Tourmaline ( ) is a crystalline silicate mineral group in which boron is compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. This gemstone comes in a wide variety of colors.
The name is derived from the Sinhalese (), which refers to the carnelian gemstones.
History Tourmaline comes from the Sinhalese toramalli, a general name for cornelian.
Brightly colored Ceylonese gem tourmalines were brought to Europe in great quantities by the Dutch East India Company, to satisfy a demand for curiosities and gems. Tourmaline was sometimes called the "Ceylonese Magnet" because it could attract and then repel hot ashes due to its pyroelectric properties.
Tourmalines were used by chemists in the 19th century to polarize light by shining rays onto a cut and polished surface of the gem.
Species and varieties Commonly encountered species and varieties of tourmaline include the following: Schorl species Brownish-black to black— Dravite species (from the Drave district of Carinthia) Dark yellow to brownish-black— Elbaite species (named after the island of Elba, Italy) Red or pinkish-red— variety Light blue to bluish-green— variety (from indigo) Green— variety Colorless— variety ()
Schorl
The most common species of tourmaline is , the sodium iron (divalent) endmember of the group. 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), and the 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 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". 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.
Dravite
, also called , is the sodium magnesium rich tourmaline endmember. Uvite, in comparison, is a calcium magnesium tourmaline. Dravite forms multiple series, with other tourmaline members, including schorl and elbaite.
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 village Dobrova near Unterdrauburg in the Drava river area, Carinthia, Austro-Hungarian Empire. Today this tourmaline locality (type locality for dravite) at Dobrova (near Dravograd), 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 , which is in good agreement (except for the OH content) with the endmember formula of dravite as known today.
Dravite varieties include the deep green chromium dravite and the vanadium dravite.
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.Ertl, 2008. 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 . 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. In 1933 Winchell published an updated formula for elbaite, , which is commonly used to date written as . 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
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 , where:Hawthorne, F.C. & Henry, D.J. (1999). "Classification of the minerals of the tourmaline group" . European Journal of Mineralogy, 11, pp. 201–215.
X = Ca, Na, K, ▢ = vacancy Y = Li, Mg, Fe2+, Mn2+, Zn, Al, Cr3+, V3+, Fe3+, Ti4+, ▢ = vacancy Z = Mg, Al, Fe3+, Cr3+, V3+ T = Si, Al, B B = B, ▢ = vacancy V = OH, O W = OH, F, O
+ The 42 minerals in the group (endmember formulas) recognized by the International Mineralogical Association Species Name Ideal Endmember Formula IMA Number Symbol Adachiite CaFe2+3Al6(Si5AlO18)(BO3)3(OH)3OH 2012-101 Adc Alumino-oxy-rossmanite ▢Al3Al6(Si5AlO18)(BO3)3(OH)3O 2020-008 Aorsm Bosiite NaFe3+3(Al4Mg2)Si6O18(BO3)3(OH)3O 2014-094 Bos Celleriite ▢(Mn2+2Al)Al6(Si6O18)(BO3)3(OH)3(OH) 2019-089 Cll Chromium-dravite NaMg3Cr6Si6O18(BO3)3(OH)3OH 1982-055 Cdrv Chromo-alumino-povondraite NaCr3(Al4Mg2)Si6O18(BO3)3(OH)3O 2013-089 Capov Darrellhenryite NaLiAl2Al6Si6O18(BO3)3(OH)3O 2012-026 Dhry Dravite NaMg3Al6Si6O18(BO3)3(OH)3OH - 1884 - Drv Dutrowite Na(Fe2.5Ti0.5)Al6Si6O18(BO3)3(OH)3O 2019-082 Dtw ElbaiteNa(Li1.5,Al1.5)Al6Si6O18(BO3)3(OH)3OH - 1913 - Elb ErtliteNaAl3Al6(Si4B2O18)(BO3)3(OH)3O 2023-086 Etl Ferro-bosiite NaFe3+3(Al4Fe2+2)Si6O18(BO3)3(OH)3O 2022-069 Fbos Feruvite CaFe2+3(MgAl5)Si6O18(BO3)3(OH)3OH 1987-057 Fer Fluor-buergerite NaFe3+3Al6Si6O18(BO3)3O3F 1965-005 Fbu Fluor-dravite NaMg3Al6Si6O18(BO3)3(OH)3F 2009-089 Fdrv Fluor-elbaite Na(Li1.5,Al1.5)Al6Si6O18(BO3)3(OH)3F 2011-071 Felb Fluor-liddicoatite