Tourmaline
| Tourmaline | |
 Schorl Tourmaline | |
| General | |
|---|---|
| Category | Silicate mineral 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 | |
| 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 Scalehardness | 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] |
| Ultravioletfluorescence | pink stones—inert to very weak red to violet in long and short wave[1] |
| Absorption spectra | a 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] |
Tourmaline is a crystal silicate mineral compounded with elements such as aluminium, iron,magnesium, sodium, lithium, or potassium. Tourmaline is classed as a semi-precious stone and the gem comes in a wide variety of colors. The name comes from the Sinhalese word "turamali" or "toramalli", which applied to different gemstones found in Ceylon (now Sri Lanka).
History
Brightly colored Sri Lankan gem tourmalines were brought to Europe in great quantities by theDutch 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
- 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
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 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, 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" (Ertl, 2006). Up to about 1600, additional names used in the German languagewere "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 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
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).
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 Arfwedson for the first time (Ertl, 2008). Elba Island, Italy,was one of the first localities where colored and colorless Li-tourmalines were extensively chemically analysed. In 1850 Rammelsberg described fluorine 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 Vernadsky proposed the name “Elbait” for Li-,Na-, and Al-rich tourmaline from Elba Island, Italy, with the simplified formula (Li,Na)HAl6B2Si4O21 (Ertl, 2008). Most likely the type material for elbaite was found at Fonte del Prete, San Piero in Campo, Campo nell'Elba, Elba Island, Livorno Province, Tuscany, Italy (Ertl, 2008). In 1933 Winchell published an updated formula for elbaite, H8Na2Li3Al3B6Al12Si12O62, which is commonly used to date written as Na(Li1.5Al1.5)Al6(BO3)3[Si6O18](OH)3(OH) (Ertl, 2008). 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, U.S.A.
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]
Y = Li, Mg, Fe2+, Mn2+, Zn, Al, Cr3+, V3+, Fe3+, Ti4+, vacancy
Z = Mg, Al, Fe3+, Cr3+, V3+
B = B, vacancy
W = OH, F, O
| 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. 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 oftenpyroelectric as well.
Color
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 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 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. 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 graniteand 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. Tourmaline is a durable mineral and can be found in minor amounts as grains in sandstone and conglomerate.
Tourmaline localities
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 inTanzania, Nigeria, Kenya, Madagascar, Mozambique, Namibia, Afghanistan, Pakistan, andSri Lanka, and Malawi.[5]
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, 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 ofMinas 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.
Africa
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 fromTanzania. 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, 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
In the Nuristan region of Kunar province extra fine Indicolite (blue tourmaline) and Verderite (green tourmaline) are found. Gem quality Tourmaline are faceted (cut) from 0.50-10 gram sizes and have unusually high clarity and intense shades of color
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