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Tourmaline



Tourmaline

Schorl Tourmaline
General
Categorymineral group
Chemical formula(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
ColorMost commonly black, but can range from brown, violet, green, pink, or in a dual-colored pink and green.
Crystal habitParallel and elongated. Acicular prisms, sometimes radiating. Massive. Scattered grains (in granite).
Crystal systemTrigonal
CleavageIndistinct
FractureUneven, small conchoidal, brittle
Mohs Scale hardness7 - 7.5
LusterVitreous, sometimes resinous
Polish lusterVitreous[1]
Refractive indexnω=1.635 - 1.675, nε=1.610 - 1.650
Optical PropertiesDouble refractive, uniaxial negative[1]
Birefringence-0.018 to -0.040; typically about .020 but in dark stones it may reach .040[1]
Dispersion.017[1]
Pleochroismtypically 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
Ultraviolet fluorescencepink stones - inert to very weak red to violet in long and short wave[1]
Absorption spectraa strong narrow band at 498nm, 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]
StreakWhite
Specific gravity3.06 (+.20 -.06)[1]
Density2.82 - 3.32

Additional recommended knowledge

Contents

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. Another effect which made Tourmaline popular in the 18th century were its unusual electrical properties: Tourmaline crystals when warmed exhibit static electricity.

Tourmaline Species and Varieties

 

  • Dravite Species: from the Drave district of Carinthia)
    • Dark yellow to brownish black - dravite
  • Schorl Species:
    • Bluish or brownish black to Black - schorl
  • Elbaite Species: named after the island of Elba, Italy
    • Rose or pink - rubellite variety(from ruby)
    • Dark Black - indicolite variety (from indigo)
    • Light blue - Brazilian sapphire variety
    • Green - verdelite or Brazilian emerald variety
    • Colorless - achroite variety (from the Greek for "colorless")

Schorl

The most common variety 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 the year 1400 (AD) 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, a lot of black tourmaline was found. The first relatively detailed 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" (Ertl, 2006). Up to about 1600, additional names used in the German language were "Schurel", "Schörle", and "Schurl". From the 18th century on, the name "Schörl" was mainly used in the German-speaking area. In English, the names "shorl" and "shirl" were used in the 18th century for schorl. In the 19th century the names "common schorl", "schörl", "schorl" and "iron tourmaline" were used in the Anglo-Saxon area (Ertl, 2006). The word tourmaline has two entymologies, both from the Sinhalese word turamali, meaning "stone attracting ash" (a reference to its pyroelectric properties) or according to other sources "mixed gemstones".

Dravite

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 Mg-rich (and Na-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 (Ertl, 2007). 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 NaMg3(Al,Mg)6B3Si6O27(OH), which is in good agreement (except for the OH content) with the endmember formula of dravite as known today (Ertl, 2007).

Chemical Composition 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

XY3Z6(T6O18)(BO3)3V3W,

where:[3]

X = Ca, Na, K, vacancy

Y = Li, Mg, Fe2+, Mn2+, Zn, Al, Cr3+, V3+, Fe3+, Ti4+

Z = Mg, Al, Fe3+, Cr3+, V3+

T = Si, Al, B

B = B, vacancy

V = OH, O

W = OH, F, O

 

The 14 recognized minerals in the group (endmember formulas)
Buergerite NaFe3+3Al6Si6O18(BO3)3O3F
Chromdravite NaMg3Cr6Si6O18(BO3)3(OH)4
Dravite NaMg3Al6Si6O18(BO3)3(OH)4
Elbaite Na(Li1.5,Al1.5)Al6Si6O18(BO3)3(OH)4
Feruvite CaFe2+3(MgAl5)Si6O18(BO3)3(OH)4
Foitite (Fe2+2Al)Al6Si6O18(BO3)3(OH)4
Liddicoatite Ca(Li2Al)Al6Si6O18(BO3)3(OH)3F
Magnesiofoitite (Mg2Al)Al6Si6O18(BO3)3(OH)4
Olenite NaAl3Al6Si6O18(BO3)3O3OH
Povondraite NaFe3+3(Fe3+4Mg2)Si6O18(BO3)3(OH)3O
Rossmanite (LiAl2)Al6Si6O18(BO3)3(OH)4
Schorl NaFe2+3Al6Si6O18(BO3)3(OH)4
Uvite CaMg3(MgAl5)Si6O18(BO3)3(OH)3F
Vanadiumdravite NaMg3V6Si6O18(BO3)3(OH)4

Physical Properties

Crystal Structure

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. Interestingly, 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 has a wide 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 practically any color: blue, green, red, yellow, pink etc. Rarely, it is colorless. Bi-colored and multicolored crystals are relatively common, reflecting variations of fluid chemistry during crystallisation. 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 appear to change color when viewed from different directions.

