paraiba tourmaline

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
  • Red or pinkish-red—rubellite variety (from ruby)
  • Dark black—schorl (from indigo)
  • Light blue to bluish green—Brazilian indicolite variety
  • Green—verdelite or Brazilian emerald variety
  • Colorless—achroite variety (from the Greek "άχρωμος" meaning "colorless")

[edit]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 used in the Anglo-Saxon area.^[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".

[edit]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 NaMg₃(Al,Mg)₆B₃Si₆O₂₇(OH), which is in good agreement (except for the OH content) with the endmember formula of dravite as known today.^[4]

[edit]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.^[5] Elba Island, Italy, was one of the first localities where colored and colorless Li-tourmalines were extensively chemically analysed. In 1850 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 Vernadsky proposed the name Elbait for lithium-, sodium-, and aluminum-rich tourmaline from Elba Island, Italy, with the simplified formula (Li,Na)HAl₆B₂Si₄O₂₁.^[5] 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.^[5] In 1933 Winchell published an updated formula for elbaite, H₈Na₂Li₃Al₃B₆Al₁₂Si₁₂O₆₂, which is commonly used to date written as Na(Li_1.5Al_1.5)Al₆(BO₃)₃[Si₆O₁₈](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, USA.

[edit]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

XY₃Z₆(T₆O₁₈)(BO₃)₃V₃W,

where:^[6]

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

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