Ca(Li2,Al)Al6Si6O18(BO3)3(OH)3F 1976-041 Fld Fluor-rossmanite ▢(LiAl2)Al6Si6O18(BO3)3(OH)3F 2023-111 Frsm Fluor-schorl NaFe2+3Al6Si6O18(BO3)3(OH)3F 2010-067 Fsrl Fluor-tsilaisite NaMn2+3Al6Si6O18(BO3)3(OH)3F 2012-044 Ftl Fluor-uvite CaMg3(Al5Mg)Si6O18(BO3)3(OH)3F - 1930 - Fluvt Foitite ▢(Fe2+2Al)Al6Si6O18(BO3)3(OH)3OH 1992-034 Foi Liddicoatite Ca(Li2,Al)Al6Si6O18(BO3)3(OH)3(OH) 2025-047 Ld Lucchesiite Ca(Fe2+)3Al6Si6O18(BO3)3(OH)3O 2015-043 Lcc Magnesio-dutrowite Na(Mg2.5Ti0.5)Al6Si6O18(BO3)3(OH)3O 2023-015 Mdtw Magnesio-foitite ▢(Mg2Al)Al6Si6O18(BO3)3(OH)3OH 1998-037 Mfoi Magnesio-lucchesite Ca(Mg3Al6Si6O18(BO3)3(OH)3O 2019-025 Mlcc Maruyamaite K(MgAl2)(Al5Mg)Si6O18(BO3)3(OH)3O 2013-123 Mry Olenite NaAl3Al6Si6O18(BO3)3O3OH 1985-006 Ole Oxy-chromium-dravite NaCr3(Mg2Cr4)Si6O18(BO3)3(OH)3O 2011-097 Ocdrv Oxy-dravite Na(Al2Mg)(Al5Mg)Si6O18(BO3)3(OH)3O 2012-004 Odrv Oxy-foitite ▢(Fe2+Al2)Al6Si6O18(BO3)3(OH)3O 2016-069 Ofoi Oxy-schorl Na(Fe2+2Al)Al6Si6O18(BO3)3(OH)3O 2011-011 Osrl Oxy-vanadium-dravite NaV3(V4Mg2)Si6O18(BO3)3(OH)3O 1999-050 Ovdrv Povondraite NaFe3+3(Fe3+4Mg2)Si6O18(BO3)3(OH)3O 1979 Pov Princivalleite Na(Mn2Al)Al6Si6O18(BO3)3(OH)3O 2020-056 Pva Rossmanite ▢(LiAl2)Al6Si6O18(BO3)3(OH)3OH 1996-018 Rsm Schorl NaFe2+3Al6Si6O18(BO3)3(OH)3OH - 1505 - Srl Tsilaisite NaMn2+3Al6Si6O18(BO3)3(OH)3OH 2011-047 Tsl Uvite CaMg3(Al5Mg)Si6O18(BO3)3(OH)3OH 2000-030 Uvt Vanadio-oxy-chromium-dravite NaV3(Cr4Mg2)Si6O18(BO3)3(OH)3O 2012-034 Vocdrv Vanadio-oxy-dravite NaV3(Al4Mg2)Si6O18(BO3)3(OH)3O 2012-074 Vodrv
Mineral species that were named before the IMA was founded in 1958 do not have an IMA number.
The IMA commission on new mineral names published a list of approved symbols for each mineral species in 2021.Warr, L.N. "IMA–CNMNC approved mineral symbols." Mineralogical Magazine, 2021, v. 85, p. 291–320. doi:10.1180/mgm.2021.43.
A revised nomenclature for the tourmaline group was published in 2011.Erratum: American Mineralogist (2013), Volume 98, page 524.Frank C. Hawthorne and Dona M. Dirlam. "Tourmaline: Tourmaline the Indicator Mineral: From Atomic Arrangement to Viking Navigation." Elements, October 2011, v. 7, p. (5): 307–312, doi:10.2113/gselements.7.5.307.
Physical properties Crystal structure
Tourmaline is a six-member ring cyclosilicate having a trigonal crystal system. It occurs as long, slender to thick prismatic and columnar crystals that are usually triangular in cross-section, often with curved striated faces. The style of termination at the ends of crystals is sometimes 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.
A crystal of tourmaline is built up of units consisting of a six-member silica ring that binds above to a large cation, such as sodium. The ring binds below to a layer of metal ions and hydroxyls or halogens, which structurally resembles a fragment of kaolin. This in turn binds to three triangular borate ions. Units joined end to end form columns running the length of the crystal. Each column binds with two other columns offset one-third and two-thirds of the vertical length of a single unit to form bundles of three columns. Bundles are packed together to form the final crystal structure. Because the neighboring columns are offset, the basic structural unit is not a unit cell: The actual unit cell of this structure includes portions of several units belonging to adjacent columns.
Color
Tourmaline has a variety of colors. Iron-rich tourmalines are usually 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 and is prized in jewelry. An excellent example of watermelon tourmaline jewelry is a brooch piece (1969, gold, watermelon tourmaline, diamonds) by Andrew Grima (British, b. Italy, 1921–2007), in the collection of Kimberly Klosterman and on display at the Cincinnati Art Museum. Some forms of tourmaline are dichroic; they change color when viewed from different directions.
The pink color of tourmalines from many localities 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 Mn3+ ions occurs, which is responsible for the deepening of the pink to red color.Reinitz & Rossman, 1988.
Magnetism