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 impact the value. Heavily-included tourmalines, such as rubellite and Brazilian paraiba are sometimes clarity enhanced, which must be disclosed to the buyer[citation needed]. A clarity-enhanced tourmaline (especially paraiba) is worth much less than a non-treated gem.[4]

Geology

Tourmaline is found in two main geological occurrences. Igneous rocks, in particular granite and granite pegmatite 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. Also, tourmaline is a durable mineral and can be found in minor amounts as grains in sandstone and conglomerate.  

Tourmaline Localities

Today, 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, significant amounts of tourmaline are also mined in Tanzania, Nigeria, Kenya, Madagascar, Mozambique, Namibia, Afghanistan, Pakistan, and Sri Lanka, and Malawi.[5]

USA

Some fine gem and specimen material has been produced in the US, with the first discoveries having been made 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 interesting bicolors. During the early 1900s, Maine and California were the world's largest producers of gem tourmalines. The Empress Dowager Tz'u Hsi, the last Empress 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. [6] 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 [7].

Brazil

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. The new type of tourmaline, which soon became known as paraiba tourmaline, came in unusually vivid blues and greens. These colors were often described as "neon" since they appeared to glow. Brazilian paraiba tourmaline is usually heavily included. 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. The demand and excitement for this new material, which has fetched more than $50,000 per carat, earned more respect for the other colors of tourmaline.

Africa

In the late 90s, 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 unique variety that is also highly valued is chrome tourmaline, a rare type of dravite tourmaline from Tanzania which occurs in a very rich green color caused by chromium, the same element which causes the green in emerald. Of the standard Elbaite colors, generally blue indicolite gems are the most expensive, followed by green verdelite and pink to red rubellite. There are also yellow tourmalines, sometimes known as canary tourmaline. Ironically the rarest variety, colorless achroite, is not appreciated and is the least expensive of the transparent tourmalines.

References

  1. ^ a b c d e f g h i Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6
  2. ^ Mindat tourmaline group Accessed September 12 2005. This website details specifically and clearly how the complicated chemical formula is structured.
  3. ^ Hawthorne, F.C. & Henry, D.J. (1999). Classification of the minerals of the tourmaline group. European Journal of Mineralogy, 11, pp 201-215.[1]
  4. ^ Gemstone Enhancement: Heat, Irradiation, Impregnation, Dyeing, and Other Treatments by Kurt Nassau; Butterworth Publishers 1984
  5. ^ Manual of Mineralogy (after Dana), 19th Edition, By Hurlbut and Klien - John Wiley and Sons, Publishers
  6. ^ Exploring and Mining Gems and Gold in the West, By Fred Rynerson - Naturegraph Publishers.
  7. ^ GIA: Gems and Gemology, Vol. XII, Winter 1968-69, Page 358, Paul Willard Johnson: "Common Gems of San Diego"
  • Ertl, A., Pertlik, F. & Bernhardt, H.-J. (1997) Investigations on olenite with excess boron from the Koralpe, Styria, Austria, Österreichische Akademie der Wissenschaften, Mathematisch-Naturwissenschaftliche Klasse, Abt. I, Anzeiger, 134, pp 3-10. Article Online
  • Ertl, A. (2006) About the etymology and the type localities of schorl Mitteilungen der Österreichischen Mineralogischen Gesellschaft, 152, 2006, pp 7–16. Article Online
  • Ertl, A. (2007) About the type locality and the nomenclature of dravite Mitteilungen der Österreichischen Mineralogischen Gesellschaft, 153, 2007, pp 265–271. Article Online
  • Schumann, Walter (2006) Gemstones of the World 3rd Edition Sterling Publishing, New York; pp 126-127
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Tourmaline". A list of authors is available in Wikipedia.
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