Tourmaline
From Wikipedia, the free encyclopedia
| Tourmaline |
Schorl Tourmaline |
| General |
| Category |
Cyclosilicate |
Formula
(repeating unit) |
(Ca,K,Na,[])(Al,Fe,Li,Mg,Mn)3(Al,Cr, Fe,V)6
(BO3)3(Si,Al,B)6O18(OH,F)4
[1][2] |
| Identification |
| Color |
Most commonly black, but can range from brown, violet, green, pink, or in a dual-colored pink and green. |
| Crystal habit |
Parallel and elongated. Acicular prisms, sometimes radiating. Massive. Scattered grains (in granite). |
| Crystal system |
Trigonal |
| Cleavage |
Indistinct |
| Fracture |
Uneven, small conchoidal, brittle |
| Mohs scale hardness |
7–7.5 |
| Luster |
Vitreous, sometimes resinous |
| Streak |
White |
| Specific gravity |
3.06 (+.20 -.06)[1] |
| Density |
2.82–3.32 |
| Polish luster |
Vitreous[1] |
| Optical properties |
Double refractive, uniaxial negative[1] |
| Refractive index |
nω=1.635–1.675, nε=1.610–1.650 |
| Birefringence |
-0.018 to -0.040; typically about .020 but in dark stones it may reach .040[1] |
| Pleochroism |
typically moderate to strong[1]
Red Tourmaline: Definite; dark red,light red
Green Tourmaline: Strong; dark green, yellow-green
Brown Tourmaline: Definite; dark brown, light brown
Blue Tourmaline: Strong; dark blue, light blue |
| Dispersion |
.017[1] |
| Ultraviolet fluorescence |
pink stones—inert to very weak red to violet in long and short wave[1] |
| Absorption spectra |
a strong narrow band at 498 nm, and almost complete absorption of
red down to 640nm in blue and green stones; red and pink stones show
lines at 458 and 451nm as well as a broad band in the green spectrum[1] |
Tourmaline (
tur-mah-Leen) is a
crystal boron silicate mineral compounded with elements such as
aluminium, iron,
magnesium,
sodium,
lithium, or
potassium. Tourmaline is classified as a
semi-precious stone and the
gemstone comes in a wide variety of colors. The name comes from the
Sinhalese word "Thuramali" (තුරමලි) or "Thoramalli" (තෝරමල්ලි), which applied to different gemstones found in
Sri Lanka.
History
Brightly colored
Sri Lankan gem tourmalines were brought to Europe in great quantities by the
Dutch East India Company to satisfy a demand for curiosities and gems. At the time it was not realised that
schorl and tourmaline were the same mineral.
Tourmaline species and varieties
Commonly encountered species and varieties
- Schorl species:
- Bluish or brownish black to Black—schorl
- Dravite species: from the Drave district of Carinthia
- Dark yellow to brownish black—dravite
- Elbaite species: named after the island of Elba, Italy
- Red or pinkish-red—rubellite variety (from ruby)
- Light blue to bluish green—Brazilian indicolite variety (from indigo)
- Green—verdelite or Brazilian emerald variety
- Colorless—achroite variety (from the Greek "άχρωμος" meaning "colorless")
Schorl
The most common species of tourmaline is
schorl. It may
account for 95% or more of all tourmaline in nature. The early history
of the mineral schorl shows that the name "schorl" was in use prior to
1400 because a village known today as
Zschorlau (in
Saxony, Germany) was then named "Schorl" (or minor variants of this name). This village had a nearby
tin mine where, in addition to
cassiterite,
black tourmaline was found. The first description of schorl with the
name "schürl" and its occurrence (various tin mines in the
Saxony Ore Mountains) was written by
Johannes Mathesius (1504–1565) in 1562 under the title "Sarepta oder Bergpostill".
[3] Up to about 1600, additional names used in the
German language were "Schurel", "Schörle", and "Schurl". Beginning in the 18th century, the name
Schörl was mainly used in the German-speaking area. In English, the names
shorl and
shirl were used in the 18th century. In the 19th century the names
common schorl,
schörl,
schorl and
iron tourmaline were the English words used for this mineral.
[3] The word tourmaline has two
etymologies, both from the
Sinhalese word
turamali, meaning "stone attracting ash" (a reference to its
pyroelectric properties) or according to other sources "mixed gemstones".
Dravite
Black Dravite on a grey matrix
The name
dravite was used for the first time by
Gustav Tschermak (1836–1927), Professor of
Mineralogy and
Petrography at the
University of Vienna, in his book
Lehrbuch der Mineralogie (published in 1884) for
magnesium-rich (and
sodium-rich) tourmaline from the village Unterdrauburg,
Drava river area,
Carinthia,
Austro-Hungarian Empire.
Today this tourmaline locality (type locality for dravite) at the
village Dravograd (near Dobrova pri Dravogradu), is a part of the
Republic of Slovenia.
[4]
Tschermak gave this tourmaline the name dravite, for the Drava river
area, which is the district along the Drava River (in German: Drau, in
Latin: Drave) in
Austria and
Slovenia. The chemical composition which was given by Tschermak in 1884 for this dravite approximately corresponds to the formula NaMg
3(Al,Mg)
6B
3Si
6O
27(OH), which is in good agreement (except for the
OH content) with the endmember formula of dravite as known today.
