, and in the western San Gabriel Mountains,
Los Angeles County, California. Abundant gem crystals
in the Pinacate volcanic field, Sonora, Mexico. At Vesuvius,
Campania, and on Mt. Etna, Sicily, Italy. From Ylämaa,
near Lappeenranta, Finland. In the Langesundsfjord-
Larvik-Tvedalen area, Norway. On Surtsey Island, south
of Iceland.
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