[4]
Elbaite
A lithium-tourmaline (
elbaite) was one of three pegmatitic minerals from
Utö,
Sweden, in which the new alkali element
lithium (Li) was determined in 1818 by
Johan August Arfwedson for the first time.
[5] Elba Island,
Italy, was one of the first localities where colored and colorless Li-tourmalines were extensively chemically analysed. In 1850
Karl Friedrich August Rammelsberg described
fluorine
(F) in tourmaline for the first time. In 1870 he proved that all
varieties of tourmaline contain chemically bound water. In 1889
Scharitzer proposed the substitution of (OH) by F in red Li-tourmaline
from
Sušice,
Czech Republic. In 1914
Vladimir Vernadsky proposed the name
Elbait for lithium-, sodium-, and aluminum-rich tourmaline from Elba Island, Italy, with the simplified formula (Li,Na)HAl
6B
2Si
4O
21.
[5] Most likely the type material for elbaite was found at Fonte del Prete, San Piero in Campo,
Campo nell'Elba,
Elba Island,
Province of Livorno,
Tuscany,
Italy.
[5] In 1933 Winchell published an updated formula for elbaite, H
8Na
2Li
3Al
3B
6Al
12Si
12O
62, which is commonly used to date written as Na(Li
1.5Al
1.5)Al
6(BO
3)
3[Si
6O
18](OH)
3(OH).
[5]
The first crystal structure determination of a Li-rich tourmaline was
published in 1972 by Donnay and Barton, performed on a pink elbaite from
San Diego County,
California, United States.
Chemical composition of the tourmaline group
The tourmaline mineral group is chemically one of the most complicated groups of
silicate minerals. Its composition varies widely because of
isomorphous replacement (solid solution), and its general formula can be written as
XY
3Z
6(T
6O
18)(BO
3)
3V
3W,
where:
[6]
X =
Ca,
Na,
K,
៛ = vacancy
Y =
Li,
Mg,
Fe2+,
Mn2+,
Zn,
Al,
Cr3+,
V3+,
Fe3+,
Ti4+, vacancy
Z = Mg, Al, Fe
3+, Cr
3+, V
3+
T =
Si, Al,
B
B = B, vacancy
V =
OH, O
W = OH,
F, O
Large pink elbaite crystal on quartz, Cryo-Genie Mine, San Diego Co., California, US.
A revised nomenclature for the tourmaline group was published in 2011.
[7][8][9]
Physical properties
Crystal structure
Tri-color elbaite crystals on quartz, Himalaya Mine, San Diego Co., California, US
Tourmaline belongs to the
trigonal crystal system and occurs as long, slender to thick prismatic and columnar
crystals
that are usually triangular in cross-section. The style of termination
at the ends of crystals is asymmetrical, called hemimorphism. Small
slender prismatic crystals are common in a fine-grained
granite called
aplite,
often forming radial daisy-like patterns. Tourmaline is distinguished
by its three-sided prisms; no other common mineral has three sides.
Prisms faces often have heavy vertical striations that produce a rounded
triangular effect. Tourmaline is rarely perfectly
euhedral. An exception was the fine dravite tourmalines of
Yinnietharra, in western Australia. The deposit was discovered in the 1970s, but is now exhausted. All hemimorphic crystals are
piezoelectric, and are often
pyroelectric as well.
Color
Tourmaline gemstones - Mozambique
Tourmaline has a variety of colors. Usually, iron-rich tourmalines
are black to bluish-black to deep brown, while magnesium-rich varieties
are brown to yellow, and lithium-rich tourmalines are almost any color:
blue, green, red, yellow, pink, etc. Rarely, it is colorless. Bi-colored
and multicolored crystals are common, reflecting variations of fluid
chemistry during crystallization. Crystals may be green at one end and
pink at the other, or green on the outside and pink inside; this type is
called
watermelon tourmaline. Some forms of tourmaline are
dichroic, in that they change color when viewed from different directions.
Two dark green rectangular tourmaline stones and one oval tourmaline stone.
The pink color of tourmalines from many fields is the result of
prolonged natural irradiation. During their growth, these tourmaline
crystals incorporated
Mn2+ and were initially very pale. Due to natural
gamma ray exposure from
radioactive decay of
40K in their
granitic environment, gradual formation of Mn
3+ ions occurs, which is responsible for the deepening of the pink to red color.
[10]
Treatments
Some tourmaline gems, especially pink to red colored stones, are altered by
irradiation
to improve their color. Irradiation is almost impossible to detect in
tourmalines, and does not, currently, impact the value. Heat treatment
is also used to enhance tourmaline. Heavily-included tourmalines, such
as rubellite and Brazilian paraiba, are sometimes clarity-enhanced. A
clarity-enhanced tourmaline (especially paraiba) is worth much less than
a non-treated gem.
[11]
Geology
Tourmaline is found in
granite and granite
pegmatites and in
metamorphic rocks such as
schist and
marble.
Schorl and lithium-rich tourmalines are usually found in granite and
granite pegmatite. Magnesium-rich tourmalines, dravites, are generally
restricted to schists and marble. Tourmaline is a durable mineral and
can be found in minor amounts as grains in
sandstone and
conglomerate, and is part of the
ZTR index for highly-weathered sediments.
Tourmaline localities
Bi-colored tourmaline crystal, 0.8 inches (2 cm) long.
Gem and specimen tourmaline is mined chiefly in Brazil and Africa.
Some placer material suitable for gem use comes from Sri Lanka. In
addition to Brazil, tourmaline is mined in
Tanzania,
Nigeria,
Kenya,
Madagascar,
Mozambique,
Namibia, Afghanistan, Pakistan,
Sri Lanka, and
Malawi.
[12]
United States
Some fine gems and specimen material has been produced in the United
States, with the first discoveries in 1822, in the state of
Maine.
California became a large producer of tourmaline in the early 1900s.
The Maine deposits tend to produce crystals in raspberry pink-red as
well as minty greens. The California deposits are known for bright
pinks, as well as bicolors. During the early 1900s, Maine and California
were the world's largest producers of gem tourmalines. The Empress
Dowager
Tz'u Hsi
of China loved pink tourmaline and bought large quantities for
gemstones and carvings from the then new Himalaya Mine, located in
San Diego County, California.
[13] It is not clear when the first tourmaline was found in California.
Native Americans
have used pink and green tourmaline as funeral gifts for centuries. The
first documented case was in 1890 when Charles Russel Orcutt found pink
tourmaline at what later became the Stewart Mine at
Pala,
San Diego.
[14]
Brazil
Watermelon Tourmaline mineral on quartz matrix (crystal approximately 2 cm wide at face)
Almost every color of tourmaline can be found in Brazil, especially in the Brazilian states of
Minas Gerais and
Bahia. In 1989, miners discovered a unique and brightly colored variety of tourmaline in the state of
Paraíba.
[citation needed]
The new type of tourmaline, which soon became known as paraiba
tourmaline, came in blue and green. Brazilian paraiba tourmaline is
usually contains abundant inclusions. Much of the paraiba tourmaline
from Brazil actually comes from the neighboring state of
Rio Grande do Norte.
Material from Rio Grande do Norte is often somewhat less intense in
color, but many fine gems are found there. It was determined that the
element copper was important in the coloration of the stone.
[15]
World's largest
A large cut tourmaline from Paraiba, measuring 36.44 x 33.75 x
21.85 mm (1.43 x 1.33 x 0.86 in) and weighing 191.87 carats, was
included in the
Guinness World Records.
[16] The large natural gem, owned by Billionaire Business Enterprises,
[16] is a bluish-green in color. The flawless oval shaped cut stone was presented in Montreal, Quebec, Canada on 14 October 2009.
[17]
Africa
Tourmaline mineral (approximately 10 cm tall)
In the late 1990s, copper-containing tourmaline was found in
Nigeria.
The material was generally paler and less saturated than the Brazilian
materials, although the material generally was much less included. A
more recent African discovery from
Mozambique has also produced beautiful tourmaline colored by copper, similar to the Brazilian
paraiba.
While its colors are somewhat less bright than top Brazilian material,
Mozambique paraiba is often less included and has been found in larger
sizes. The Mozambique paraiba material usually is more intensely colored
than the Nigerian. There is a significant overlap in color and clarity
with Mozambique paraiba and Brazilian paraiba, especially with the
material from Rio Grande do Norte. While less expensive than top quality
Brazilian paraiba, some Mozambique material sells for well over $5,000
per
carat, which still is extremely high compared to other tourmalines.
Another highly valuable variety is chrome tourmaline, a rare type of dravite tourmaline from
Tanzania.
Chrome tourmaline is a rich green color due to the presence of chromium
atoms in the crystal; chromium also produces the green color of
emeralds. Of the standard elbaite colors, blue indicolite gems are typically the most valuable,
[18] followed by green verdelite and pink to red rubellite.
[citation needed] There are also yellow tourmalines, sometimes known as canary tourmaline.
Zambia
is rich in both red and yellow tourmaline, which are relatively
inexpensive in that country. Ironically the rarest variety, colorless
achroite, is not appreciated and is the least expensive of the
transparent tourmalines.
Afghanistan
Extra fine indicolite (blue tourmaline) and verdelite (green tourmaline) are found in the
Nuristan region (Ghazi Abad district) and Pech Valley (Pech and Chapa Dara districts) of
Kunar province. Gem-quality tourmalines are faceted (cut) from 0.50–10 gram sizes and have unusually high clarity and intense shades of color.
Em
sentido horário, no alto, anel Amazonas Esmeraldas, da Amsterdam Sauer,
R$ 50 mil; anel Mario Pantalena, R$ 5.900; Rua das Pedras, H.Stern, R$
9.790; anel Silvia Furmanovich, preço sob consulta; Anel de três dedos
Ara Vartanian, preço sob consulta
Diamond Rings, da Tiffany, sob consulta
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